Messages in this thread Patch in this message | | | Date | Mon, 5 Feb 2024 23:45:52 -0800 | Subject | [PATCH v2] arm64: jump_label: use constraints "Si" instead of "i" | From | Fangrui Song <> |
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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@kernel.org> Signed-off-by: Fangrui Song <maskray@google.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
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
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