[RFC 5/6] Introduce thread-safe way to handle SIGSEGV

Tom Tromey
2019-03-09T17:23:10Z

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 thread 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.

2019-03-08 Tom Tromey <tom@tromey.com>

* event-top.h (segv_handler): Declare.
* event-top.c (segv_handler): New global.
(handle_sigsegv): New function.
(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.
---
gdb/ChangeLog | 13 ++++++
gdb/cp-support.c | 114 +++++++++++++++++++++++------------------------
gdb/event-top.c | 37 +++++++++++++++
gdb/event-top.h | 3 ++
4 files changed, 109 insertions(+), 58 deletions(-)

diff --git a/gdb/cp-support.c b/gdb/cp-support.c
index 79457f991fa..ae7f7e2abc2 100644
--- a/gdb/cp-support.c
+++ b/gdb/cp-support.c
@@ -37,6 +37,9 @@
#include "common/gdb_setjmp.h"
#include "safe-ctype.h"
#include "common/selftest.h"
+#include <atomic>
+#include "event-top.h"
+#include "ser-event.h"

#define d_left(dc) (dc)->u.s_binary.left
#define d_right(dc) (dc)->u.s_binary.right
@@ -1470,7 +1473,7 @@ static int catch_demangler_crashes = 1;

/* 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. */

@@ -1489,7 +1492,43 @@ gdb_demangle_signal_handler (int signo)
gdb_demangle_attempt_core_dump = 0;
}

- 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, int core_dump_allowed,
+ int crash_signal)
+{
+ static int error_reported = 0;
+
+ 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 = 1;
+ }
}

#endif
@@ -1503,12 +1542,7 @@ 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;
+ static std::atomic<int> core_dump_allowed (-1);

if (core_dump_allowed == -1)
{
@@ -1518,23 +1552,15 @@ gdb_demangle (const char *name, int options)
gdb_demangle_attempt_core_dump = 0;
}

- 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
+ scoped_restore restore_segv
+ = make_scoped_restore (&segv_handler,
+ catch_demangler_crashes
+ ? gdb_demangle_signal_handler
+ : nullptr);

- crash_signal = SIGSETJMP (gdb_demangle_jmp_buf);
- }
+ SIGJMP_BUF jmp_buf;
+ if (catch_demangler_crashes)
+ crash_signal = SIGSETJMP (jmp_buf);
#endif

if (crash_signal == 0)
@@ -1543,42 +1569,14 @@ 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;
-
- 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 = 1;
- }
+ std::string copy = name;
+ run_on_main_thread ([=] ()
+ {
+ report_failed_demangle (copy.c_str (), core_dump_allowed,
+ crash_signal);
+ });

result = NULL;
}
diff --git a/gdb/event-top.c b/gdb/event-top.c
index 6e4031165c7..3cb2046aeb3 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -845,6 +845,29 @@ gdb_readline_no_editing_callback (gdb_client_data client_data)
}
\f

+/* See event-top.h. */
+
+thread_local void (*segv_handler) (int);
+
+/* Handler for SIGSEGV. */
+
+static void
+handle_sigsegv (int sig)
+{
+ if (segv_handler == nullptr)
+ {
+ signal (sig, nullptr);
+ raise (sig);
+ }
+ else
+ {
+ signal (sig, handle_sigsegv);
+ segv_handler (sig);
+ }
+}
+
+\f
+
/* 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
@@ -912,6 +935,20 @@ async_init_signals (void)
sigtstp_token =
create_async_signal_handler (async_sigtstp_handler, NULL);
#endif
+
+#if defined (HAVE_SIGACTION) && defined (SA_RESTART)
+ struct sigaction sa, old_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, &old_sa);
+#else
+ signal (SIGSEGV, handle_sigsegv);
+#endif
}

/* See defs.h. */
diff --git a/gdb/event-top.h b/gdb/event-top.h
index 4c3e6cb8612..849f40d9192 100644
--- a/gdb/event-top.h
+++ b/gdb/event-top.h
@@ -70,4 +70,7 @@ 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. */
+extern thread_local void (*segv_handler) (int);
+
#endif
--
2.17.2