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Message-ID: <20240201223545.728028-1-maskray@google.com>
Date: Thu, 1 Feb 2024 14:35:45 -0800
From: Fangrui Song <maskray@...gle.com>
To: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org
Cc: Jisheng Zhang <jszhang@...nel.org>, Dave Martin <Dave.Martin@....com>,
Ard Biesheuvel <ardb@...nel.org>, Peter Smith <peter.smith@....com>, llvm@...ts.linux.dev,
linux-kernel@...r.kernel.org, Fangrui Song <maskray@...gle.com>
Subject: [PATCH] arm64: jump_label: use constraints "Si" instead of "i"
The generic constraint "i" seems to be copied from x86 or arm (and with
a redundant generic operand modifier "c"). It works with -fno-PIE but
not with -fPIE/-fPIC in GCC's aarch64 port.
The machine constraint "S", which denotes a symbol or label reference
with a constant offset, supports PIC and has been available in GCC since
2012 and in Clang since 7.0. However, Clang before 19 does not support
"S" on a symbol with a constant offset [1] (e.g.
`static_key_false(&nf_hooks_needed[pf][hook])` in
include/linux/netfilter.h), so we use "i" as a fallback.
Suggested-by: Ard Biesheuvel <ardb@...nel.org>
Signed-off-by: Fangrui Song <maskray@...gle.com>
Link: https://github.com/llvm/llvm-project/pull/80255 [1]
---
Changes from
arm64: jump_label: use constraint "S" instead of "i" (https://lore.kernel.org/all/20240131065322.1126831-1-maskray@google.com/)
* Use "Si" as Ard suggested to support Clang<19
* Make branch a separate operand
---
arch/arm64/include/asm/jump_label.h | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
index 48ddc0f45d22..1f7d529be608 100644
--- a/arch/arm64/include/asm/jump_label.h
+++ b/arch/arm64/include/asm/jump_label.h
@@ -15,6 +15,10 @@
#define JUMP_LABEL_NOP_SIZE AARCH64_INSN_SIZE
+/*
+ * Prefer the constraint "S" to support PIC with GCC. Clang before 19 does not
+ * support "S" on a symbol with a constant offset, so we use "i" as a fallback.
+ */
static __always_inline bool arch_static_branch(struct static_key * const key,
const bool branch)
{
@@ -23,9 +27,9 @@ static __always_inline bool arch_static_branch(struct static_key * const key,
" .pushsection __jump_table, \"aw\" \n\t"
" .align 3 \n\t"
" .long 1b - ., %l[l_yes] - . \n\t"
- " .quad %c0 - . \n\t"
+ " .quad %0 + %1 - . \n\t"
" .popsection \n\t"
- : : "i"(&((char *)key)[branch]) : : l_yes);
+ : : "Si"(key), "i"(branch) : : l_yes);
return false;
l_yes:
@@ -40,9 +44,9 @@ static __always_inline bool arch_static_branch_jump(struct static_key * const ke
" .pushsection __jump_table, \"aw\" \n\t"
" .align 3 \n\t"
" .long 1b - ., %l[l_yes] - . \n\t"
- " .quad %c0 - . \n\t"
+ " .quad %0 + %1 - . \n\t"
" .popsection \n\t"
- : : "i"(&((char *)key)[branch]) : : l_yes);
+ : : "Si"(key), "i"(branch) : : l_yes);
return false;
l_yes:
--
2.43.0.594.gd9cf4e227d-goog
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