Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for function graph tracing

From: Pratyush Anand
Date: Wed Sep 13 2017 - 01:02:08 EST

On Wednesday 13 September 2017 08:12 AM, Will Deacon wrote:
On Tue, Sep 12, 2017 at 10:54:28AM +0100, James Morse wrote:
Hi Pratyush,

On 01/09/17 06:48, Pratyush Anand wrote:
do_task_stat() calls get_wchan(), which further does unbind_frame().
unbind_frame() restores frame->pc to original value in case function
graph tracer has modified a return address (LR) in a stack frame to hook
a function return. However, if function graph tracer has hit a filtered
function, then we can't unwind it as ftrace_push_return_trace() has
biased the index(frame->graph) with a 'huge negative'

Moreover, arm64 stack walker defines index(frame->graph) as unsigned
int, which can not compare a -ve number.

Similar problem we can have with calling of walk_stackframe() from
save_stack_trace_tsk() or dump_backtrace().

This patch fixes unwind_frame() to test the index for -ve value and
restore index accordingly before we can restore frame->pc.

I've just spotted arm64's profile_pc, which does this:
From arch/arm64/kernel/time.c:profile_pc():
frame.graph = -1; /* no task info */

Is this another elaborate way of hitting this problem?

I guess the options are skip any return-address restore in the unwinder if
frame.graph is -1. (and profile_pc may have a bug here). Or, put
current->curr_ret_stack in there.

I think we should go with latter, ie assign frame.graph = current->curr_ret_stack in profile_pc().

profile_pc() always passes tsk=NULL, so the unwinder assumes its current...
kernel/profile.c pulls the pt_regs from a per-cpu irq_regs variable, that is
updated by handle_IPI ... so it looks like this should always be current...

Hmmm... is profile_pc the *only* case where frame->graph isn't equal to
tsk->curr_ret_stack in unwind_frame? If so, maybe unwind_frame should just

Yes, it is the only place.

use that, and we could kill the graph member of struct stackframe completely?

Humm, not sure, we initialize frame->graph out of the while loop in unwind_frame()'s caller and then keep in decrementing it in looped function.