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Message-ID: <20190207121729.GE2414@zn.tnic>
Date: Thu, 7 Feb 2019 13:17:29 +0100
From: Borislav Petkov <bp@...en8.de>
To: Thomas Garnier <thgarnie@...omium.org>
Cc: kernel-hardening@...ts.openwall.com, kristen@...ux.intel.com,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
x86@...nel.org, "Peter Zijlstra (Intel)" <peterz@...radead.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Thomas Garnier <thgarnie@...gle.com>, Nadav Amit <namit@...are.com>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE
support
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?
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.
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