4.14-stable review patch. If anyone has any objections, please let me know.
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From: Josh Poimboeuf jpoimboe@redhat.com
[ 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@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... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Sasha Levin alexander.levin@verizon.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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 @@ -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);