linux-kernel - Re: [PATCH 1/2] x86/stacktrace: do not fail when regs on stack for ORC

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Date:   Fri, 1 Dec 2017 08:34:31 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/stacktrace: do not fail when regs on stack for
 ORC

On 11/30/2017, 08:59 PM, Josh Poimboeuf wrote:
> On Thu, Nov 30, 2017 at 01:57:10PM -0600, Josh Poimboeuf wrote:
>> On Thu, Nov 30, 2017 at 09:03:24AM +0100, Jiri Slaby wrote:
>>> save_stack_trace_reliable now returns "non reliable" when there are
>>> kernel pt_regs on stack. This means an interrupt or exception happened.
>>> Somewhere down the route. It is a problem for frame pointer unwinder,
>>> because the frame might now have been set up yet when the irq happened,
>>> so it might fail to unwind from the interrupted function.
>>>
>>> With ORC, this is not a problem, as ORC has out-of-band data. We can
>>> find ORC data even for the IP in interrupted function and always unwind
>>> one level up.
>>>
>>> So introduce `unwind_regs_reliable' which decides if this is an issue
>>> for the currently selected unwinder at all and change the code
>>> accordingly.
>>
>> Thanks.  I'm thinking there a few ways we can simplify things.  (Most of
>> these are actually issues with the existing code.)
>>
>> - Currently we check to make sure that there's no frame *after* the user
>>   space regs.  I think there's no way that could ever happen and the
>>   check is overkill.
>>
>> - We should probably remove the STACKTRACE_DUMP_ONCE() warnings.  There
>>   are some known places where a stack trace will fail, particularly with
>>   generated code.  I wish we had a DEBUG_WARN_ON() macro which used
>>   pr_debug(), but oh well.  At least the livepatch code has some helpful
>>   pr_warn()s, those are probably good enough.
>     ^^^^^^^
>     meant to say pr_debug()s.
> 
> Also adding the live patching mailing list as an FYI.

The changes are now in:
https://git.kernel.org/pub/scm/linux/kernel/git/jirislaby/linux.git/log/?h=devel

Letting 01 bot to run through them, then I will send a v2.

thanks,
-- 
js
suse labs