[PATCH AUTOSEL 5.4 13/17] x86/stacktrace: Fix reliable check for empty user task stacks

[Sasha Levin <sashal@kernel.org>]

From: Josh Poimboeuf <jpoimboe@redhat.com>

[ Upstream commit 039a7a30ec102ec866d382a66f87f6f7654f8140 ]

If a user task's stack is empty, or if it only has user regs, ORC
reports it as a reliable empty stack.  But arch_stack_walk_reliable()
incorrectly treats it as unreliable.

That happens because the only success path for user tasks is inside the
loop, which only iterates on non-empty stacks.  Generally, a user task
must end in a user regs frame, but an empty stack is an exception to
that rule.

Thanks to commit 71c95825289f ("x86/unwind/orc: Fix error handling in
__unwind_start()"), unwind_start() now sets state->error appropriately.
So now for both ORC and FP unwinders, unwind_done() and !unwind_error()
always means the end of the stack was successfully reached.  So the
success path for kthreads is no longer needed -- it can also be used for
empty user tasks.

Reported-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Link: https://lkml.kernel.org/r/f136a4e5f019219cbc4f4da33b30c2f44fa65b84.1594994374.git.jpoimboe@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kernel/stacktrace.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index 2d6898c2cb647..6d83b4b857e6a 100644
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -58,7 +58,6 @@ int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
 			 * or a page fault), which can make frame pointers
 			 * unreliable.
 			 */
-
 			if (IS_ENABLED(CONFIG_FRAME_POINTER))
 				return -EINVAL;
 		}
@@ -81,10 +80,6 @@ int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
 	if (unwind_error(&state))
 		return -EINVAL;
 
-	/* Success path for non-user tasks, i.e. kthreads and idle tasks */
-	if (!(task->flags & (PF_KTHREAD | PF_IDLE)))
-		return -EINVAL;
-
 	return 0;
 }
 
-- 
2.25.1