Patch "x86/stacktrace: Make zombie stack traces reliable" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

x86/stacktrace: Make zombie stack traces reliable

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-stacktrace-make-zombie-stack-traces-reliable.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Josh Poimboeuf <[email protected]>
Date: Mon, 18 Dec 2017 15:13:44 -0600
Subject: x86/stacktrace: Make zombie stack traces reliable

From: Josh Poimboeuf <[email protected]>

[ 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.

---
 arch/x86/kernel/stacktrace.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

@@ -160,8 +160,12 @@ int save_stack_trace_tsk_reliable(struct
 {
 	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);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/x86-asm-allow-again-using-asm.h-when-building-for-the-bpf-clang-target.patch queue-4.14/x86-stacktrace-make-zombie-stack-traces-reliable.patch