Re: [PATCH 02/14] x86/alternatives: Add nested alternatives macros

From: Borislav Petkov
Date: Fri May 31 2024 - 14:09:30 EST


On Fri, May 31, 2024 at 05:10:08PM +0200, Uros Bizjak wrote:
> The intention of '%P' operand modifier is primarily to be used with PIC,
> where ...

Ok, I'll fold in the following. I'd like to have this documented
somewhere in the tree too.

Thx.

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index f02867092a49..4973edbe9289 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -236,9 +236,18 @@ static inline int alternatives_text_reserved(void *start, void *end)
asm_inline volatile(ALTERNATIVE(oldinstr, newinstr, ft_flags) \
: output : "i" (0), ## input)

-/* Like alternative_io, but for replacing a direct call with another one. */
+/*
+ * Like alternative_io, but for replacing a direct call with another one.
+ *
+ * Use the %c operand modifier which is the generic way to print a bare
+ * constant expression with all syntax-specific punctuation omitted. %P
+ * is the x86-specific variant which can handle constants too, for
+ * historical reasons, but it should be used primarily for PIC
+ * references: i.e., if used for a function, it would add the PLT
+ * suffix.
+ */
#define alternative_call(oldfunc, newfunc, ft_flags, output, input...) \
- asm_inline volatile(ALTERNATIVE("call %P[old]", "call %P[new]", ft_flags) \
+ asm_inline volatile(ALTERNATIVE("call %c[old]", "call %c[new]", ft_flags) \
: output : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)

/*
@@ -249,8 +258,8 @@ static inline int alternatives_text_reserved(void *start, void *end)
*/
#define alternative_call_2(oldfunc, newfunc1, ft_flags1, newfunc2, ft_flags2, \
output, input...) \
- asm_inline volatile(ALTERNATIVE_2("call %P[old]", "call %P[new1]", ft_flags1, \
- "call %P[new2]", ft_flags2) \
+ asm_inline volatile(ALTERNATIVE_2("call %c[old]", "call %c[new1]", ft_flags1, \
+ "call %c[new2]", ft_flags2) \
: output, ASM_CALL_CONSTRAINT \
: [old] "i" (oldfunc), [new1] "i" (newfunc1), \
[new2] "i" (newfunc2), ## input)

--
Regards/Gruss,
Boris.

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