[PATCH 1/4] x86/unwind: silence warnings for non-current tasks

[Josh Poimboeuf <jpoimboe@redhat.com>]

There are a handful of callers to save_stack_trace_tsk() and
show_stack() which try to unwind the stack of a task other than current.
In such cases, it's remotely possible that the task is running on one
CPU while the unwinder is reading its stack from another CPU, causing
the unwinder to see stack corruption.

These cases seem to be mostly harmless.  The unwinder has checks which
prevent it from following bad pointers beyond the bounds of the stack.
So it's not really a bug as long as the caller understands that
unwinding another task will not always succeed.

Since stack "corruption" on another task's stack isn't necessarily a
bug, silence the warnings when unwinding tasks other than current.

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/kernel/unwind_frame.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index 4443e49..195eebf 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -207,6 +207,16 @@ bool unwind_next_frame(struct unwind_state *state)
 	return true;
 
 bad_address:
+	/*
+	 * When unwinding a non-current task, the task might actually be
+	 * running on another CPU, in which case it could be modifying its
+	 * stack while we're reading it.  This is generally not a problem and
+	 * can be ignored as long as the caller understands that unwinding
+	 * another task will not always succeed.
+	 */
+	if (state->task != current)
+		goto the_end;
+
 	if (state->regs) {
 		printk_deferred_once(KERN_WARNING
 			"WARNING: kernel stack regs at %p in %s:%d has bad 'bp' value %p\n",
-- 
2.7.4