[PATCH 1/3] jump_label: s390: avoid pointless initial NOP patching

Ard Biesheuvel
2022-06-08T10:45:39Z

Patching NOPs into other NOPs at boot time serves no purpose, so let's
use the same NOP encodings at compile time and runtime.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/s390/include/asm/jump_label.h | 5 ++---
arch/s390/kernel/jump_label.c | 23 +++++---------------
2 files changed, 7 insertions(+), 21 deletions(-)

diff --git a/arch/s390/include/asm/jump_label.h b/arch/s390/include/asm/jump_label.h
index 916cfcb36d8a..895f774bbcc5 100644
--- a/arch/s390/include/asm/jump_label.h
+++ b/arch/s390/include/asm/jump_label.h
@@ -10,7 +10,6 @@
#include <linux/stringify.h>

#define JUMP_LABEL_NOP_SIZE 6
-#define JUMP_LABEL_NOP_OFFSET 2

#ifdef CONFIG_CC_IS_CLANG
#define JUMP_LABEL_STATIC_KEY_CONSTRAINT "i"
@@ -21,12 +20,12 @@
#endif

/*
- * We use a brcl 0,2 instruction for jump labels at compile time so it
+ * We use a brcl 0,<offset> instruction for jump labels so it
* can be easily distinguished from a hotpatch generated instruction.
*/
static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
{
- asm_volatile_goto("0: brcl 0,"__stringify(JUMP_LABEL_NOP_OFFSET)"\n"
+ asm_volatile_goto("0: brcl 0,%l[label]\n"
".pushsection __jump_table,\"aw\"\n"
".balign 8\n"
".long 0b-.,%l[label]-.\n"
diff --git a/arch/s390/kernel/jump_label.c b/arch/s390/kernel/jump_label.c
index 6bec000c6c1c..d764f0d229ab 100644
--- a/arch/s390/kernel/jump_label.c
+++ b/arch/s390/kernel/jump_label.c
@@ -44,14 +44,8 @@ static void jump_label_bug(struct jump_entry *entry, struct insn *expected,
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)
+ enum jump_label_type type)
{
void *code = (void *)jump_entry_code(entry);
struct insn old, new;
@@ -63,27 +57,22 @@ static void jump_label_transform(struct jump_entry *entry,
jump_label_make_branch(entry, &old);
jump_label_make_nop(entry, &new);
}
- if (init) {
- if (memcmp(code, &orignop, sizeof(orignop)))
- jump_label_bug(entry, &orignop, &new);
- } else {
- if (memcmp(code, &old, sizeof(old)))
- jump_label_bug(entry, &old, &new);
- }
+ if (memcmp(code, &old, sizeof(old)))
+ jump_label_bug(entry, &old, &new);
s390_kernel_write(code, &new, sizeof(new));
}

void arch_jump_label_transform(struct jump_entry *entry,
enum jump_label_type type)
{
- jump_label_transform(entry, type, 0);
+ jump_label_transform(entry, type);
text_poke_sync();
}

bool arch_jump_label_transform_queue(struct jump_entry *entry,
enum jump_label_type type)
{
- jump_label_transform(entry, type, 0);
+ jump_label_transform(entry, type);
return true;
}

@@ -95,6 +84,4 @@ void arch_jump_label_transform_apply(void)
void __init_or_module arch_jump_label_transform_static(struct jump_entry *entry,
enum jump_label_type type)
{
- jump_label_transform(entry, type, 1);
- text_poke_sync();
}
--
2.30.2