linux-kernel - Re: [PATCH v3 04/29] x86: assembly, use ENDPROC for functions

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Message-ID: <20170519195008.ls6ynaoxcxilip4x@treble>
Date:   Fri, 19 May 2017 14:50:08 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v3 04/29] x86: assembly, use ENDPROC for functions

On Fri, May 19, 2017 at 11:17:24AM +0200, Jiri Slaby wrote:
> On 05/17/2017, 03:23 PM, Jiri Slaby wrote:
> >> So the initial CFI state is different between the two types of
> >> "functions".  And there are a lot of other differences.  C-type
> >> functions have to follow frame pointer conventions, for example.  So
> >> your FUNC_START macro (and objtool) would have to somehow figure out a
> >> way to make a distinction between the two.  So it would probably work
> >> out better if we kept the distinction between C-type functions and other
> >> code.
> > 
> > Ok, that makes a lot of sense.
> 
> A quick question:
> Do you consider these to be C-type functions?
> 
>   ENTRY(function_hook)
>         ret
>   END(function_hook)
> 
> or this?
> 
>   ENTRY(native_load_gs_index)
>         pushfq
>         DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI)
>         SWAPGS
>         movl    %edi, %gs
>         SWAPGS
>         popfq
>         ret
>   END(native_load_gs_index)
> 
> Both are called from C, but they do not setup frame pointer etc.

Yeah, those are valid C-type functions.  Setting up the frame pointer is
optional for leaf functions (i.e.  functions which don't call other
functions).

-- 
Josh