On a randomly chosen distro kernel build for arm64, vmlinux.o shows the
following sections, containing jump label entries, and the associated
RELA relocation records, respectively:
...
[38088] __jump_table PROGBITS 0000000000000000 00e19f30
000000000002ea10 0000000000000000 WA 0 0 8
[38089] .rela__jump_table RELA 0000000000000000 01fd8bb0
000000000008be30 0000000000000018 I 38178 38088 8
...
In other words, we have 190 KB worth of 'struct jump_entry' instances,
and 573 KB worth of RELA entries to relocate each entry's code, target
and key members. This means the RELA section occupies 10% of the .init
segment, and the two sections combined represent 5% of vmlinux's entire
memory footprint.
So let's switch from 64-bit absolute references to 32-bit relative
references for the code and target field, and a 64-bit relative
reference for the 'key' field (which may reside in another module or the
core kernel, which may be more than 4 GB way on arm64 when running with
KASLR enable): this reduces the size of the __jump_table by 33%, and
gets rid of the RELA section entirely.
Acked-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/jump_label.h | 36 +++++++++-----------
arch/arm64/kernel/jump_label.c | 6 ++--
3 files changed, 20 insertions(+), 23 deletions(-)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1940c6405d04..d17aa9614e69 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -91,6 +91,7 @@ config ARM64
select HAVE_ARCH_BITREVERSE
select HAVE_ARCH_HUGE_VMAP
select HAVE_ARCH_JUMP_LABEL
+ select HAVE_ARCH_JUMP_LABEL_RELATIVE
select HAVE_ARCH_KASAN if !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
select HAVE_ARCH_KGDB
select HAVE_ARCH_MMAP_RND_BITS
diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
index 1b5e0e843c3a..bb6d15b34668 100644
--- a/arch/arm64/include/asm/jump_label.h
+++ b/arch/arm64/include/asm/jump_label.h
@@ -26,13 +26,15 @@
#define JUMP_LABEL_NOP_SIZE AARCH64_INSN_SIZE
-static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch(struct static_key *key,
+ bool branch)
{
- asm goto("1: nop\n\t"
- ".pushsection __jump_table, \"aw\"\n\t"
- ".align 3\n\t"
- ".quad 1b, %l[l_yes], %c0\n\t"
- ".popsection\n\t"
+ asm goto("1: nop \n\t"
+ " .pushsection __jump_table, \"aw\" \n\t"
+ " .align 3 \n\t"
+ " .long 1b - ., %l[l_yes] - . \n\t"
+ " .quad %c0 - . \n\t"
+ " .popsection \n\t"
: : "i"(&((char *)key)[branch]) : : l_yes);
return false;
@@ -40,13 +42,15 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
return true;
}
-static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch_jump(struct static_key *key,
+ bool branch)
{
- asm goto("1: b %l[l_yes]\n\t"
- ".pushsection __jump_table, \"aw\"\n\t"
- ".align 3\n\t"
- ".quad 1b, %l[l_yes], %c0\n\t"
- ".popsection\n\t"
+ asm goto("1: b %l[l_yes] \n\t"
+ " .pushsection __jump_table, \"aw\" \n\t"
+ " .align 3 \n\t"
+ " .long 1b - ., %l[l_yes] - . \n\t"
+ " .quad %c0 - . \n\t"
+ " .popsection \n\t"
: : "i"(&((char *)key)[branch]) : : l_yes);
return false;
@@ -54,13 +58,5 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
return true;
}
-typedef u64 jump_label_t;
-
-struct jump_entry {
- jump_label_t code;
- jump_label_t target;
- jump_label_t key;
-};
-
#endif /* __ASSEMBLY__ */
#endif /* __ASM_JUMP_LABEL_H */
diff --git a/arch/arm64/kernel/jump_label.c b/arch/arm64/kernel/jump_label.c
index c2dd1ad3e648..903d17023d77 100644
--- a/arch/arm64/kernel/jump_label.c
+++ b/arch/arm64/kernel/jump_label.c
@@ -25,12 +25,12 @@
void arch_jump_label_transform(struct jump_entry *entry,
enum jump_label_type type)
{
- void *addr = (void *)entry->code;
+ void *addr = (void *)jump_entry_code(entry);
u32 insn;
if (type == JUMP_LABEL_JMP) {
- insn = aarch64_insn_gen_branch_imm(entry->code,
- entry->target,
+ insn = aarch64_insn_gen_branch_imm(jump_entry_code(entry),
+ jump_entry_target(entry),
AARCH64_INSN_BRANCH_NOLINK);
} else {
insn = aarch64_insn_gen_nop();
--
2.17.1
