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Message-ID: <CAJcbSZFe+Cn9+gEa4iv56sZE+WXXiHjeR8Mcte9s_HYr8QBrsw@mail.gmail.com>
Date: Thu, 7 Feb 2019 09:04:45 -0800
From: Thomas Garnier <thgarnie@...omium.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Kristen Carlson Accardi <kristen@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
"the arch/x86 maintainers" <x86@...nel.org>, "Peter Zijlstra (Intel)" <peterz@...radead.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>, Nadav Amit <namit@...are.com>,
Masahiro Yamada <yamada.masahiro@...ionext.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support
On Thu, Feb 7, 2019 at 4:17 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Jan 31, 2019 at 11:24:09AM -0800, Thomas Garnier wrote:
> > Replace the %c constraint with %P. The %c is incompatible with PIE
> > because it implies an immediate value whereas %P reference a symbol.
>
> How so?
>
> AFAIK, %c requires a constant operand and if %P is used to print a
> constant, it simply drops syntax-specific prefixes and does a bare
> constant.
>
> I guess that here
>
> https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers
>
> is not entirely correct as it should not say "If used for a constant"
> for %P but say "symbol or constant".
>
> But before/after asm doesn't show any difference. So what gives?
I assume that's an optimisation done by gcc later. The P modifier in
the documentation does state that it is used to generate PIC code.
>
> before:
> # 39 "./arch/x86/include/asm/jump_label.h" 1
> 1:
> .byte 0xe9
> .long .L241 - 2f #
> 2:
> .pushsection __jump_table, "aw"
> .balign 8
> .long 1b - ., .L241 - . #
> .quad __use_tsc + 1 - . #,
> .popsection
>
> after:
> # 39 "./arch/x86/include/asm/jump_label.h" 1
> 1:
> .byte 0xe9
> .long .L241 - 2f #
> 2:
> .pushsection __jump_table, "aw"
> .balign 8
> .long 1b - ., .L241 - . #
> .quad __use_tsc+1 - . #
> .popsection
>
> --
> Regards/Gruss,
> Boris.
>
> Good mailing practices for 400: avoid top-posting and trim the reply.
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