Messages in this thread | | | Date | Wed, 19 Apr 2017 10:12:03 -0400 | From | Steven Rostedt <> | Subject | Re: WARNING: kernel stack frame pointer has bad value |
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On Wed, 19 Apr 2017 08:44:57 -0500 Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 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.
This is because we have to handle each different config differently. This is the case with FENTRY and FRAME_POINTERS. As I like to keep this as efficient as possible. To do the above, we need to modify the return address and then restore it. And handle that for each config type.
> > The alternative would be to change the unwinder, but I would rather > avoid having to detect another special case if possible.
I'm not sure what's worse. Modifying all the special cases of ftrace, or adding a new one to the undwinder.
You can take a crack at it if you like, but it needs to be negligible in the performance of FENTRY and no frame pointers.
-- Steve
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