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@kernel.org>
Signed-off-by: Fangrui Song <maskray@google.com>
Link: https://github.com/llvm/llvm-project/pull/80255
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20240206074552.541154-1-maskray@google.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
index 48ddc0f45d2283f2c5ba0de62f5d3231999a91f3..b7716b215f91a75a078f5741fca248ac6903fb05 100644 (file)
--- 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: