coredump: sanitize the setting of signal->group_exit_code

Now that the coredumping process can be SIGKILL'ed, the setting of
->group_exit_code in do_coredump() can race with complete_signal() and
SIGKILL or 0x80 can be "lost", or wait(status) can report status ==
SIGKILL | 0x80.

But the main problem is that it is not clear to me what should we do if
binfmt->core_dump() succeeds but SIGKILL was sent, that is why this patch
comes as a separate change.

This patch adds 0x80 if ->core_dump() succeeds and the process was not
killed.  But perhaps we can (should?) re-set ->group_exit_code changed by
SIGKILL back to "siginfo->si_signo |= 0x80" in case when core_dumped == T.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Mandeep Singh Baines <msb@chromium.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Neil Horman <nhorman@redhat.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/coredump.c

@@ -390,12 +390,14 @@ static int coredump_wait(int exit_code, struct core_state *core_state)
 	return core_waiters;
 }
 
-static void coredump_finish(struct mm_struct *mm)
+static void coredump_finish(struct mm_struct *mm, bool core_dumped)
 {
 	struct core_thread *curr, *next;
 	struct task_struct *task;
 
 	spin_lock_irq(&current->sighand->siglock);
+	if (core_dumped && !__fatal_signal_pending(current))
+		current->signal->group_exit_code |= 0x80;
 	current->signal->group_exit_task = NULL;
 	current->signal->flags = SIGNAL_GROUP_EXIT;
 	spin_unlock_irq(&current->sighand->siglock);
@@ -480,6 +482,7 @@ void do_coredump(siginfo_t *siginfo)
 	int ispipe;
 	struct files_struct *displaced;
 	bool need_nonrelative = false;
+	bool core_dumped = false;
 	static atomic_t core_dump_count = ATOMIC_INIT(0);
 	struct coredump_params cprm = {
 		.siginfo = siginfo,
@@ -638,9 +641,7 @@ void do_coredump(siginfo_t *siginfo)
 		goto close_fail;
 	if (displaced)
 		put_files_struct(displaced);
-	retval = binfmt->core_dump(&cprm);
+	core_dumped = binfmt->core_dump(&cprm);
-	if (retval)
-		current->signal->group_exit_code |= 0x80;
 
 	if (ispipe && core_pipe_limit)
 		wait_for_dump_helpers(cprm.file);
@@ -653,7 +654,7 @@ fail_dropcount:
 fail_unlock:
 	kfree(cn.corename);
 fail_corename:
-	coredump_finish(mm);
+	coredump_finish(mm, core_dumped);
 	revert_creds(old_cred);
 fail_creds:
 	put_cred(cred);