Messages in this thread | | | Date | Wed, 19 Apr 2017 08:44:57 -0500 | From | Josh Poimboeuf <> | Subject | Re: WARNING: kernel stack frame pointer has bad value |
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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
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