binutils-gdb/gdb/nat/linux-procfs.h

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/* Linux-specific PROCFS manipulation routines.
Copyright (C) 2011-2018 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef COMMON_LINUX_PROCFS_H
#define COMMON_LINUX_PROCFS_H
#include <unistd.h>
/* Return the TGID of LWPID from /proc/pid/status. Returns -1 if not
Linux: on attach, attach to lwps listed under /proc/$pid/task/ ... instead of relying on libthread_db. I wrote a test that attaches to a program that constantly spawns short-lived threads, which exposed several issues. This is one of them. On Linux, we need to attach to all threads of a process (thread group) individually. We currently rely on libthread_db to list the threads, but that is problematic, because libthread_db relies on reading data structures out of the inferior (which may well be corrupted). If threads are being created or exiting just while we try to attach, we may trip on inconsistencies in the inferior's thread list. To work around that, when we see a seemingly corrupt list, we currently retry a few times: static void thread_db_find_new_threads_2 (ptid_t ptid, int until_no_new) { ... if (until_no_new) { /* Require 4 successive iterations which do not find any new threads. The 4 is a heuristic: there is an inherent race here, and I have seen that 2 iterations in a row are not always sufficient to "capture" all threads. */ ... That heuristic may well fail, and when it does, we end up with threads in the program that aren't under GDB's control. That's obviously bad and results in quite mistifying failures, like e.g., the process dying for seeminly no reason when a thread that wasn't attached trips on a breakpoint. There's really no reason to rely on libthread_db for this nowadays when we have /proc mounted. In that case, which is the usual case, we can list the LWPs from /proc/PID/task/. In fact, GDBserver is already doing this. The patch factors out that code that knows to walk the task/ directory out of GDBserver, and makes GDB use it too. Like GDBserver, the patch makes GDB attach to LWPs and _not_ wait for them to stop immediately. Instead, we just tag the LWP as having an expected stop. Because we can only set the ptrace options when the thread stops, we need a new flag in the lwp structure to keep track of whether we've already set the ptrace options, just like in GDBserver. Note that nothing issues any ptrace command to the threads between the PTRACE_ATTACH and the stop, so this is safe (unlike one scenario described in gdbserver's linux-low.c). When we attach to a program that has threads exiting while we attach, it's easy to race with a thread just exiting as we try to attach to it, like: #1 - get current list of threads #2 - attach to each listed thread #3 - ooops, attach failed, thread is already gone As this is pretty normal, we shouldn't be issuing a scary warning in step #3. When #3 happens, PTRACE_ATTACH usually fails with ESRCH, but sometimes we'll see EPERM as well. That happens when the kernel still has the thread in its task list, but the thread is marked as dead. Unfortunately, EPERM is ambiguous and we'll get it also on other scenarios where the thread isn't dead, and in those cases, it's useful to get a warning. To distiguish the cases, when we get an EPERM failure, we open /proc/PID/status, and check the thread's state -- if the /proc file no longer exists, or the state is "Z (Zombie)" or "X (Dead)", we ignore the EPERM error silently; otherwise, we'll warn. Unfortunately, there seems to be a kernel race here. Sometimes I get EPERM, and then the /proc state still indicates "R (Running)"... If we wait a bit and retry, we do end up seeing X or Z state, or get an ESRCH. I thought of making GDB retry the attach a few times, but even with a 500ms wait and 4 retries, I still see the warning sometimes. I haven't been able to identify the kernel path that causes this yet, but in any case, it looks like a kernel bug to me. As this just results failure to suppress a warning that we've been printing since about forever anyway, I'm just making the test cope with it, and issue an XFAIL. gdb/gdbserver/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-low.c (linux_attach_fail_reason_string): Move to nat/linux-ptrace.c, and rename. (linux_attach_lwp): Update comment. (attach_proc_task_lwp_callback): New function. (linux_attach): Adjust to rename and use linux_proc_attach_tgid_threads. (linux_attach_fail_reason_string): Delete declaration. gdb/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-nat.c (attach_proc_task_lwp_callback): New function. (linux_nat_attach): Use linux_proc_attach_tgid_threads. (wait_lwp, linux_nat_filter_event): If not set yet, set the lwp's ptrace option flags. * linux-nat.h (struct lwp_info) <must_set_ptrace_flags>: New field. * nat/linux-procfs.c: Include <dirent.h>. (linux_proc_get_int): New parameter "warn". Handle it. (linux_proc_get_tgid): Adjust. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this. (linux_proc_pid_get_state): New function, factored out from (linux_proc_pid_has_state): ... this. Add new parameter "warn" and handle it. (linux_proc_pid_is_gone): New function. (linux_proc_pid_is_stopped): Adjust. (linux_proc_pid_is_zombie_maybe_warn) (linux_proc_pid_is_zombie_nowarn): New functions. (linux_proc_pid_is_zombie): Use linux_proc_pid_is_zombie_maybe_warn. (linux_proc_attach_tgid_threads): New function. * nat/linux-procfs.h (linux_proc_get_tgid): Update comment. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this, and update comment. (linux_proc_pid_is_gone): New declaration. (linux_proc_pid_is_zombie): Update comment. (linux_proc_pid_is_zombie_nowarn): New declaration. (linux_proc_attach_lwp_func): New typedef. (linux_proc_attach_tgid_threads): New declaration. * nat/linux-ptrace.c (linux_ptrace_attach_fail_reason): Adjust to use nowarn functions. (linux_ptrace_attach_fail_reason_string): Move here from gdbserver/linux-low.c and rename. (ptrace_supports_feature): If the current ptrace options are not known yet, check them now, instead of asserting. * nat/linux-ptrace.h (linux_ptrace_attach_fail_reason_string): Declare.
2014-12-16 17:12:24 +01:00
found. Failure to open the /proc file results in a warning. */
extern int linux_proc_get_tgid (pid_t lwpid);
Linux: on attach, attach to lwps listed under /proc/$pid/task/ ... instead of relying on libthread_db. I wrote a test that attaches to a program that constantly spawns short-lived threads, which exposed several issues. This is one of them. On Linux, we need to attach to all threads of a process (thread group) individually. We currently rely on libthread_db to list the threads, but that is problematic, because libthread_db relies on reading data structures out of the inferior (which may well be corrupted). If threads are being created or exiting just while we try to attach, we may trip on inconsistencies in the inferior's thread list. To work around that, when we see a seemingly corrupt list, we currently retry a few times: static void thread_db_find_new_threads_2 (ptid_t ptid, int until_no_new) { ... if (until_no_new) { /* Require 4 successive iterations which do not find any new threads. The 4 is a heuristic: there is an inherent race here, and I have seen that 2 iterations in a row are not always sufficient to "capture" all threads. */ ... That heuristic may well fail, and when it does, we end up with threads in the program that aren't under GDB's control. That's obviously bad and results in quite mistifying failures, like e.g., the process dying for seeminly no reason when a thread that wasn't attached trips on a breakpoint. There's really no reason to rely on libthread_db for this nowadays when we have /proc mounted. In that case, which is the usual case, we can list the LWPs from /proc/PID/task/. In fact, GDBserver is already doing this. The patch factors out that code that knows to walk the task/ directory out of GDBserver, and makes GDB use it too. Like GDBserver, the patch makes GDB attach to LWPs and _not_ wait for them to stop immediately. Instead, we just tag the LWP as having an expected stop. Because we can only set the ptrace options when the thread stops, we need a new flag in the lwp structure to keep track of whether we've already set the ptrace options, just like in GDBserver. Note that nothing issues any ptrace command to the threads between the PTRACE_ATTACH and the stop, so this is safe (unlike one scenario described in gdbserver's linux-low.c). When we attach to a program that has threads exiting while we attach, it's easy to race with a thread just exiting as we try to attach to it, like: #1 - get current list of threads #2 - attach to each listed thread #3 - ooops, attach failed, thread is already gone As this is pretty normal, we shouldn't be issuing a scary warning in step #3. When #3 happens, PTRACE_ATTACH usually fails with ESRCH, but sometimes we'll see EPERM as well. That happens when the kernel still has the thread in its task list, but the thread is marked as dead. Unfortunately, EPERM is ambiguous and we'll get it also on other scenarios where the thread isn't dead, and in those cases, it's useful to get a warning. To distiguish the cases, when we get an EPERM failure, we open /proc/PID/status, and check the thread's state -- if the /proc file no longer exists, or the state is "Z (Zombie)" or "X (Dead)", we ignore the EPERM error silently; otherwise, we'll warn. Unfortunately, there seems to be a kernel race here. Sometimes I get EPERM, and then the /proc state still indicates "R (Running)"... If we wait a bit and retry, we do end up seeing X or Z state, or get an ESRCH. I thought of making GDB retry the attach a few times, but even with a 500ms wait and 4 retries, I still see the warning sometimes. I haven't been able to identify the kernel path that causes this yet, but in any case, it looks like a kernel bug to me. As this just results failure to suppress a warning that we've been printing since about forever anyway, I'm just making the test cope with it, and issue an XFAIL. gdb/gdbserver/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-low.c (linux_attach_fail_reason_string): Move to nat/linux-ptrace.c, and rename. (linux_attach_lwp): Update comment. (attach_proc_task_lwp_callback): New function. (linux_attach): Adjust to rename and use linux_proc_attach_tgid_threads. (linux_attach_fail_reason_string): Delete declaration. gdb/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-nat.c (attach_proc_task_lwp_callback): New function. (linux_nat_attach): Use linux_proc_attach_tgid_threads. (wait_lwp, linux_nat_filter_event): If not set yet, set the lwp's ptrace option flags. * linux-nat.h (struct lwp_info) <must_set_ptrace_flags>: New field. * nat/linux-procfs.c: Include <dirent.h>. (linux_proc_get_int): New parameter "warn". Handle it. (linux_proc_get_tgid): Adjust. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this. (linux_proc_pid_get_state): New function, factored out from (linux_proc_pid_has_state): ... this. Add new parameter "warn" and handle it. (linux_proc_pid_is_gone): New function. (linux_proc_pid_is_stopped): Adjust. (linux_proc_pid_is_zombie_maybe_warn) (linux_proc_pid_is_zombie_nowarn): New functions. (linux_proc_pid_is_zombie): Use linux_proc_pid_is_zombie_maybe_warn. (linux_proc_attach_tgid_threads): New function. * nat/linux-procfs.h (linux_proc_get_tgid): Update comment. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this, and update comment. (linux_proc_pid_is_gone): New declaration. (linux_proc_pid_is_zombie): Update comment. (linux_proc_pid_is_zombie_nowarn): New declaration. (linux_proc_attach_lwp_func): New typedef. (linux_proc_attach_tgid_threads): New declaration. * nat/linux-ptrace.c (linux_ptrace_attach_fail_reason): Adjust to use nowarn functions. (linux_ptrace_attach_fail_reason_string): Move here from gdbserver/linux-low.c and rename. (ptrace_supports_feature): If the current ptrace options are not known yet, check them now, instead of asserting. * nat/linux-ptrace.h (linux_ptrace_attach_fail_reason_string): Declare.
2014-12-16 17:12:24 +01:00
/* Return the TracerPid of LWPID from /proc/pid/status. Returns -1 if
not found. Does not warn on failure to open the /proc file. */
Linux: on attach, attach to lwps listed under /proc/$pid/task/ ... instead of relying on libthread_db. I wrote a test that attaches to a program that constantly spawns short-lived threads, which exposed several issues. This is one of them. On Linux, we need to attach to all threads of a process (thread group) individually. We currently rely on libthread_db to list the threads, but that is problematic, because libthread_db relies on reading data structures out of the inferior (which may well be corrupted). If threads are being created or exiting just while we try to attach, we may trip on inconsistencies in the inferior's thread list. To work around that, when we see a seemingly corrupt list, we currently retry a few times: static void thread_db_find_new_threads_2 (ptid_t ptid, int until_no_new) { ... if (until_no_new) { /* Require 4 successive iterations which do not find any new threads. The 4 is a heuristic: there is an inherent race here, and I have seen that 2 iterations in a row are not always sufficient to "capture" all threads. */ ... That heuristic may well fail, and when it does, we end up with threads in the program that aren't under GDB's control. That's obviously bad and results in quite mistifying failures, like e.g., the process dying for seeminly no reason when a thread that wasn't attached trips on a breakpoint. There's really no reason to rely on libthread_db for this nowadays when we have /proc mounted. In that case, which is the usual case, we can list the LWPs from /proc/PID/task/. In fact, GDBserver is already doing this. The patch factors out that code that knows to walk the task/ directory out of GDBserver, and makes GDB use it too. Like GDBserver, the patch makes GDB attach to LWPs and _not_ wait for them to stop immediately. Instead, we just tag the LWP as having an expected stop. Because we can only set the ptrace options when the thread stops, we need a new flag in the lwp structure to keep track of whether we've already set the ptrace options, just like in GDBserver. Note that nothing issues any ptrace command to the threads between the PTRACE_ATTACH and the stop, so this is safe (unlike one scenario described in gdbserver's linux-low.c). When we attach to a program that has threads exiting while we attach, it's easy to race with a thread just exiting as we try to attach to it, like: #1 - get current list of threads #2 - attach to each listed thread #3 - ooops, attach failed, thread is already gone As this is pretty normal, we shouldn't be issuing a scary warning in step #3. When #3 happens, PTRACE_ATTACH usually fails with ESRCH, but sometimes we'll see EPERM as well. That happens when the kernel still has the thread in its task list, but the thread is marked as dead. Unfortunately, EPERM is ambiguous and we'll get it also on other scenarios where the thread isn't dead, and in those cases, it's useful to get a warning. To distiguish the cases, when we get an EPERM failure, we open /proc/PID/status, and check the thread's state -- if the /proc file no longer exists, or the state is "Z (Zombie)" or "X (Dead)", we ignore the EPERM error silently; otherwise, we'll warn. Unfortunately, there seems to be a kernel race here. Sometimes I get EPERM, and then the /proc state still indicates "R (Running)"... If we wait a bit and retry, we do end up seeing X or Z state, or get an ESRCH. I thought of making GDB retry the attach a few times, but even with a 500ms wait and 4 retries, I still see the warning sometimes. I haven't been able to identify the kernel path that causes this yet, but in any case, it looks like a kernel bug to me. As this just results failure to suppress a warning that we've been printing since about forever anyway, I'm just making the test cope with it, and issue an XFAIL. gdb/gdbserver/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-low.c (linux_attach_fail_reason_string): Move to nat/linux-ptrace.c, and rename. (linux_attach_lwp): Update comment. (attach_proc_task_lwp_callback): New function. (linux_attach): Adjust to rename and use linux_proc_attach_tgid_threads. (linux_attach_fail_reason_string): Delete declaration. gdb/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-nat.c (attach_proc_task_lwp_callback): New function. (linux_nat_attach): Use linux_proc_attach_tgid_threads. (wait_lwp, linux_nat_filter_event): If not set yet, set the lwp's ptrace option flags. * linux-nat.h (struct lwp_info) <must_set_ptrace_flags>: New field. * nat/linux-procfs.c: Include <dirent.h>. (linux_proc_get_int): New parameter "warn". Handle it. (linux_proc_get_tgid): Adjust. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this. (linux_proc_pid_get_state): New function, factored out from (linux_proc_pid_has_state): ... this. Add new parameter "warn" and handle it. (linux_proc_pid_is_gone): New function. (linux_proc_pid_is_stopped): Adjust. (linux_proc_pid_is_zombie_maybe_warn) (linux_proc_pid_is_zombie_nowarn): New functions. (linux_proc_pid_is_zombie): Use linux_proc_pid_is_zombie_maybe_warn. (linux_proc_attach_tgid_threads): New function. * nat/linux-procfs.h (linux_proc_get_tgid): Update comment. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this, and update comment. (linux_proc_pid_is_gone): New declaration. (linux_proc_pid_is_zombie): Update comment. (linux_proc_pid_is_zombie_nowarn): New declaration. (linux_proc_attach_lwp_func): New typedef. (linux_proc_attach_tgid_threads): New declaration. * nat/linux-ptrace.c (linux_ptrace_attach_fail_reason): Adjust to use nowarn functions. (linux_ptrace_attach_fail_reason_string): Move here from gdbserver/linux-low.c and rename. (ptrace_supports_feature): If the current ptrace options are not known yet, check them now, instead of asserting. * nat/linux-ptrace.h (linux_ptrace_attach_fail_reason_string): Declare.
2014-12-16 17:12:24 +01:00
extern pid_t linux_proc_get_tracerpid_nowarn (pid_t lwpid);
/* Detect `T (stopped)' in `/proc/PID/status'.
Other states including `T (tracing stop)' are reported as false. */
extern int linux_proc_pid_is_stopped (pid_t pid);
Fix race exposed by gdb.threads/killed.exp On GNU/Linux, this test sometimes FAILs like this: (gdb) run Starting program: /home/pedro/gdb/mygit/build/gdb/testsuite/gdb.threads/killed [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib64/libthread_db.so.1". ptrace: No such process. (gdb) Program terminated with signal SIGKILL, Killed. The program no longer exists. FAIL: gdb.threads/killed.exp: run program to completion (timeout) Note the suspicious "No such process" line (that's errno==ESRCH). Adding debug output we see: linux_nat_wait: [process -1], [TARGET_WNOHANG] LLW: enter LNW: waitpid(-1, ...) returned 18465, ERRNO-OK LLW: waitpid 18465 received Stopped (signal) (stopped) LNW: waitpid(-1, ...) returned 18461, ERRNO-OK LLW: waitpid 18461 received Trace/breakpoint trap (stopped) LLW: Handling extended status 0x03057f LHEW: Got clone event from LWP 18461, new child is LWP 18465 LNW: waitpid(-1, ...) returned 0, ERRNO-OK RSRL: resuming stopped-resumed LWP LWP 18465 at 0x3b36af4b51: step=0 RSRL: resuming stopped-resumed LWP LWP 18461 at 0x3b36af4b51: step=0 sigchld ptrace: No such process. (gdb) linux_nat_wait: [process -1], [TARGET_WNOHANG] LLW: enter LNW: waitpid(-1, ...) returned 18465, ERRNO-OK LLW: waitpid 18465 received Killed (terminated) LLW: LWP 18465 exited. LNW: waitpid(-1, ...) returned 18461, No child processes LLW: waitpid 18461 received Killed (terminated) Process 18461 exited LNW: waitpid(-1, ...) returned -1, No child processes LLW: exit sigchld infrun: target_wait (-1, status) = infrun: 18461 [process 18461], infrun: status->kind = signalled, signal = GDB_SIGNAL_KILL infrun: TARGET_WAITKIND_SIGNALLED Program terminated with signal SIGKILL, Killed. The program no longer exists. infrun: stop_waiting FAIL: gdb.threads/killed.exp: run program to completion (timeout) The issue is that here: RSRL: resuming stopped-resumed LWP LWP 18465 at 0x3b36af4b51: step=0 RSRL: resuming stopped-resumed LWP LWP 18461 at 0x3b36af4b51: step=0 The first line shows we had just resumed LWP 18465, which does: void * child_func (void *dummy) { kill (pid, SIGKILL); exit (1); } So if the kernel manages to schedule that thread fast enough, the process may be killed before GDB has a chance to resume LWP 18461. GDBserver has code at the tail end of linux_resume_one_lwp to cope with this: ~~~ ptrace (step ? PTRACE_SINGLESTEP : PTRACE_CONT, lwpid_of (thread), (PTRACE_TYPE_ARG3) 0, /* Coerce to a uintptr_t first to avoid potential gcc warning of coercing an 8 byte integer to a 4 byte pointer. */ (PTRACE_TYPE_ARG4) (uintptr_t) signal); current_thread = saved_thread; if (errno) { /* ESRCH from ptrace either means that the thread was already running (an error) or that it is gone (a race condition). If it's gone, we will get a notification the next time we wait, so we can ignore the error. We could differentiate these two, but it's tricky without waiting; the thread still exists as a zombie, so sending it signal 0 would succeed. So just ignore ESRCH. */ if (errno == ESRCH) return; perror_with_name ("ptrace"); } ~~~ However, that's not a complete fix, because between starting to handle the resume request and getting that PTRACE_CONTINUE, we run other ptrace calls that can also fail with ESRCH, and that end up throwing an error (with perror_with_name). In the case above, I indeed sometimes see resume_stopped_resumed_lwps fail in the registers read: resume_stopped_resumed_lwps (struct lwp_info *lp, void *data) { ... CORE_ADDR pc = regcache_read_pc (regcache); Or e.g., in 32-bit mode, i386_linux_resume has several calls that can throw too. Whether to ignore ptrace errors or not depends on context that is only available somewhere up the call chain. So the fix is to let ptrace errors throw as they do today, and wrap the resume request in a TRY/CATCH that swallows it iff the lwp that we were trying to resume is no longer ptrace-stopped. gdb/gdbserver/ChangeLog: 2015-03-19 Pedro Alves <palves@redhat.com> * linux-low.c (linux_resume_one_lwp): Rename to ... (linux_resume_one_lwp_throw): ... this. Don't handle ESRCH here, instead call perror_with_name. (check_ptrace_stopped_lwp_gone): New function. (linux_resume_one_lwp): Reimplement as wrapper around linux_resume_one_lwp_throw that swallows errors if the LWP is gone. gdb/ChangeLog: 2015-03-19 Pedro Alves <palves@redhat.com> * linux-nat.c (linux_resume_one_lwp): Rename to ... (linux_resume_one_lwp_throw): ... this. Don't handle ESRCH here, instead call perror_with_name. (check_ptrace_stopped_lwp_gone): New function. (linux_resume_one_lwp): Reimplement as wrapper around linux_resume_one_lwp_throw that swallows errors if the LWP is gone. (resume_stopped_resumed_lwps): Try register reads in TRY/CATCH and swallows errors if the LWP is gone. Use linux_resume_one_lwp_throw instead of linux_resume_one_lwp.
2015-03-19 16:12:33 +01:00
extern int linux_proc_pid_is_trace_stopped_nowarn (pid_t pid);
Linux: on attach, attach to lwps listed under /proc/$pid/task/ ... instead of relying on libthread_db. I wrote a test that attaches to a program that constantly spawns short-lived threads, which exposed several issues. This is one of them. On Linux, we need to attach to all threads of a process (thread group) individually. We currently rely on libthread_db to list the threads, but that is problematic, because libthread_db relies on reading data structures out of the inferior (which may well be corrupted). If threads are being created or exiting just while we try to attach, we may trip on inconsistencies in the inferior's thread list. To work around that, when we see a seemingly corrupt list, we currently retry a few times: static void thread_db_find_new_threads_2 (ptid_t ptid, int until_no_new) { ... if (until_no_new) { /* Require 4 successive iterations which do not find any new threads. The 4 is a heuristic: there is an inherent race here, and I have seen that 2 iterations in a row are not always sufficient to "capture" all threads. */ ... That heuristic may well fail, and when it does, we end up with threads in the program that aren't under GDB's control. That's obviously bad and results in quite mistifying failures, like e.g., the process dying for seeminly no reason when a thread that wasn't attached trips on a breakpoint. There's really no reason to rely on libthread_db for this nowadays when we have /proc mounted. In that case, which is the usual case, we can list the LWPs from /proc/PID/task/. In fact, GDBserver is already doing this. The patch factors out that code that knows to walk the task/ directory out of GDBserver, and makes GDB use it too. Like GDBserver, the patch makes GDB attach to LWPs and _not_ wait for them to stop immediately. Instead, we just tag the LWP as having an expected stop. Because we can only set the ptrace options when the thread stops, we need a new flag in the lwp structure to keep track of whether we've already set the ptrace options, just like in GDBserver. Note that nothing issues any ptrace command to the threads between the PTRACE_ATTACH and the stop, so this is safe (unlike one scenario described in gdbserver's linux-low.c). When we attach to a program that has threads exiting while we attach, it's easy to race with a thread just exiting as we try to attach to it, like: #1 - get current list of threads #2 - attach to each listed thread #3 - ooops, attach failed, thread is already gone As this is pretty normal, we shouldn't be issuing a scary warning in step #3. When #3 happens, PTRACE_ATTACH usually fails with ESRCH, but sometimes we'll see EPERM as well. That happens when the kernel still has the thread in its task list, but the thread is marked as dead. Unfortunately, EPERM is ambiguous and we'll get it also on other scenarios where the thread isn't dead, and in those cases, it's useful to get a warning. To distiguish the cases, when we get an EPERM failure, we open /proc/PID/status, and check the thread's state -- if the /proc file no longer exists, or the state is "Z (Zombie)" or "X (Dead)", we ignore the EPERM error silently; otherwise, we'll warn. Unfortunately, there seems to be a kernel race here. Sometimes I get EPERM, and then the /proc state still indicates "R (Running)"... If we wait a bit and retry, we do end up seeing X or Z state, or get an ESRCH. I thought of making GDB retry the attach a few times, but even with a 500ms wait and 4 retries, I still see the warning sometimes. I haven't been able to identify the kernel path that causes this yet, but in any case, it looks like a kernel bug to me. As this just results failure to suppress a warning that we've been printing since about forever anyway, I'm just making the test cope with it, and issue an XFAIL. gdb/gdbserver/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-low.c (linux_attach_fail_reason_string): Move to nat/linux-ptrace.c, and rename. (linux_attach_lwp): Update comment. (attach_proc_task_lwp_callback): New function. (linux_attach): Adjust to rename and use linux_proc_attach_tgid_threads. (linux_attach_fail_reason_string): Delete declaration. gdb/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-nat.c (attach_proc_task_lwp_callback): New function. (linux_nat_attach): Use linux_proc_attach_tgid_threads. (wait_lwp, linux_nat_filter_event): If not set yet, set the lwp's ptrace option flags. * linux-nat.h (struct lwp_info) <must_set_ptrace_flags>: New field. * nat/linux-procfs.c: Include <dirent.h>. (linux_proc_get_int): New parameter "warn". Handle it. (linux_proc_get_tgid): Adjust. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this. (linux_proc_pid_get_state): New function, factored out from (linux_proc_pid_has_state): ... this. Add new parameter "warn" and handle it. (linux_proc_pid_is_gone): New function. (linux_proc_pid_is_stopped): Adjust. (linux_proc_pid_is_zombie_maybe_warn) (linux_proc_pid_is_zombie_nowarn): New functions. (linux_proc_pid_is_zombie): Use linux_proc_pid_is_zombie_maybe_warn. (linux_proc_attach_tgid_threads): New function. * nat/linux-procfs.h (linux_proc_get_tgid): Update comment. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this, and update comment. (linux_proc_pid_is_gone): New declaration. (linux_proc_pid_is_zombie): Update comment. (linux_proc_pid_is_zombie_nowarn): New declaration. (linux_proc_attach_lwp_func): New typedef. (linux_proc_attach_tgid_threads): New declaration. * nat/linux-ptrace.c (linux_ptrace_attach_fail_reason): Adjust to use nowarn functions. (linux_ptrace_attach_fail_reason_string): Move here from gdbserver/linux-low.c and rename. (ptrace_supports_feature): If the current ptrace options are not known yet, check them now, instead of asserting. * nat/linux-ptrace.h (linux_ptrace_attach_fail_reason_string): Declare.
2014-12-16 17:12:24 +01:00
/* Return non-zero if PID is a zombie. Failure to open the
/proc/pid/status file results in a warning. */
extern int linux_proc_pid_is_zombie (pid_t pid);
Linux: on attach, attach to lwps listed under /proc/$pid/task/ ... instead of relying on libthread_db. I wrote a test that attaches to a program that constantly spawns short-lived threads, which exposed several issues. This is one of them. On Linux, we need to attach to all threads of a process (thread group) individually. We currently rely on libthread_db to list the threads, but that is problematic, because libthread_db relies on reading data structures out of the inferior (which may well be corrupted). If threads are being created or exiting just while we try to attach, we may trip on inconsistencies in the inferior's thread list. To work around that, when we see a seemingly corrupt list, we currently retry a few times: static void thread_db_find_new_threads_2 (ptid_t ptid, int until_no_new) { ... if (until_no_new) { /* Require 4 successive iterations which do not find any new threads. The 4 is a heuristic: there is an inherent race here, and I have seen that 2 iterations in a row are not always sufficient to "capture" all threads. */ ... That heuristic may well fail, and when it does, we end up with threads in the program that aren't under GDB's control. That's obviously bad and results in quite mistifying failures, like e.g., the process dying for seeminly no reason when a thread that wasn't attached trips on a breakpoint. There's really no reason to rely on libthread_db for this nowadays when we have /proc mounted. In that case, which is the usual case, we can list the LWPs from /proc/PID/task/. In fact, GDBserver is already doing this. The patch factors out that code that knows to walk the task/ directory out of GDBserver, and makes GDB use it too. Like GDBserver, the patch makes GDB attach to LWPs and _not_ wait for them to stop immediately. Instead, we just tag the LWP as having an expected stop. Because we can only set the ptrace options when the thread stops, we need a new flag in the lwp structure to keep track of whether we've already set the ptrace options, just like in GDBserver. Note that nothing issues any ptrace command to the threads between the PTRACE_ATTACH and the stop, so this is safe (unlike one scenario described in gdbserver's linux-low.c). When we attach to a program that has threads exiting while we attach, it's easy to race with a thread just exiting as we try to attach to it, like: #1 - get current list of threads #2 - attach to each listed thread #3 - ooops, attach failed, thread is already gone As this is pretty normal, we shouldn't be issuing a scary warning in step #3. When #3 happens, PTRACE_ATTACH usually fails with ESRCH, but sometimes we'll see EPERM as well. That happens when the kernel still has the thread in its task list, but the thread is marked as dead. Unfortunately, EPERM is ambiguous and we'll get it also on other scenarios where the thread isn't dead, and in those cases, it's useful to get a warning. To distiguish the cases, when we get an EPERM failure, we open /proc/PID/status, and check the thread's state -- if the /proc file no longer exists, or the state is "Z (Zombie)" or "X (Dead)", we ignore the EPERM error silently; otherwise, we'll warn. Unfortunately, there seems to be a kernel race here. Sometimes I get EPERM, and then the /proc state still indicates "R (Running)"... If we wait a bit and retry, we do end up seeing X or Z state, or get an ESRCH. I thought of making GDB retry the attach a few times, but even with a 500ms wait and 4 retries, I still see the warning sometimes. I haven't been able to identify the kernel path that causes this yet, but in any case, it looks like a kernel bug to me. As this just results failure to suppress a warning that we've been printing since about forever anyway, I'm just making the test cope with it, and issue an XFAIL. gdb/gdbserver/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-low.c (linux_attach_fail_reason_string): Move to nat/linux-ptrace.c, and rename. (linux_attach_lwp): Update comment. (attach_proc_task_lwp_callback): New function. (linux_attach): Adjust to rename and use linux_proc_attach_tgid_threads. (linux_attach_fail_reason_string): Delete declaration. gdb/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-nat.c (attach_proc_task_lwp_callback): New function. (linux_nat_attach): Use linux_proc_attach_tgid_threads. (wait_lwp, linux_nat_filter_event): If not set yet, set the lwp's ptrace option flags. * linux-nat.h (struct lwp_info) <must_set_ptrace_flags>: New field. * nat/linux-procfs.c: Include <dirent.h>. (linux_proc_get_int): New parameter "warn". Handle it. (linux_proc_get_tgid): Adjust. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this. (linux_proc_pid_get_state): New function, factored out from (linux_proc_pid_has_state): ... this. Add new parameter "warn" and handle it. (linux_proc_pid_is_gone): New function. (linux_proc_pid_is_stopped): Adjust. (linux_proc_pid_is_zombie_maybe_warn) (linux_proc_pid_is_zombie_nowarn): New functions. (linux_proc_pid_is_zombie): Use linux_proc_pid_is_zombie_maybe_warn. (linux_proc_attach_tgid_threads): New function. * nat/linux-procfs.h (linux_proc_get_tgid): Update comment. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this, and update comment. (linux_proc_pid_is_gone): New declaration. (linux_proc_pid_is_zombie): Update comment. (linux_proc_pid_is_zombie_nowarn): New declaration. (linux_proc_attach_lwp_func): New typedef. (linux_proc_attach_tgid_threads): New declaration. * nat/linux-ptrace.c (linux_ptrace_attach_fail_reason): Adjust to use nowarn functions. (linux_ptrace_attach_fail_reason_string): Move here from gdbserver/linux-low.c and rename. (ptrace_supports_feature): If the current ptrace options are not known yet, check them now, instead of asserting. * nat/linux-ptrace.h (linux_ptrace_attach_fail_reason_string): Declare.
2014-12-16 17:12:24 +01:00
/* Return non-zero if PID is a zombie. Does not warn on failure to
open the /proc file. */
extern int linux_proc_pid_is_zombie_nowarn (pid_t pid);
/* Return non-zero if /proc/PID/status indicates that PID is gone
(i.e., in Z/Zombie or X/Dead state). Failure to open the /proc
file is assumed to indicate the thread is gone. */
extern int linux_proc_pid_is_gone (pid_t pid);
/* Return a string giving the thread's name or NULL if the
information is unavailable. The returned value points to a statically
allocated buffer. The value therefore becomes invalid at the next
linux_proc_tid_get_name call. */
extern const char *linux_proc_tid_get_name (ptid_t ptid);
Linux: on attach, attach to lwps listed under /proc/$pid/task/ ... instead of relying on libthread_db. I wrote a test that attaches to a program that constantly spawns short-lived threads, which exposed several issues. This is one of them. On Linux, we need to attach to all threads of a process (thread group) individually. We currently rely on libthread_db to list the threads, but that is problematic, because libthread_db relies on reading data structures out of the inferior (which may well be corrupted). If threads are being created or exiting just while we try to attach, we may trip on inconsistencies in the inferior's thread list. To work around that, when we see a seemingly corrupt list, we currently retry a few times: static void thread_db_find_new_threads_2 (ptid_t ptid, int until_no_new) { ... if (until_no_new) { /* Require 4 successive iterations which do not find any new threads. The 4 is a heuristic: there is an inherent race here, and I have seen that 2 iterations in a row are not always sufficient to "capture" all threads. */ ... That heuristic may well fail, and when it does, we end up with threads in the program that aren't under GDB's control. That's obviously bad and results in quite mistifying failures, like e.g., the process dying for seeminly no reason when a thread that wasn't attached trips on a breakpoint. There's really no reason to rely on libthread_db for this nowadays when we have /proc mounted. In that case, which is the usual case, we can list the LWPs from /proc/PID/task/. In fact, GDBserver is already doing this. The patch factors out that code that knows to walk the task/ directory out of GDBserver, and makes GDB use it too. Like GDBserver, the patch makes GDB attach to LWPs and _not_ wait for them to stop immediately. Instead, we just tag the LWP as having an expected stop. Because we can only set the ptrace options when the thread stops, we need a new flag in the lwp structure to keep track of whether we've already set the ptrace options, just like in GDBserver. Note that nothing issues any ptrace command to the threads between the PTRACE_ATTACH and the stop, so this is safe (unlike one scenario described in gdbserver's linux-low.c). When we attach to a program that has threads exiting while we attach, it's easy to race with a thread just exiting as we try to attach to it, like: #1 - get current list of threads #2 - attach to each listed thread #3 - ooops, attach failed, thread is already gone As this is pretty normal, we shouldn't be issuing a scary warning in step #3. When #3 happens, PTRACE_ATTACH usually fails with ESRCH, but sometimes we'll see EPERM as well. That happens when the kernel still has the thread in its task list, but the thread is marked as dead. Unfortunately, EPERM is ambiguous and we'll get it also on other scenarios where the thread isn't dead, and in those cases, it's useful to get a warning. To distiguish the cases, when we get an EPERM failure, we open /proc/PID/status, and check the thread's state -- if the /proc file no longer exists, or the state is "Z (Zombie)" or "X (Dead)", we ignore the EPERM error silently; otherwise, we'll warn. Unfortunately, there seems to be a kernel race here. Sometimes I get EPERM, and then the /proc state still indicates "R (Running)"... If we wait a bit and retry, we do end up seeing X or Z state, or get an ESRCH. I thought of making GDB retry the attach a few times, but even with a 500ms wait and 4 retries, I still see the warning sometimes. I haven't been able to identify the kernel path that causes this yet, but in any case, it looks like a kernel bug to me. As this just results failure to suppress a warning that we've been printing since about forever anyway, I'm just making the test cope with it, and issue an XFAIL. gdb/gdbserver/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-low.c (linux_attach_fail_reason_string): Move to nat/linux-ptrace.c, and rename. (linux_attach_lwp): Update comment. (attach_proc_task_lwp_callback): New function. (linux_attach): Adjust to rename and use linux_proc_attach_tgid_threads. (linux_attach_fail_reason_string): Delete declaration. gdb/ 2015-01-09 Pedro Alves <palves@redhat.com> * linux-nat.c (attach_proc_task_lwp_callback): New function. (linux_nat_attach): Use linux_proc_attach_tgid_threads. (wait_lwp, linux_nat_filter_event): If not set yet, set the lwp's ptrace option flags. * linux-nat.h (struct lwp_info) <must_set_ptrace_flags>: New field. * nat/linux-procfs.c: Include <dirent.h>. (linux_proc_get_int): New parameter "warn". Handle it. (linux_proc_get_tgid): Adjust. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this. (linux_proc_pid_get_state): New function, factored out from (linux_proc_pid_has_state): ... this. Add new parameter "warn" and handle it. (linux_proc_pid_is_gone): New function. (linux_proc_pid_is_stopped): Adjust. (linux_proc_pid_is_zombie_maybe_warn) (linux_proc_pid_is_zombie_nowarn): New functions. (linux_proc_pid_is_zombie): Use linux_proc_pid_is_zombie_maybe_warn. (linux_proc_attach_tgid_threads): New function. * nat/linux-procfs.h (linux_proc_get_tgid): Update comment. (linux_proc_get_tracerpid): Rename to ... (linux_proc_get_tracerpid_nowarn): ... this, and update comment. (linux_proc_pid_is_gone): New declaration. (linux_proc_pid_is_zombie): Update comment. (linux_proc_pid_is_zombie_nowarn): New declaration. (linux_proc_attach_lwp_func): New typedef. (linux_proc_attach_tgid_threads): New declaration. * nat/linux-ptrace.c (linux_ptrace_attach_fail_reason): Adjust to use nowarn functions. (linux_ptrace_attach_fail_reason_string): Move here from gdbserver/linux-low.c and rename. (ptrace_supports_feature): If the current ptrace options are not known yet, check them now, instead of asserting. * nat/linux-ptrace.h (linux_ptrace_attach_fail_reason_string): Declare.
2014-12-16 17:12:24 +01:00
/* Callback function for linux_proc_attach_tgid_threads. If the PTID
thread is not yet known, try to attach to it and return true,
otherwise return false. */
typedef int (*linux_proc_attach_lwp_func) (ptid_t ptid);
/* If PID is a tgid, scan the /proc/PID/task/ directory for existing
threads, and call FUNC for each thread found. */
extern void linux_proc_attach_tgid_threads (pid_t pid,
linux_proc_attach_lwp_func func);
GNU/Linux: Stop using libthread_db/td_ta_thr_iter TL;DR - GDB can hang if something refreshes the thread list out of the target while the target is running. GDB hangs inside td_ta_thr_iter. The fix is to not use that libthread_db function anymore. Long version: Running the testsuite against my all-stop-on-top-of-non-stop series is still exposing latent non-stop bugs. I was originally seeing this with the multi-create.exp test, back when we were still using libthread_db thread event breakpoints. The all-stop-on-top-of-non-stop series forces a thread list refresh each time GDB needs to start stepping over a breakpoint (to pause all threads). That test hits the thread event breakpoint often, resulting in a bunch of step-over operations, thus a bunch of thread list refreshes while some threads in the target are running. The commit adds a real non-stop mode test that triggers the issue, based on multi-create.exp, that does an explicit "info threads" when a breakpoint is hit. IOW, it does the same things the as-ns series was doing when testing multi-create.exp. The bug is a race, so it unfortunately takes several runs for the test to trigger it. In fact, even when setting the test running in a loop, it sometimes takes several minutes for it to trigger for me. The race is related to libthread_db's td_ta_thr_iter. This is libthread_db's entry point for walking the thread list of the inferior. Sometimes, when GDB refreshes the thread list from the target, libthread_db's td_ta_thr_iter can somehow see glibc's thread list as a cycle, and get stuck in an infinite loop. The issue is that when a thread exits, its thread control structure in glibc is moved from a "used" list to a "cache" list. These lists are simply circular linked lists where the "next/prev" pointers are embedded in the thread control structure itself. The "next" pointer of the last element of the list points back to the list's sentinel "head". There's only one set of "next/prev" pointers for both lists; thus a thread can only be in one of the lists at a time, not in both simultaneously. So when thread C exits, simplifying, the following happens. A-C are threads. stack_used and stack_cache are the list's heads. Before: stack_used -> A -> B -> C -> (&stack_used) stack_cache -> (&stack_cache) After: stack_used -> A -> B -> (&stack_used) stack_cache -> C -> (&stack_cache) td_ta_thr_iter starts by iterating at the list's head's next, and iterates until it sees a thread whose next pointer points to the list's head again. Thus in the before case above, C's next points to stack_used, indicating end of list. In the same case, the stack_cache list is empty. For each thread being iterated, td_ta_thr_iter reads the whole thread object out of the inferior. This includes the thread's "next" pointer. In the scenario above, it may happen that td_ta_thr_iter is iterating thread B and has already read B's thread structure just before thread C exits and its control structure moves to the cached list. Now, recall that td_ta_thr_iter is running in the context of GDB, and there's no locking between GDB and the inferior. From it's local copy of B, td_ta_thr_iter believes that the next thread after B is thread C, so it happilly continues iterating to C, a thread that has already exited, and is now in the stack cache list. After iterating C, td_ta_thr_iter finds the stack_cache head, which because it is not stack_used, td_ta_thr_iter assumes it's just another thread. After this, unless the reverse race triggers, GDB gets stuck in td_ta_thr_iter forever walking the stack_cache list, as no thread in thatlist has a next pointer that points back to stack_used (the terminating condition). Before fully understanding the issue, I tried adding cycle detection to GDB's td_ta_thr_iter callback. However, td_ta_thr_iter skips calling the callback in some cases, which means that it's possible that the callback isn't called at all, making it impossible for GDB to break the loop. I did manage to get GDB stuck in that state more than once. Fortunately, we can avoid the issue altogether. We don't really need td_ta_thr_iter for live debugging nowadays, given PTRACE_EVENT_CLONE. We already know how to map and lwp id to a thread id without iterating (thread_from_lwp), so use that more. gdb/ChangeLog: 2015-02-20 Pedro Alves <palves@redhat.com> * linux-nat.c (linux_handle_extended_wait): Call thread_db_notice_clone whenever a new clone LWP is detected. (linux_stop_and_wait_all_lwps, linux_unstop_all_lwps): New functions. * linux-nat.h (thread_db_attach_lwp): Delete declaration. (thread_db_notice_clone, linux_stop_and_wait_all_lwps) (linux_unstop_all_lwps): Declare. * linux-thread-db.c (struct thread_get_info_inout): Delete. (thread_get_info_callback): Delete. (thread_from_lwp): Use td_thr_get_info and record_thread. (thread_db_attach_lwp): Delete. (thread_db_notice_clone): New function. (try_thread_db_load_1): If /proc is mounted and shows the process'es task list, walk over all LWPs and call thread_from_lwp instead of relying on td_ta_thr_iter. (attach_thread): Don't call check_thread_signals here. Split the tail part of the function (which adds the thread to the core GDB thread list) to ... (record_thread): ... this function. Call check_thread_signals here. (thread_db_wait): Don't call thread_db_find_new_threads_1. Always call thread_from_lwp. (thread_db_update_thread_list): Rename to ... (thread_db_update_thread_list_org): ... this. (thread_db_update_thread_list): New function. (thread_db_find_thread_from_tid): Delete. (thread_db_get_ada_task_ptid): Simplify. * nat/linux-procfs.c: Include <sys/stat.h>. (linux_proc_task_list_dir_exists): New function. * nat/linux-procfs.h (linux_proc_task_list_dir_exists): Declare. gdb/gdbserver/ChangeLog: 2015-02-20 Pedro Alves <palves@redhat.com> * thread-db.c: Include "nat/linux-procfs.h". (thread_db_init): Skip listing new threads if the kernel supports PTRACE_EVENT_CLONE and /proc/PID/task/ is accessible. gdb/testsuite/ChangeLog: 2015-02-20 Pedro Alves <palves@redhat.com> * gdb.threads/multi-create-ns-info-thr.exp: New file.
2015-02-20 21:21:59 +01:00
/* Return true if the /proc/PID/task/ directory exists. */
extern int linux_proc_task_list_dir_exists (pid_t pid);
/* Return the full absolute name of the executable file that was run
to create the process PID. The returned value persists until this
function is next called. */
extern char *linux_proc_pid_to_exec_file (int pid);
#endif /* COMMON_LINUX_PROCFS_H */