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

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Date:   Tue, 14 Apr 2020 20:42:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH V3 5/9] objtool: Add return address unwind hints

On Tue, Apr 14, 2020 at 08:31:23PM +0200, Alexandre Chartre wrote:
> On 4/14/20 7:56 PM, Peter Zijlstra wrote:

> > 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.

But which intra-function calls are you worried about here? The RSB
stuffing ones we have to explicitly forget and the retpoline ones we
can't follow because they're indirect calls.

So again, who cares about that stack?