Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks

[Madhavan T. Venkataraman]

On 4/9/21 5:53 PM, Josh Poimboeuf wrote:
> On Fri, Apr 09, 2021 at 05:32:27PM -0500, Josh Poimboeuf wrote:
>> On Fri, Apr 09, 2021 at 05:05:58PM -0500, Madhavan T. Venkataraman wrote:
>>>> FWIW, over the years we've had zero issues with encoding the frame
>>>> pointer on x86.  After you save pt_regs, you encode the frame pointer to
>>>> point to it.  Ideally in the same macro so it's hard to overlook.
>>>>
>>>
>>> I had the same opinion. In fact, in my encoding scheme, I have additional
>>> checks to make absolutely sure that it is a true encoding and not stack
>>> corruption. The chances of all of those values accidentally matching are,
>>> well, null.
>>
>> Right, stack corruption -- which is already exceedingly rare -- would
>> have to be combined with a miracle or two in order to come out of the
>> whole thing marked as 'reliable' :-)
>>
>> And really, we already take a similar risk today by "trusting" the frame
>> pointer value on the stack to a certain extent.
> 
> Oh yeah, I forgot to mention some more benefits of encoding the frame
> pointer (or marking pt_regs in some other way):
> 
> a) Stack addresses can be printed properly: '%pS' for printing regs->pc
>    and '%pB' for printing call returns.
> 
>    Using '%pS' for call returns (as arm64 seems to do today) will result
>    in printing the wrong function when you have tail calls to noreturn
>    functions on the stack (which is actually quite common for calls to
>    panic(), die(), etc).
> 
>    More details:
> 
>    https://lkml.kernel.org/r/20210403155948.ubbgtwmlsdyar7yp@treble
> 
> b) Stack dumps to the console can dump the exception registers they find
>    along the way.  This is actually quite nice for debugging.
> 
> 

Great.

I am preparing version 3 taking into account comments from yourself,
Mark Rutland and Mark Brown.

Stay tuned.

Madhavan