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 <tom@tromey.com> * 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
This commit is contained in:
Tom Tromey
parent
9411c49ecc
commit
3b3978bca2
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@ -1,3 +1,16 @@
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2019-11-26 Tom Tromey <tom@tromey.com>
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* event-top.h (thread_local_segv_handler): Declare.
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* event-top.c (thread_local_segv_handler): New global.
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(install_handle_sigsegv, handle_sigsegv): New functions.
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(async_init_signals): Install SIGSEGV handler.
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* cp-support.c (gdb_demangle_jmp_buf): Change type. Now
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thread-local.
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(report_failed_demangle): New function.
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(gdb_demangle): Make core_dump_allowed atomic. Remove signal
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handler-setting code, instead use segv_handler. Run warning code
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on main thread.
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2019-11-26 Tom Tromey <tom@tromey.com>
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* run-on-main-thread.c: New file.
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140
gdb/cp-support.c
140
gdb/cp-support.c
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@ -38,6 +38,9 @@
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#include "safe-ctype.h"
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#include "gdbsupport/selftest.h"
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#include "gdbsupport/gdb-sigmask.h"
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#include <atomic>
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#include "event-top.h"
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#include "run-on-main-thread.h"
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#define d_left(dc) (dc)->u.s_binary.left
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#define d_right(dc) (dc)->u.s_binary.right
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@ -1476,11 +1479,11 @@ static bool catch_demangler_crashes = true;
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/* Stack context and environment for demangler crash recovery. */
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static SIGJMP_BUF gdb_demangle_jmp_buf;
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static thread_local SIGJMP_BUF *gdb_demangle_jmp_buf;
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/* If nonzero, attempt to dump core from the signal handler. */
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/* If true, attempt to dump core from the signal handler. */
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static int gdb_demangle_attempt_core_dump = 1;
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static std::atomic<bool> gdb_demangle_attempt_core_dump;
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/* Signal handler for gdb_demangle. */
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@ -1492,10 +1495,46 @@ gdb_demangle_signal_handler (int signo)
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if (fork () == 0)
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dump_core ();
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gdb_demangle_attempt_core_dump = 0;
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gdb_demangle_attempt_core_dump = false;
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}
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SIGLONGJMP (gdb_demangle_jmp_buf, signo);
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SIGLONGJMP (*gdb_demangle_jmp_buf, signo);
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}
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/* A helper for gdb_demangle that reports a demangling failure. */
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static void
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report_failed_demangle (const char *name, bool core_dump_allowed,
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int crash_signal)
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{
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static bool error_reported = false;
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if (!error_reported)
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{
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std::string short_msg
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= string_printf (_("unable to demangle '%s' "
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"(demangler failed with signal %d)"),
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name, crash_signal);
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std::string long_msg
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= string_printf ("%s:%d: %s: %s", __FILE__, __LINE__,
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"demangler-warning", short_msg.c_str ());
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target_terminal::scoped_restore_terminal_state term_state;
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target_terminal::ours_for_output ();
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begin_line ();
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if (core_dump_allowed)
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fprintf_unfiltered (gdb_stderr,
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_("%s\nAttempting to dump core.\n"),
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long_msg.c_str ());
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else
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warn_cant_dump_core (long_msg.c_str ());
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demangler_warning (__FILE__, __LINE__, "%s", short_msg.c_str ());
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error_reported = true;
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}
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}
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#endif
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@ -1509,45 +1548,27 @@ gdb_demangle (const char *name, int options)
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int crash_signal = 0;
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#ifdef HAVE_WORKING_FORK
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#if defined (HAVE_SIGACTION) && defined (SA_RESTART)
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struct sigaction sa, old_sa;
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#else
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sighandler_t ofunc;
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#endif
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static int core_dump_allowed = -1;
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if (core_dump_allowed == -1)
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{
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core_dump_allowed = can_dump_core (LIMIT_CUR);
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if (!core_dump_allowed)
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gdb_demangle_attempt_core_dump = 0;
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}
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scoped_restore restore_segv
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= make_scoped_restore (&thread_local_segv_handler,
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catch_demangler_crashes
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? gdb_demangle_signal_handler
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: nullptr);
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bool core_dump_allowed = gdb_demangle_attempt_core_dump;
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SIGJMP_BUF jmp_buf;
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scoped_restore restore_jmp_buf
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= make_scoped_restore (&gdb_demangle_jmp_buf, &jmp_buf);
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if (catch_demangler_crashes)
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{
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#if defined (HAVE_SIGACTION) && defined (SA_RESTART)
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sa.sa_handler = gdb_demangle_signal_handler;
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sigemptyset (&sa.sa_mask);
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#ifdef HAVE_SIGALTSTACK
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sa.sa_flags = SA_ONSTACK;
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#else
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sa.sa_flags = 0;
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#endif
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sigaction (SIGSEGV, &sa, &old_sa);
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#else
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ofunc = signal (SIGSEGV, gdb_demangle_signal_handler);
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#endif
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/* The signal handler may keep the signal blocked when we longjmp out
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of it. If we have sigprocmask, we can use it to unblock the signal
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afterwards and we can avoid the performance overhead of saving the
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signal mask just in case the signal gets triggered. Otherwise, just
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tell sigsetjmp to save the mask. */
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#ifdef HAVE_SIGPROCMASK
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crash_signal = SIGSETJMP (gdb_demangle_jmp_buf, 0);
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crash_signal = SIGSETJMP (*gdb_demangle_jmp_buf, 0);
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#else
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crash_signal = SIGSETJMP (gdb_demangle_jmp_buf, 1);
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crash_signal = SIGSETJMP (*gdb_demangle_jmp_buf, 1);
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#endif
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}
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#endif
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@ -1558,16 +1579,8 @@ gdb_demangle (const char *name, int options)
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#ifdef HAVE_WORKING_FORK
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if (catch_demangler_crashes)
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{
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#if defined (HAVE_SIGACTION) && defined (SA_RESTART)
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sigaction (SIGSEGV, &old_sa, NULL);
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#else
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signal (SIGSEGV, ofunc);
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#endif
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if (crash_signal != 0)
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{
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static int error_reported = 0;
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{
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#ifdef HAVE_SIGPROCMASK
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/* If we got the signal, SIGSEGV may still be blocked; restore it. */
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sigset_t segv_sig_set;
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@ -1576,35 +1589,18 @@ gdb_demangle (const char *name, int options)
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gdb_sigmask (SIG_UNBLOCK, &segv_sig_set, NULL);
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#endif
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if (!error_reported)
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{
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std::string short_msg
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= string_printf (_("unable to demangle '%s' "
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"(demangler failed with signal %d)"),
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name, crash_signal);
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/* If there was a failure, we can't report it here, because
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we might be in a background thread. Instead, arrange for
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the reporting to happen on the main thread. */
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std::string copy = name;
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run_on_main_thread ([=] ()
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{
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report_failed_demangle (copy.c_str (), core_dump_allowed,
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crash_signal);
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});
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std::string long_msg
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= string_printf ("%s:%d: %s: %s", __FILE__, __LINE__,
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"demangler-warning", short_msg.c_str ());
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target_terminal::scoped_restore_terminal_state term_state;
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target_terminal::ours_for_output ();
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begin_line ();
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if (core_dump_allowed)
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fprintf_unfiltered (gdb_stderr,
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_("%s\nAttempting to dump core.\n"),
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long_msg.c_str ());
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else
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warn_cant_dump_core (long_msg.c_str ());
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demangler_warning (__FILE__, __LINE__, "%s", short_msg.c_str ());
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error_reported = 1;
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}
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result = NULL;
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}
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result = NULL;
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}
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}
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#endif
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@ -2211,4 +2207,6 @@ display the offending symbol."),
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selftests::register_test ("cp_remove_params",
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selftests::test_cp_remove_params);
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#endif
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gdb_demangle_attempt_core_dump = can_dump_core (LIMIT_CUR);
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}
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@ -848,6 +848,45 @@ gdb_readline_no_editing_callback (gdb_client_data client_data)
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}
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/* See event-top.h. */
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thread_local void (*thread_local_segv_handler) (int);
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static void handle_sigsegv (int sig);
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/* Install the SIGSEGV handler. */
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static void
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install_handle_sigsegv ()
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{
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#if defined (HAVE_SIGACTION)
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struct sigaction sa;
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sa.sa_handler = handle_sigsegv;
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sigemptyset (&sa.sa_mask);
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#ifdef HAVE_SIGALTSTACK
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sa.sa_flags = SA_ONSTACK;
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#else
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sa.sa_flags = 0;
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#endif
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sigaction (SIGSEGV, &sa, nullptr);
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#else
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signal (SIGSEGV, handle_sigsegv);
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#endif
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}
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/* Handler for SIGSEGV. */
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static void
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handle_sigsegv (int sig)
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{
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install_handle_sigsegv ();
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if (thread_local_segv_handler == nullptr)
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abort ();
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thread_local_segv_handler (sig);
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}
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/* The serial event associated with the QUIT flag. set_quit_flag sets
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this, and check_quit_flag clears it. Used by interruptible_select
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to be able to do interruptible I/O with no race with the SIGINT
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@ -915,6 +954,8 @@ async_init_signals (void)
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sigtstp_token =
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create_async_signal_handler (async_sigtstp_handler, NULL);
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#endif
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install_handle_sigsegv ();
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}
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/* See defs.h. */
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@ -70,4 +70,10 @@ extern void gdb_rl_callback_handler_install (const char *prompt);
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currently installed. */
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extern void gdb_rl_callback_handler_reinstall (void);
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/* The SIGSEGV handler for this thread, or NULL if there is none. GDB
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always installs a global SIGSEGV handler, and then lets threads
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indicate their interest in handling the signal by setting this
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thread-local variable. */
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extern thread_local void (*thread_local_segv_handler) (int);
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#endif
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