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

@@ -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

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 */

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();