linux-kernel - Re: [PATCH] x86/entry: add compatibility with IAS

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Message-ID: <20200713221736.GA1904448@rani.riverdale.lan>
Date:   Mon, 13 Jul 2020 18:17:36 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Jian Cai <caij2003@...il.com>
Cc:     jiancai@...gle.com, ndesaulniers@...gle.com, manojgupta@...gle.com,
        sedat.dilek@...il.com, Brian Gerst <brgerst@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86/entry: add compatibility with IAS

On Mon, Jul 13, 2020 at 02:38:01PM -0700, Jian Cai wrote:
> Clang's integrated assembler does not allow symbols with non-absolute
> values to be reassigned. This patch allows the affected code to be
> compatible with IAS.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1043
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Suggested-by: Brian Gerst <brgerst@...il.com>
> Suggested-by: Arvind Sankar <nivedita@...m.mit.edu>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Jian Cai <caij2003@...il.com>
> ---
>  arch/x86/include/asm/idtentry.h | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
> index f3d70830bf2a..7d22684eafdf 100644
> --- a/arch/x86/include/asm/idtentry.h
> +++ b/arch/x86/include/asm/idtentry.h
> @@ -469,16 +469,15 @@ __visible noinstr void func(struct pt_regs *regs,			\
>  	.align 8
>  SYM_CODE_START(irq_entries_start)
>      vector=FIRST_EXTERNAL_VECTOR
> -    pos = .
>      .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR)
> +0 :
>  	UNWIND_HINT_IRET_REGS

I know I had it this way, but I think it may be slightly safer to put
the label immediately after UNWIND_HINT_IRET_REGS instead of before,
just in case anyone adds a 0: inside that macro.