[PATCH v6 7/8] arm64/kernel: jump_label: use relative references

[Ard Biesheuvel]

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: this reduces the size of the __jump_table by 50%, and gets
rid of the RELA section entirely.

Note that this requires some extra care in the sorting routine, given
that the offsets change when entries are moved around in the jump_entry
table.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
 arch/arm64/include/asm/jump_label.h | 27 ++++++++++++--------
 arch/arm64/kernel/jump_label.c      | 22 +++++++++++++---
 2 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
index 9d6e46355c89..5cec68616125 100644
--- a/arch/arm64/include/asm/jump_label.h
+++ b/arch/arm64/include/asm/jump_label.h
@@ -30,8 +30,8 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
 {
 	asm goto("1: nop\n\t"
 		 ".pushsection __jump_table,  \"aw\"\n\t"
-		 ".align 3\n\t"
-		 ".quad 1b, %l[l_yes], %c0\n\t"
+		 ".align 2\n\t"
+		 ".long 1b - ., %l[l_yes] - ., %c0 - .\n\t"
 		 ".popsection\n\t"
 		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
 
@@ -44,8 +44,8 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
 {
 	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"
+		 ".align 2\n\t"
+		 ".long 1b - ., %l[l_yes] - ., %c0 - .\n\t"
 		 ".popsection\n\t"
 		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
 
@@ -57,19 +57,26 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
 typedef u64 jump_label_t;
 
 struct jump_entry {
-	jump_label_t code;
-	jump_label_t target;
-	jump_label_t key;
+	s32 code;
+	s32 target;
+	s32 key;
 };
 
 static inline jump_label_t jump_entry_code(const struct jump_entry *entry)
 {
-	return entry->code;
+	return (jump_label_t)&entry->code + entry->code;
+}
+
+static inline jump_label_t jump_entry_target(const struct jump_entry *entry)
+{
+	return (jump_label_t)&entry->target + entry->target;
 }
 
 static inline struct static_key *jump_entry_key(const struct jump_entry *entry)
 {
-	return (struct static_key *)((unsigned long)entry->key & ~1UL);
+	unsigned long key = (unsigned long)&entry->key + entry->key;
+
+	return (struct static_key *)(key & ~1UL);
 }
 
 static inline bool jump_entry_is_branch(const struct jump_entry *entry)
@@ -87,7 +94,7 @@ static inline void jump_entry_set_module_init(struct jump_entry *entry)
 	entry->code = 0;
 }
 
-#define jump_label_swap		NULL
+void jump_label_swap(void *a, void *b, int size);
 
 #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..2b8e459e91f7 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();
@@ -50,4 +50,20 @@ void arch_jump_label_transform_static(struct jump_entry *entry,
 	 */
 }
 
+void jump_label_swap(void *a, void *b, int size)
+{
+	long delta = (unsigned long)a - (unsigned long)b;
+	struct jump_entry *jea = a;
+	struct jump_entry *jeb = b;
+	struct jump_entry tmp = *jea;
+
+	jea->code	= jeb->code - delta;
+	jea->target	= jeb->target - delta;
+	jea->key	= jeb->key - delta;
+
+	jeb->code	= tmp.code + delta;
+	jeb->target	= tmp.target + delta;
+	jeb->key	= tmp.key + delta;
+}
+
 #endif	/* HAVE_JUMP_LABEL */
-- 
2.11.0