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

1 files changed, 41 insertions, 0 deletions

diff --git a/gdb/event-top.c b/gdb/event-top.c
index 6c6e0ff..9ec599e 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.  */