Re: [RFC PATCH v2 1/4] arm64: Implement infrastructure for stack trace reliability checks

["Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>]

On 4/8/21 2:30 PM, Madhavan T. Venkataraman wrote:
> 
> 
> On 4/8/21 12:17 PM, Mark Brown wrote:
>> On Mon, Apr 05, 2021 at 03:43:10PM -0500, madvenka@linux.microsoft.com wrote:
>>
>>> These checks will involve checking the return PC to see if it falls inside
>>> any special functions where the stack trace is considered unreliable.
>>> Implement the infrastructure needed for this.
>>
>> Following up again based on an off-list discussion with Mark Rutland:
>> while I think this is a reasonable implementation for specifically
>> listing functions that cause problems we could make life easier for
>> ourselves by instead using annotations at the call sites to put things
>> into sections which indicate that they're unsafe for unwinding, we can
>> then check for any address in one of those sections (or possibly do the
>> reverse and check for any address in a section we specifically know is
>> safe) rather than having to enumerate problematic functions in the
>> unwinder.  This also has the advantage of not having a list that's
>> separate to the functions themselves so it's less likely that the
>> unwinder will get out of sync with the rest of the code as things evolve.
>>
>> We already have SYM_CODE_START() annotations in the code for assembly
>> functions that aren't using the standard calling convention which should
>> help a lot here, we could add a variant of that for things that we know
>> are safe on stacks (like those we expect to find at the bottom of
>> stacks).
>>
> 
> As I already mentioned before, I like the idea of sections. The only reason that I did
> not try it was that I have to address FTRACE trampolines and the kretprobe_trampoline
> (and optprobes in the future).
> 
> I have the following options:
> 
> 1. Create a common section (I will have to come up with an appropriate name) and put
>    all such functions in that one section.
> 
> 2. Create one section for each logical type (exception section, ftrace section and
>    kprobe section) or some such.
> 

For now, I will start with idea 2. I will create a special section for each class of
functions (EL1 exception handlers, FTRACE trampolines, KPROBE trampolines). Instead of a
special functions array, I will implement a special_sections array. The rest of the code
should just fall into place.

Let me know if you prefer something different.

Thanks.

Madhavan

> 3. Use the section idea only for the el1 exceptions. For the others use the current
>    special_functions[] approach.
> 
> Which one do you and Mark Rutland prefer? Or, is there another choice?
> 
> Madhavan
>