[Linux] Ask kernel to kill inferior when GDB terminates

This patch enhances GDB on GNU/Linux systems in the situation where
we are debugging an inferior that was created from GDB (as opposed
to attached to), by asking the kernel to kill the inferior if GDB
terminates without doing it itself.

This would typically happen when GDB encounters a problem and
crashes, or when it gets killed by an external process. This can
be observed by starting a program under GDB, and then killing
GDB with signal 9. After GDB is killed, the inferior still remains.

This patch also fixes GDBserver similarly.

This fix is conditional on the kernel supporting the PTRACE_O_EXITKILL
feature.  On older kernels, the behavior remains unchanged.

gdb/ChangeLog:

        * nat/linux-ptrace.h (PTRACE_O_EXITKILL): Define if not
        already defined.
        (linux_enable_event_reporting): Add parameter "attached".
        * nat/linux-ptrace.c (linux_test_for_exitkill): New forward
        declaration.  New function.
        (linux_check_ptrace_features): Add linux_test_for_exitkill call.
        (linux_enable_event_reporting): Add new parameter "attached".
        Do not call ptrace with the PTRACE_O_EXITKILL if ATTACHED is
        nonzero.
        * linux-nat.c (linux_init_ptrace): Add parameter "attached".
        Use it.  Update function description.
        (linux_child_post_attach, linux_child_post_startup_inferior):
        Update call to linux_enable_event_reporting.

gdb/gdbserver/ChangeLog:

        * linux-low.c (linux_low_filter_event): Update call to
        linux_enable_event_reporting following the addition of
        a new parameter to that function.

Tested on x86_64-linux, native and native-gdbserver.

I also verified by hand that the inferior gets killed when killing
GDB in the "run" case, while the inferior remains in the "attach"
case. Same for GDBserver.

2 files changed, 37 insertions, 4 deletions

diff --git a/gdb/nat/linux-ptrace.c b/gdb/nat/linux-ptrace.c
index 8bc3f16..a0e0c32 100644
--- a/gdb/nat/linux-ptrace.c
+++ b/gdb/nat/linux-ptrace.c
@@ -307,6 +307,7 @@ linux_child_function (gdb_byte *child_stack)
 
 static void linux_test_for_tracesysgood (int child_pid);
 static void linux_test_for_tracefork (int child_pid);
+static void linux_test_for_exitkill (int child_pid);
 
 /* Determine ptrace features available on this target.  */
 
@@ -338,6 +339,8 @@ linux_check_ptrace_features (void)
 
   linux_test_for_tracefork (child_pid);
 
+  linux_test_for_exitkill (child_pid);
+
   /* Clean things up and kill any pending children.  */
   do
     {
@@ -449,19 +452,44 @@ linux_test_for_tracefork (int child_pid)
 	     "(%d, status 0x%x)"), ret, status);
 }
 
-/* Enable reporting of all currently supported ptrace events.  */
+/* Determine if PTRACE_O_EXITKILL can be used.  */
+
+static void
+linux_test_for_exitkill (int child_pid)
+{
+  int ret;
+
+  ret = ptrace (PTRACE_SETOPTIONS, child_pid, (PTRACE_TYPE_ARG3) 0,
+		(PTRACE_TYPE_ARG4) PTRACE_O_EXITKILL);
+
+  if (ret == 0)
+    current_ptrace_options |= PTRACE_O_EXITKILL;
+}
+
+/* Enable reporting of all currently supported ptrace events.
+   ATTACHED should be nonzero if we have attached to the inferior.  */
 
 void
-linux_enable_event_reporting (pid_t pid)
+linux_enable_event_reporting (pid_t pid, int attached)
 {
+  int ptrace_options;
+
   /* Check if we have initialized the ptrace features for this
      target.  If not, do it now.  */
   if (current_ptrace_options == -1)
     linux_check_ptrace_features ();
 
+  ptrace_options = current_ptrace_options;
+  if (attached)
+    {
+      /* When attached to our inferior, we do not want the inferior
+	 to die with us if we terminate unexpectedly.  */
+      ptrace_options &= ~PTRACE_O_EXITKILL;
+    }
+
   /* Set the options.  */
   ptrace (PTRACE_SETOPTIONS, pid, (PTRACE_TYPE_ARG3) 0,
-	  (PTRACE_TYPE_ARG4) (uintptr_t) current_ptrace_options);
+	  (PTRACE_TYPE_ARG4) (uintptr_t) ptrace_options);
 }
 
 /* Disable reporting of all currently supported ptrace events.  */
diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
index 31a77cd..588d38a 100644
--- a/gdb/nat/linux-ptrace.h
+++ b/gdb/nat/linux-ptrace.h
@@ -69,6 +69,11 @@ struct buffer;
 
 #endif /* PTRACE_EVENT_FORK */
 
+#ifndef PTRACE_O_EXITKILL
+/* Only defined in Linux Kernel 3.8 or later.  */
+#define PTRACE_O_EXITKILL	0x00100000
+#endif
+
 #if (defined __bfin__ || defined __frv__ || defined __sh__) \
     && !defined PTRACE_GETFDPIC
 #define PTRACE_GETFDPIC		31
@@ -85,7 +90,7 @@ struct buffer;
 
 extern void linux_ptrace_attach_fail_reason (pid_t pid, struct buffer *buffer);
 extern void linux_ptrace_init_warnings (void);
-extern void linux_enable_event_reporting (pid_t pid);
+extern void linux_enable_event_reporting (pid_t pid, int attached);
 extern void linux_disable_event_reporting (pid_t pid);
 extern int linux_supports_tracefork (void);
 extern int linux_supports_traceclone (void);