linux-kernel - Re: [PATCH 07/22] x86/uaccess: Remove ELF function annotation from copy_user_handle

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Message-ID: <CAKwvOdn2ZEe6y=TZ2OyphR7CrC3yJgDEUVtwSX_5fXGtYAQ=bA@mail.gmail.com>
Date:   Tue, 16 Jul 2019 11:16:48 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH 07/22] x86/uaccess: Remove ELF function annotation from copy_user_handle_tail()

On Sun, Jul 14, 2019 at 5:37 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> After an objtool improvement, it's complaining about the CLAC in
> copy_user_handle_tail():
>
>   arch/x86/lib/copy_user_64.o: warning: objtool: .altinstr_replacement+0x12: redundant UACCESS disable
>   arch/x86/lib/copy_user_64.o: warning: objtool:   copy_user_handle_tail()+0x6: (alt)
>   arch/x86/lib/copy_user_64.o: warning: objtool:   copy_user_handle_tail()+0x2: (alt)
>   arch/x86/lib/copy_user_64.o: warning: objtool:   copy_user_handle_tail()+0x0: <=== (func)
>
> copy_user_handle_tail() is incorrectly marked as a callable function, so
> objtool is rightfully concerned about the CLAC with no corresponding
> STAC.
>
> Remove the ELF function annotation.  The copy_user_handle_tail() code
> path is already verified by objtool because it's jumped to by other
> callable asm code (which does the corresponding STAC).

What is CLAC and STAC?

>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  arch/x86/lib/copy_user_64.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
> index 378a1f70ae7d..4fe1601dbc5d 100644
> --- a/arch/x86/lib/copy_user_64.S
> +++ b/arch/x86/lib/copy_user_64.S
> @@ -239,7 +239,7 @@ copy_user_handle_tail:
>         ret
>
>         _ASM_EXTABLE_UA(1b, 2b)
> -ENDPROC(copy_user_handle_tail)
> +END(copy_user_handle_tail)
>
>  /*
>   * copy_user_nocache - Uncached memory copy with exception handling
> --
> 2.20.1
>


-- 
Thanks,
~Nick Desaulniers