From: Josh Poimboeuf <jpoimboe@redhat.com>
Date: Mon, 18 Dec 2017 15:13:44 -0600
Subject: x86/stacktrace: Make zombie stack traces reliable
Git-commit: 6454b3bdd138dfc640deb5e7b9a0668fca2d55dd
Patch-mainline: v4.15-rc6
References: bnc#1058115
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@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: live-patching@vger.kernel.org
Fixes: af085d9084b4 ("stacktrace/x86: add function for detecting reliable stack traces")
Link: http://lkml.kernel.org/r/e4b09e630e99d0c1080528f0821fc9d9dbaeea82.1513631620.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
arch/x86/kernel/stacktrace.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -164,8 +164,12 @@ 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);