linux-kernel - Re: [PATCH 04/12] tools/nolibc: use attribute((naked)) if available

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Message-ID: <20240803092558.GB29127@1wt.eu>
Date: Sat, 3 Aug 2024 11:25:58 +0200
From: Willy Tarreau <w@....eu>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 04/12] tools/nolibc: use attribute((naked)) if available

On Sun, Jul 28, 2024 at 12:09:58PM +0200, Thomas Weißschuh wrote:
> The current entrypoint attributes optimize("Os", "omit-frame-pointer")
> are intended to avoid all compiler generated code, like function
> porologue and epilogue.
> This is the exact usecase implemented by the attribute "naked".
> 
> Unfortunately this is not implemented by GCC for all targets,
> so only use it where available.
> This also provides compatibility with clang, which recognizes the
> "naked" attribute but not the previously used attribute "optimized".
> 
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
>  tools/include/nolibc/compiler.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/tools/include/nolibc/compiler.h b/tools/include/nolibc/compiler.h
> index fe3701863634..f77bb7d3e1a8 100644
> --- a/tools/include/nolibc/compiler.h
> +++ b/tools/include/nolibc/compiler.h
> @@ -9,6 +9,15 @@
>  #define __entrypoint __attribute__((optimize("Os", "omit-frame-pointer")))
>  #define __entrypoint_epilogue() __builtin_unreachable()
>  
> +#if defined(__has_attribute)
> +#  if __has_attribute(naked)
> +#    undef  __entrypoint
> +#    define __entrypoint __attribute__((naked))
> +#    undef __entrypoint_epilogue
> +#    define __entrypoint_epilogue()
> +#  endif
> +#endif /* defined(__has_attribute) */

I would find it cleaner to enclose the previous declaration with the #if
rather than #undef everything just after it has been defined. Also it's
not very common to undo declarations just after they've been done, and
it makes quick code analysis harder.

I think that it can resolve to roughly this:

#if defined(__has_attribute) && __has_attribute(naked)
#  define __entrypoint __attribute__((naked))
#  define __entrypoint_epilogue()
#else
#  define __entrypoint __attribute__((optimize("Os", "omit-frame-pointer")))
#  define __entrypoint_epilogue() __builtin_unreachable()
#endif
 
What do you think ?

Willy