[PATCH v2] arm64: jump_label: use constraints "Si" instead of "i"

From: Fangrui Song
Date: Tue Feb 06 2024 - 02:46:11 EST


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@xxxxxxxxxx>
Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
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@xxxxxxxxxx/)

* Use "Si" as Ard suggested to support Clang<19
* Make branch a separate operand

Changes from v1:

* Use asmSymbolicName for readability
---
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..b7716b215f91 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 (%[key] - .) + %[bit0] \n\t"
" .popsection \n\t"
- : : "i"(&((char *)key)[branch]) : : l_yes);
+ : : [key]"Si"(key), [bit0]"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 (%[key] - .) + %[bit0] \n\t"
" .popsection \n\t"
- : : "i"(&((char *)key)[branch]) : : l_yes);
+ : : [key]"Si"(key), [bit0]"i"(branch) : : l_yes);

return false;
l_yes:
--
2.43.0.594.gd9cf4e227d-goog