jump_label: Implement generic support for relative references

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 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@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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: Will Deacon <will.deacon@arm.com>
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/20180919065144.25010-3-ard.biesheuvel@linaro.org

1 files changed, 21 insertions, 1 deletions

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 834e43de0daf..898a1d0c38dc 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -47,14 +47,34 @@ 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)
 {
 	unsigned long size;
+	void *swapfn = NULL;
+
+	if (IS_ENABLED(CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE))
+		swapfn = jump_label_swap;
 
 	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, swapfn);
 }
 
 static void jump_label_update(struct static_key *key);