linux-kernel - Re: [RESEND PATCH v1] x86/build: add -fno-builtin flag to prevent shadowing

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Message-ID: <CAMZ6Rq+q44vAeqa7HN8mfoNXzv9GndnQDm1e4_kyMXf_+oADdQ@mail.gmail.com>
Date:   Tue, 10 May 2022 08:12:33 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Tom Rix <trix@...hat.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [RESEND PATCH v1] x86/build: add -fno-builtin flag to prevent shadowing

Hi Nick,

On Tue. 10 May 2022 at 04:50, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> On Mon, May 9, 2022 at 8:01 AM Vincent MAILHOL
> <mailhol.vincent@...adoo.fr> wrote:
> >
> > Instead, I am thinking of just using -fno-builtin-ffs to remove
> > the annoying -Wshadow warning. Would that make more sense?
>
> Perhaps a pragma would be the best tool to silence this instance of
> -Wshadow?  I understand what GCC is trying to express, but the kernel
> does straddle a weird place between -ffreestanding and a "hosted" env.

I was a bit reluctant to propose the use of pragma because I received
negative feedback in another patch for using the __diag_ignore()
c.f.:
https://lore.kernel.org/all/YmhZSZWg9YZZLRHA@yury-laptop/

But the context here is a bit different, I guess. If I receive your support, I
am fully OK to silence this with some #pragma.

The patch would look as below (I just need to test with clang
before submitting).

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index a288ecd230ab..e44911253bdf 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -269,6 +269,9 @@ static __always_inline unsigned long
__fls(unsigned long word)
 #undef ADDR

 #ifdef __KERNEL__
+__diag_push();
+__diag_ignore_all("-Wshadow",
+                  "-fno-builtin-foo would remove optimization, just
silence it instead");
 /**
  * ffs - find first set bit in word
  * @x: the word to search
@@ -309,6 +312,7 @@ static __always_inline int ffs(int x)
 #endif
        return r + 1;
 }
+__diag_pop(); /* ignore -Wshadow */

 /**
  * fls - find last set bit in word