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Subject[tip:core/core] arm64/kernel: jump_label: Switch to relative references
Commit-ID:  c296146c058c87e9ed53001b6a3988519dbbb6a5
Gitweb: https://git.kernel.org/tip/c296146c058c87e9ed53001b6a3988519dbbb6a5
Author: Ard Biesheuvel <ard.biesheuvel@linaro.org>
AuthorDate: Tue, 18 Sep 2018 23:51:38 -0700
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 27 Sep 2018 17:56:47 +0200

arm64/kernel: jump_label: Switch to relative references

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.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-s390@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Jessica Yu <jeyu@kernel.org>
Link: https://lkml.kernel.org/r/20180919065144.25010-4-ard.biesheuvel@linaro.org

---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/jump_label.h | 38 ++++++++++++++++++-------------------
arch/arm64/kernel/jump_label.c | 6 +++---
3 files changed, 22 insertions(+), 23 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1b1a0e95c751..16e1e2c1e77b 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -104,6 +104,7 @@ 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 7e2b3e360086..472023498d71 100644
--- a/arch/arm64/include/asm/jump_label.h
+++ b/arch/arm64/include/asm/jump_label.h
@@ -26,13 +26,16 @@

#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_volatile_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_volatile_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 +43,16 @@ l_yes:
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_volatile_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_volatile_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 +60,5 @@ l_yes:
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 e0756416e567..646b9562ee64 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();
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