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

From: Sasha Levin
Date: Mon Jul 27 2020 - 19:27:56 EST

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

[ 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@xxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Wang ShaoBo <bobo.shaobowang@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/f136a4e5f019219cbc4f4da33b30c2f44fa65b84.1594994374.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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.
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;