Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support

From: Borislav Petkov
Date: Thu Feb 07 2019 - 07:17:47 EST


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.

Good mailing practices for 400: avoid top-posting and trim the reply.