[PATCH AUTOSEL for 4.14 026/110] x86/stacktrace: Make zombie stack traces reliable

[Sasha Levin]

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

[ Upstream commit 6454b3bdd138dfc640deb5e7b9a0668fca2d55dd ]

Commit:

  1959a60182f4 ("x86/dumpstack: Pin the target stack when dumping it")

changed the behavior of stack traces for zombies.  Before that commit,
/proc/<pid>/stack reported the last execution path of the zombie before
it died:

  [<ffffffff8105b877>] do_exit+0x6f7/0xa80
  [<ffffffff8105bc79>] do_group_exit+0x39/0xa0
  [<ffffffff8105bcf0>] __wake_up_parent+0x0/0x30
  [<ffffffff8152dd09>] system_call_fastpath+0x16/0x1b
  [<00007fd128f9c4f9>] 0x7fd128f9c4f9
  [<ffffffffffffffff>] 0xffffffffffffffff

After the commit, it just reports an empty stack trace.

The new behavior is actually probably more correct.  If the stack
refcount has gone down to zero, then the task has already gone through
do_exit() and isn't going to run anymore.  The stack could be freed at
any time and is basically gone, so reporting an empty stack makes sense.

However, save_stack_trace_tsk_reliable() treats such a missing stack
condition as an error.  That can cause livepatch transition stalls if
there are any unreaped zombies.  Instead, just treat it as a reliable,
empty stack.

Reported-and-tested-by: Miroslav Benes <mbenes@xxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: live-patching@xxxxxxxxxxxxxxx
Fixes: af085d9084b4 ("stacktrace/x86: add function for detecting reliable stack traces")
Link: http://lkml.kernel.org/r/e4b09e630e99d0c1080528f0821fc9d9dbaeea82.1513631620.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
---
 arch/x86/kernel/stacktrace.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index 60244bfaf88f..4565f31bd398 100644
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -160,8 +160,12 @@ int save_stack_trace_tsk_reliable(struct task_struct *tsk,
 {
 	int ret;
 
+	/*
+	 * If the task doesn't have a stack (e.g., a zombie), the stack is
+	 * "reliably" empty.
+	 */
 	if (!try_get_task_stack(tsk))
-		return -EINVAL;
+		return 0;
 
 	ret = __save_stack_trace_reliable(trace, tsk);
 
-- 
2.11.0