[PATCH 3.16 07/86] s390/jump label: use different nop instruction

Ben Hutchings
2019-05-16T15:58:49Z

From: Heiko Carstens <heiko.carstens@de.ibm.com>

commit d5caa4dbf9bd2ad8cd7f6be0ca76722be947182b upstream.

Use a brcl 0,2 instruction for jump label nops during compile time,
so we don't mix up the different nops during mcount/hotpatch call
site detection.
The initial jump label code instruction replacement will exchange
these instructions with either a branch or a brcl 0,0 instruction.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/s390/include/asm/jump_label.h | 7 ++++++-
arch/s390/kernel/jump_label.c | 19 +++++++++++++------
2 files changed, 19 insertions(+), 7 deletions(-)

--- a/arch/s390/include/asm/jump_label.h
+++ b/arch/s390/include/asm/jump_label.h
@@ -4,6 +4,7 @@
#include <linux/types.h>

#define JUMP_LABEL_NOP_SIZE 6
+#define JUMP_LABEL_NOP_OFFSET 2

#ifdef CONFIG_64BIT
#define ASM_PTR ".quad"
@@ -13,9 +14,13 @@
#define ASM_ALIGN ".balign 4"
#endif

+/*
+ * We use a brcl 0,2 instruction for jump labels at compile time so it
+ * can be easily distinguished from a hotpatch generated instruction.
+ */
static __always_inline bool arch_static_branch(struct static_key *key)
{
- asm_volatile_goto("0: brcl 0,0\n"
+ asm_volatile_goto("0: brcl 0,"__stringify(JUMP_LABEL_NOP_OFFSET)"\n"
".pushsection __jump_table, \"aw\"\n"
ASM_ALIGN "\n"
ASM_PTR " 0b, %l[label], %0\n"
--- a/arch/s390/kernel/jump_label.c
+++ b/arch/s390/kernel/jump_label.c
@@ -49,6 +49,11 @@ static void jump_label_bug(struct jump_e
panic("Corrupted kernel text");
}

+static struct insn orignop = {
+ .opcode = 0xc004,
+ .offset = JUMP_LABEL_NOP_OFFSET >> 1,
+};
+
static void __jump_label_transform(struct jump_entry *entry,
enum jump_label_type type,
int init)
@@ -59,14 +64,16 @@ static void __jump_label_transform(struc
jump_label_make_nop(entry, &old);
jump_label_make_branch(entry, &new);
} else {
- if (init)
- jump_label_make_nop(entry, &old);
- else
- jump_label_make_branch(entry, &old);
+ jump_label_make_branch(entry, &old);
jump_label_make_nop(entry, &new);
}
- if (memcmp((void *)entry->code, &old, sizeof(old)))
- jump_label_bug(entry, &old);
+ if (init) {
+ if (memcmp((void *)entry->code, &orignop, sizeof(orignop)))
+ jump_label_bug(entry, &old);
+ } else {
+ if (memcmp((void *)entry->code, &old, sizeof(old)))
+ jump_label_bug(entry, &old);
+ }
probe_kernel_write((void *)entry->code, &new, sizeof(new));
}