Re: [RFC PATCH v1 0/9] arm64: livepatch: Use DWARF Call Frame Information for frame pointer validation

[Madhavan T. Venkataraman]

On 4/8/22 07:06, Peter Zijlstra wrote:
> On Thu, Apr 07, 2022 at 03:25:09PM -0500, madvenka@xxxxxxxxxxxxxxxxxxx wrote:
>> The solution
>> ============
>>
>> The goal here is to use the absolute minimum CFI needed to compute the FP at
>> every instruction address. The unwinder can compute the FP in each frame,
>> compare the actual FP with the computed one and validate the actual FP.
>>
>> Objtool is enhanced to parse the CFI, extract just the rules required,
>> encode them in compact data structures and create special sections for
>> the rules. The unwinder uses the special sections to find the rules for
>> a given instruction address and compute the FP.
>>
>> Objtool can be invoked as follows:
>>
>> 	objtool dwarf generate <object-file>
>>
>> The version of the DWARF standard supported in this work is version 4. The
>> official documentation for this version is here:
>>
>> 	https://dwarfstd.org/doc/DWARF4.pdf
>>
>> Section 6.4 contains the description of the CFI.
> 
> The problem is of course that DWARF is only available for compiler
> generated code and doesn't cover assembly code, of which is there is
> always lots.
> 

Yes. But assembly functions are of two types:

SYM_CODE_*() functions
SYM_FUNC_*() functions

SYM_CODE functions are, by definition, special functions that don't follow any ABI rules.
They don't set up a frame. Based on the opinion of ARM64 experts, these need to be
recognized by the unwinder and, if they are present in a stack trace, the stack trace
must be considered unreliable. I have, in fact, submitted a patch to implement that.

So, only SYM_FUNC*() functions are relevant for this part. I will look into these for arm64
and check if any of them can occur frequently in stack traces. If any of them is likely
to occur frequently in stack traces, I must address them. If there are only a few such
functions, unwind hints may be sufficient. I will get back to you on this.

> I suppose you can go add DWARF annotations to all the assembly, but IIRC
> those are pretty terrible. We were *REALLY* happy to delete all that
> nasty from the x86 code.
> 

DWARF annotations are a PITA to maintain. I will never recommend that!

> On top of that, AFAIK compilers don't generally consider DWARF
> generation to be a correctness issue. For them it's debug info and
> having it be correct is nice but not required. So using it as input for
> something that's required to be correct, seems unfortunate.

It is only debug info. But if that info can be verified, then it is usable for livepatch
purposes. I am thinking of implementing a verifier since DWARF reliability is a valid
concern. I will keep you posted.

Thanks!

Madhavan