[PATCH 2/5] kernel/jump_label: implement generic support for relative references

[Ard Biesheuvel]

To reduce the size taken up by absolute references in jump label
entries themselves and the associated relocation records in the
.init segment, add support for emitting them as 32-bit relative
references instead.

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/Kconfig               |  3 +++
 include/linux/jump_label.h | 28 ++++++++++++++++++++
 kernel/jump_label.c        | 20 +++++++++++++-
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 2b8b70820002..22fa3792626e 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -348,6 +348,9 @@ config HAVE_PERF_USER_STACK_DUMP
 config HAVE_ARCH_JUMP_LABEL
 	bool
 
+config HAVE_ARCH_JUMP_LABEL_RELATIVE
+	bool
+
 config HAVE_RCU_TABLE_FREE
 	bool
 
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index 86ec0652d3b1..aa203dffe72c 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -121,6 +121,32 @@ struct static_key {
 #include <asm/jump_label.h>
 
 #ifndef __ASSEMBLY__
+#ifdef CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE
+
+struct jump_entry {
+	int code;
+	int target;
+	int key;
+};
+
+static inline unsigned long jump_entry_code(const struct jump_entry *entry)
+{
+	return (unsigned long)&entry->code + entry->code;
+}
+
+static inline unsigned long 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)
+{
+	unsigned long key = (unsigned long)&entry->key + entry->key;
+
+	return (struct static_key *)(key & ~1UL);
+}
+
+#else
 
 struct jump_entry; /* defined by the architecture */
 
@@ -139,6 +165,8 @@ static inline struct static_key *jump_entry_key(const struct jump_entry *entry)
 	return (struct static_key *)((unsigned long)entry->key & ~1UL);
 }
 
+#endif
+
 static inline bool jump_entry_is_branch(const struct jump_entry *entry)
 {
 	return (unsigned long)entry->key & 1UL;
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index c3524c9b3004..285eff13ecd1 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -49,6 +49,22 @@ static int jump_label_cmp(const void *a, const void *b)
 	return 0;
 }
 
+static 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;
+}
+
 static void
 jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)
 {
@@ -56,7 +72,9 @@ jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)
 
 	size = (((unsigned long)stop - (unsigned long)start)
 					/ sizeof(struct jump_entry));
-	sort(start, size, sizeof(struct jump_entry), jump_label_cmp, NULL);
+	sort(start, size, sizeof(struct jump_entry), jump_label_cmp,
+	     IS_ENABLED(CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE) ? jump_label_swap
+							      : NULL);
 }
 
 static void jump_label_update(struct static_key *key);
-- 
2.11.0