Re: [PATCH V3 5/9] objtool: Add return address unwind hints

Alexandre Chartre
2020-04-14T18:29:15Z

On 4/14/20 7:56 PM, Peter Zijlstra wrote:
> On Tue, Apr 14, 2020 at 06:40:12PM +0200, Alexandre Chartre wrote:
>>
>> On 4/14/20 6:16 PM, Peter Zijlstra wrote:
>>> On Tue, Apr 14, 2020 at 12:36:14PM +0200, Alexandre Chartre wrote:
>>>> Add the UNWIND_HINT_RADDR_DELETE and UNWIND_HINT_RADDR_ALTER unwind
>>>> hint macros to flag instructions which remove or modify return
>>>> addresses.
>>>
>>> I'm confused by this thing; why? AFAICT the rest of the patches are
>>> actually simpler without this one.
>>>
>>
>> This is required to indicate to objtool that assembly instructions are
>> changing return addresses. For example, in patch 8:
>>
>> For retpoline:
>>
>> @@ -88,6 +96,7 @@
>> lfence
>> jmp .Lspec_trap_\@
>> .Ldo_rop_\@:
>> + UNWIND_HINT_RADDR_ALTER
>> mov \reg, (%_ASM_SP)
>> ret
>> .endm
>>
>> The unwind hint indicates that the return address has been altered, so the
>> code won't return to the return address which was on the stack.
>
> But if you hadn't added that return stack stuff in the first place,
> you'd not have needed that HINT.
>
> So what actual problem is it solving?
>

The return stack stuff is here to correctly handle intra-function call so that
we can figure out where the ret of an intra-function call should return. We
don't have this challenge with regular functions because we know that a ret
inside such function just indicates the end of the function.

But when there's an intra-function call, a ret instruction can either:
- continue after the intra-function call (if the stack was unchanged)
- jump somewhere else (if the return address was changed) and eventually
return to the next return address
- indicate the end of the function (if the return address was removed).

So, all this is needed to correctly follow the flow of the code and properly
record stack changes.

alex.