From 3b3978bca2a204a772563c8e121e4a02be72e802 Mon Sep 17 00:00:00 2001 From: Tom Tromey Date: Mon, 4 Mar 2019 15:12:04 -0700 Subject: [PATCH] Introduce thread-safe way to handle SIGSEGV The gdb demangler installs a SIGSEGV handler in order to protect gdb from demangler bugs. However, this is not thread-safe, as signal handlers are global to the process. This patch changes gdb to always install a global SIGSEGV handler, and then lets threads indicate their interest in handling the signal by setting a thread-local variable. This patch then arranges for the demangler code to use this; being sure to arrange for calls to warning and the like to be done on the main thread. One thing I wondered while writing this patch is if there are any systems that do not have "sigaction". If gdb could assume this, it would simplify this code. gdb/ChangeLog 2019-11-26 Tom Tromey * event-top.h (thread_local_segv_handler): Declare. * event-top.c (thread_local_segv_handler): New global. (install_handle_sigsegv, handle_sigsegv): New functions. (async_init_signals): Install SIGSEGV handler. * cp-support.c (gdb_demangle_jmp_buf): Change type. Now thread-local. (report_failed_demangle): New function. (gdb_demangle): Make core_dump_allowed atomic. Remove signal handler-setting code, instead use segv_handler. Run warning code on main thread. Change-Id: Ic832bbb033b64744e4b44f14b41db7e4168ce427 --- gdb/ChangeLog | 13 +++++ gdb/cp-support.c | 140 +++++++++++++++++++++++------------------------ gdb/event-top.c | 41 ++++++++++++++ gdb/event-top.h | 6 ++ 4 files changed, 129 insertions(+), 71 deletions(-) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 80a3ea0a2a..48e7ca1ca0 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,16 @@ +2019-11-26 Tom Tromey + + * event-top.h (thread_local_segv_handler): Declare. + * event-top.c (thread_local_segv_handler): New global. + (install_handle_sigsegv, handle_sigsegv): New functions. + (async_init_signals): Install SIGSEGV handler. + * cp-support.c (gdb_demangle_jmp_buf): Change type. Now + thread-local. + (report_failed_demangle): New function. + (gdb_demangle): Make core_dump_allowed atomic. Remove signal + handler-setting code, instead use segv_handler. Run warning code + on main thread. + 2019-11-26 Tom Tromey * run-on-main-thread.c: New file. diff --git a/gdb/cp-support.c b/gdb/cp-support.c index f1ddc74976..4de2a98e87 100644 --- a/gdb/cp-support.c +++ b/gdb/cp-support.c @@ -38,6 +38,9 @@ #include "safe-ctype.h" #include "gdbsupport/selftest.h" #include "gdbsupport/gdb-sigmask.h" +#include +#include "event-top.h" +#include "run-on-main-thread.h" #define d_left(dc) (dc)->u.s_binary.left #define d_right(dc) (dc)->u.s_binary.right @@ -1476,11 +1479,11 @@ static bool catch_demangler_crashes = true; /* Stack context and environment for demangler crash recovery. */ -static SIGJMP_BUF gdb_demangle_jmp_buf; +static thread_local SIGJMP_BUF *gdb_demangle_jmp_buf; -/* If nonzero, attempt to dump core from the signal handler. */ +/* If true, attempt to dump core from the signal handler. */ -static int gdb_demangle_attempt_core_dump = 1; +static std::atomic gdb_demangle_attempt_core_dump; /* Signal handler for gdb_demangle. */ @@ -1492,10 +1495,46 @@ gdb_demangle_signal_handler (int signo) if (fork () == 0) dump_core (); - gdb_demangle_attempt_core_dump = 0; + gdb_demangle_attempt_core_dump = false; } - SIGLONGJMP (gdb_demangle_jmp_buf, signo); + SIGLONGJMP (*gdb_demangle_jmp_buf, signo); +} + +/* A helper for gdb_demangle that reports a demangling failure. */ + +static void +report_failed_demangle (const char *name, bool core_dump_allowed, + int crash_signal) +{ + static bool error_reported = false; + + if (!error_reported) + { + std::string short_msg + = string_printf (_("unable to demangle '%s' " + "(demangler failed with signal %d)"), + name, crash_signal); + + std::string long_msg + = string_printf ("%s:%d: %s: %s", __FILE__, __LINE__, + "demangler-warning", short_msg.c_str ()); + + target_terminal::scoped_restore_terminal_state term_state; + target_terminal::ours_for_output (); + + begin_line (); + if (core_dump_allowed) + fprintf_unfiltered (gdb_stderr, + _("%s\nAttempting to dump core.\n"), + long_msg.c_str ()); + else + warn_cant_dump_core (long_msg.c_str ()); + + demangler_warning (__FILE__, __LINE__, "%s", short_msg.c_str ()); + + error_reported = true; + } } #endif @@ -1509,45 +1548,27 @@ gdb_demangle (const char *name, int options) int crash_signal = 0; #ifdef HAVE_WORKING_FORK -#if defined (HAVE_SIGACTION) && defined (SA_RESTART) - struct sigaction sa, old_sa; -#else - sighandler_t ofunc; -#endif - static int core_dump_allowed = -1; - - if (core_dump_allowed == -1) - { - core_dump_allowed = can_dump_core (LIMIT_CUR); - - if (!core_dump_allowed) - gdb_demangle_attempt_core_dump = 0; - } + scoped_restore restore_segv + = make_scoped_restore (&thread_local_segv_handler, + catch_demangler_crashes + ? gdb_demangle_signal_handler + : nullptr); + bool core_dump_allowed = gdb_demangle_attempt_core_dump; + SIGJMP_BUF jmp_buf; + scoped_restore restore_jmp_buf + = make_scoped_restore (&gdb_demangle_jmp_buf, &jmp_buf); if (catch_demangler_crashes) { -#if defined (HAVE_SIGACTION) && defined (SA_RESTART) - sa.sa_handler = gdb_demangle_signal_handler; - sigemptyset (&sa.sa_mask); -#ifdef HAVE_SIGALTSTACK - sa.sa_flags = SA_ONSTACK; -#else - sa.sa_flags = 0; -#endif - sigaction (SIGSEGV, &sa, &old_sa); -#else - ofunc = signal (SIGSEGV, gdb_demangle_signal_handler); -#endif - /* The signal handler may keep the signal blocked when we longjmp out of it. If we have sigprocmask, we can use it to unblock the signal afterwards and we can avoid the performance overhead of saving the signal mask just in case the signal gets triggered. Otherwise, just tell sigsetjmp to save the mask. */ #ifdef HAVE_SIGPROCMASK - crash_signal = SIGSETJMP (gdb_demangle_jmp_buf, 0); + crash_signal = SIGSETJMP (*gdb_demangle_jmp_buf, 0); #else - crash_signal = SIGSETJMP (gdb_demangle_jmp_buf, 1); + crash_signal = SIGSETJMP (*gdb_demangle_jmp_buf, 1); #endif } #endif @@ -1558,16 +1579,8 @@ gdb_demangle (const char *name, int options) #ifdef HAVE_WORKING_FORK if (catch_demangler_crashes) { -#if defined (HAVE_SIGACTION) && defined (SA_RESTART) - sigaction (SIGSEGV, &old_sa, NULL); -#else - signal (SIGSEGV, ofunc); -#endif - if (crash_signal != 0) - { - static int error_reported = 0; - + { #ifdef HAVE_SIGPROCMASK /* If we got the signal, SIGSEGV may still be blocked; restore it. */ sigset_t segv_sig_set; @@ -1576,35 +1589,18 @@ gdb_demangle (const char *name, int options) gdb_sigmask (SIG_UNBLOCK, &segv_sig_set, NULL); #endif - if (!error_reported) - { - std::string short_msg - = string_printf (_("unable to demangle '%s' " - "(demangler failed with signal %d)"), - name, crash_signal); + /* If there was a failure, we can't report it here, because + we might be in a background thread. Instead, arrange for + the reporting to happen on the main thread. */ + std::string copy = name; + run_on_main_thread ([=] () + { + report_failed_demangle (copy.c_str (), core_dump_allowed, + crash_signal); + }); - std::string long_msg - = string_printf ("%s:%d: %s: %s", __FILE__, __LINE__, - "demangler-warning", short_msg.c_str ()); - - target_terminal::scoped_restore_terminal_state term_state; - target_terminal::ours_for_output (); - - begin_line (); - if (core_dump_allowed) - fprintf_unfiltered (gdb_stderr, - _("%s\nAttempting to dump core.\n"), - long_msg.c_str ()); - else - warn_cant_dump_core (long_msg.c_str ()); - - demangler_warning (__FILE__, __LINE__, "%s", short_msg.c_str ()); - - error_reported = 1; - } - - result = NULL; - } + result = NULL; + } } #endif @@ -2211,4 +2207,6 @@ display the offending symbol."), selftests::register_test ("cp_remove_params", selftests::test_cp_remove_params); #endif + + gdb_demangle_attempt_core_dump = can_dump_core (LIMIT_CUR); } diff --git a/gdb/event-top.c b/gdb/event-top.c index 6c6e0ff3ba..9ec599e8c4 100644 --- a/gdb/event-top.c +++ b/gdb/event-top.c @@ -848,6 +848,45 @@ gdb_readline_no_editing_callback (gdb_client_data client_data) } +/* See event-top.h. */ + +thread_local void (*thread_local_segv_handler) (int); + +static void handle_sigsegv (int sig); + +/* Install the SIGSEGV handler. */ +static void +install_handle_sigsegv () +{ +#if defined (HAVE_SIGACTION) + struct sigaction sa; + sa.sa_handler = handle_sigsegv; + sigemptyset (&sa.sa_mask); +#ifdef HAVE_SIGALTSTACK + sa.sa_flags = SA_ONSTACK; +#else + sa.sa_flags = 0; +#endif + sigaction (SIGSEGV, &sa, nullptr); +#else + signal (SIGSEGV, handle_sigsegv); +#endif +} + +/* Handler for SIGSEGV. */ + +static void +handle_sigsegv (int sig) +{ + install_handle_sigsegv (); + + if (thread_local_segv_handler == nullptr) + abort (); + thread_local_segv_handler (sig); +} + + + /* The serial event associated with the QUIT flag. set_quit_flag sets this, and check_quit_flag clears it. Used by interruptible_select to be able to do interruptible I/O with no race with the SIGINT @@ -915,6 +954,8 @@ async_init_signals (void) sigtstp_token = create_async_signal_handler (async_sigtstp_handler, NULL); #endif + + install_handle_sigsegv (); } /* See defs.h. */ diff --git a/gdb/event-top.h b/gdb/event-top.h index 1dc7b13d4f..e844125cbb 100644 --- a/gdb/event-top.h +++ b/gdb/event-top.h @@ -70,4 +70,10 @@ extern void gdb_rl_callback_handler_install (const char *prompt); currently installed. */ extern void gdb_rl_callback_handler_reinstall (void); +/* The SIGSEGV handler for this thread, or NULL if there is none. GDB + always installs a global SIGSEGV handler, and then lets threads + indicate their interest in handling the signal by setting this + thread-local variable. */ +extern thread_local void (*thread_local_segv_handler) (int); + #endif