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Subject[PATCH v7 10/10] x86/kernel: jump_table: use relative references
Date
Similar to the arm64 case, 64-bit x86 can benefit from using 32-bit
relative references rather than 64-bit absolute ones when emitting
struct jump_entry instances. Not only does this reduce the memory
footprint of the entries themselves by 50%, it also removes the need
for carrying relocation metadata on relocatable builds (i.e., for KASLR)
which saves a fair chunk of .init space as well (although the savings
are not as dramatic as on arm64)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/x86/include/asm/jump_label.h | 35 ++++++++++++--------
arch/x86/kernel/jump_label.c | 16 +++++++++
tools/objtool/special.c | 4 +--
3 files changed, 39 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 009ff2699d07..35fc2c5ec846 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -36,8 +36,8 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
asm_volatile_goto("1:"
".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
".pushsection __jump_table, \"aw\" \n\t"
- _ASM_ALIGN "\n\t"
- _ASM_PTR "1b, %l[l_yes], %c0 + %c1 \n\t"
+ ".balign 4\n\t"
+ ".long 1b - ., %l[l_yes] - ., %c0 + %c1 - .\n\t"
".popsection \n\t"
: : "i" (key), "i" (branch) : : l_yes);

@@ -52,8 +52,8 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
"2:\n\t"
".pushsection __jump_table, \"aw\" \n\t"
- _ASM_ALIGN "\n\t"
- _ASM_PTR "1b, %l[l_yes], %c0 + %c1 \n\t"
+ ".balign 4\n\t"
+ ".long 1b - ., %l[l_yes] - ., %c0 + %c1 - .\n\t"
".popsection \n\t"
: : "i" (key), "i" (branch) : : l_yes);

@@ -69,19 +69,26 @@ typedef u32 jump_label_t;
#endif

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 (unsigned long)&entry->code + entry->code;
+}
+
+static inline jump_label_t jump_entry_target(const struct jump_entry *entry)
+{
+ return (unsigned long)&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)
@@ -99,7 +106,7 @@ 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);

#else /* __ASSEMBLY__ */

@@ -114,8 +121,8 @@ static inline void jump_entry_set_module_init(struct jump_entry *entry)
.byte STATIC_KEY_INIT_NOP
.endif
.pushsection __jump_table, "aw"
- _ASM_ALIGN
- _ASM_PTR .Lstatic_jump_\@, \target, \key
+ .balign 4
+ .long .Lstatic_jump_\@ - ., \target - ., \key - .
.popsection
.endm

@@ -130,8 +137,8 @@ static inline void jump_entry_set_module_init(struct jump_entry *entry)
.Lstatic_jump_after_\@:
.endif
.pushsection __jump_table, "aw"
- _ASM_ALIGN
- _ASM_PTR .Lstatic_jump_\@, \target, \key + 1
+ .balign 4
+ .long .Lstatic_jump_\@ - ., \target - ., \key + 1 - .
.popsection
.endm

diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
index d64296092ef5..cc5034b42335 100644
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -149,4 +149,20 @@ __init_or_module void arch_jump_label_transform_static(struct jump_entry *entry,
__jump_label_transform(entry, type, text_poke_early, 1);
}

+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
diff --git a/tools/objtool/special.c b/tools/objtool/special.c
index 84f001d52322..98ae55b39037 100644
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -30,9 +30,9 @@
#define EX_ORIG_OFFSET 0
#define EX_NEW_OFFSET 4

-#define JUMP_ENTRY_SIZE 24
+#define JUMP_ENTRY_SIZE 12
#define JUMP_ORIG_OFFSET 0
-#define JUMP_NEW_OFFSET 8
+#define JUMP_NEW_OFFSET 4

#define ALT_ENTRY_SIZE 13
#define ALT_ORIG_OFFSET 0
--
2.11.0
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