Re: WARNING: kernel stack frame pointer has bad value

[Josh Poimboeuf]

On Tue, Apr 18, 2017 at 11:37:14PM -0400, Steven Rostedt wrote:
> Josh,
> 
> I'm starting to get a bunch of these warnings, and I'm thinking they
> are false positives. The stack frame error is recorded at a call from
> entry_SYSCALL_64_fastpath, where I would expect the bp to not be valid.
> 
> To trigger this, I only need to go into /sys/kernel/debug/tracing and
> echo function > current_tracer then cat trace. Maybe function tracer
> stack frames is messing it up some how, but it always fails at the
> entry call.
> 
> Here's the dump;
> 
>  WARNING: kernel stack frame pointer at ffff8800bda0ff30 in sshd:1090 has bad value 000055b32abf1fa8
...
>  ffff8800bda0ff20: ffff8800bda0ff30 (0xffff8800bda0ff30)
>  ffff8800bda0ff28: ffffffff810dc945 (SyS_rt_sigprocmask+0x5/0x1a0)
>  ffff8800bda0ff30: 000055b32abf1fa8 (0x55b32abf1fa8)
>  ffff8800bda0ff38: ffffffff81cf502a (entry_SYSCALL_64_fastpath+0x18/0xad)
>  ffff8800bda0ff40: 000055b32abf1fa8 (0x55b32abf1fa8)
>  ffff8800bda0ff48: ffffffff810dc945 (SyS_rt_sigprocmask+0x5/0x1a0)
>  ffff8800bda0ff50: ffffffff81cf502a (entry_SYSCALL_64_fastpath+0x18/0xad)

Thanks for reporting, I hadn't seen this one yet.

The problem is that the unwinder expects the last frame pointer to be at
a certain address (0xffff8800bda0ff48 in this case), so it can know that
it reached the end.  It's confused by the save_mcount_regs macro, which
builds some fake frames -- which is good -- but then the last frame is
at a different offset than what the unwinder expects.

Would it be possible for ftrace to rewrite the stack so that it looks
like this instead?

>  ffff8800bda0ff38: ffff8800bda0ff48 (0xffff8800bda0ff48)
>  ffff8800bda0ff40: ffffffff810dc945 (SyS_rt_sigprocmask+0x5/0x1a0)
>  ffff8800bda0ff48: 000055b32abf1fa8 (0x55b32abf1fa8)
>  ffff8800bda0ff50: ffffffff81cf502a (entry_SYSCALL_64_fastpath+0x18/0xad)

In other words it would overwrite the "SyS_rt_sigprocmask+0x5/0x1a0"
value on the stack at ffff8800bda0ff48 with the original bp, instead of
appending to the existing stack.  If you would be ok with such an
approach, I could take a stab at it.

The alternative would be to change the unwinder, but I would rather
avoid having to detect another special case if possible.

-- 
Josh