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From: ard.biesheuvel@linaro.org (Ard Biesheuvel)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH v3 3/9] arm64/kernel: jump_label: switch to relative references
Date: Tue, 18 Sep 2018 23:51:38 -0700
Message-ID: <20180919065144.25010-4-ard.biesheuvel@linaro.org> (raw)
In-Reply-To: <20180919065144.25010-1-ard.biesheuvel@linaro.org>

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@arm.com>
Signed-off-by: Ard Biesheuvel <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 @@ 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_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 @@ 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 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();
-- 
2.17.1

  parent reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-19  6:51 [PATCH v3 0/9] add support for relative references in jump tables Ard Biesheuvel
2018-09-19  6:51 ` [PATCH v3 1/9] kernel/jump_label: abstract jump_entry member accessors Ard Biesheuvel
2018-09-19  6:51 ` [PATCH v3 2/9] kernel/jump_label: implement generic support for relative references Ard Biesheuvel
2018-09-19  6:51 ` Ard Biesheuvel [this message]
2018-09-19  6:51 ` [PATCH v3 4/9] x86: add support for 64-bit place relative relocations Ard Biesheuvel
2018-09-19  6:51 ` [PATCH v3 5/9] x86: jump_label: switch to jump_entry accessors Ard Biesheuvel
2018-09-19  6:51 ` [PATCH v3 6/9] x86/kernel: jump_table: use relative references Ard Biesheuvel
2018-09-19  6:51 ` [PATCH v3 7/9] jump_label: annotate entries that operate on __init code earlier Ard Biesheuvel
2018-09-19  6:51 ` [PATCH v3 8/9] jump_table: move entries into ro_after_init region Ard Biesheuvel
2018-09-30 15:42   ` Guenter Roeck
2018-09-19  6:51 ` [PATCH v3 9/9] s390/jump_label: switch to relative references Ard Biesheuvel
2018-09-19 13:08 ` [PATCH v3 0/9] add support for relative references in jump tables Peter Zijlstra
2018-09-27 15:22   ` Ard Biesheuvel
2018-09-27 15:46   ` Kees Cook

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