linux-kernel - Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET

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Date:   Thu, 2 Apr 2020 09:50:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Thierry <jthierry@...hat.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, x86@...nel.org, mhiramat@...nel.org,
        mbenes@...e.cz, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET

On Thu, Apr 02, 2020 at 07:41:46AM +0100, Julien Thierry wrote:
> On 4/1/20 6:09 PM, Peter Zijlstra wrote:

> > The code in question (x86's sync_core()), is an exception return to
> > self. It pushes an exception frame that points to right after the
> > exception return instruction.
> > 
> > This is the only usage of IRET in STT_FUNC symbols.
> > 
> > So rather than teaching objtool how to interpret the whole
> > push;push;push;push;push;iret sequence, teach it how big the frame is
> > (arch_exception_frame_size) and let it continue.
> > 
> > All the other (real) IRETs are in STT_NOTYPE in the entry assembly.
> > 
> 
> Right, I see.. However I'm not completely convinced by this. I must admit I
> haven't followed the whole conversation, but what was the issue with the
> HINT_IRET_SELF? It seemed more elegant, but I might be missing some context.

https://lkml.kernel.org/r/20200331211755.pb7f3wa6oxzjnswc@treble

Josh didn't think it was worth it, I think.

> Otherwise, it might be worth having a comment in the code to point that this
> only handles the sync_core() case.

I can certainly do that. Does ARM have any ERETs sprinkled around in
places it should not have? That is, is this going to be a problem for
you?

> Also, instead of adding a special "arch_exception_frame_size", I could
> suggest:
> - Picking this patch [1] from a completely arbitrary source
> - Getting rid of INSN_STACK type, any instruction could then include stack
> ops on top of their existing semantics, they can just have an empty list if
> they don't touch SP/BP
> - x86 decoder adds a stack_op to the iret to modify the stack pointer by the
> right amount

That's not the worst idea, lemme try that.