[PATCH 2/4] x86/alternatives: Get rid of __optimize_nops()

Borislav Petkov
2024-01-30T11:00:05Z

From: "Borislav Petkov (AMD)" <bp@alien8.de>

There's no need to carve out bits of the NOP optimization functionality
and look at JMP opcodes - simply do one more NOPs optimization pass
at the end of the patching.

A lot simpler code.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
---
arch/x86/kernel/alternative.c | 52 +++++++----------------------------
1 file changed, 10 insertions(+), 42 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d633eb59f2b6..2dd1c7fe0949 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -216,47 +216,12 @@ static int skip_nops(u8 *buf, int offset, int len)
return offset;
}

-/*
- * Optimize a sequence of NOPs, possibly preceded by an unconditional jump
- * to the end of the NOP sequence into a single NOP.
- */
-static bool __init_or_module
-__optimize_nops(const u8 * const instr, u8 *buf, size_t len, struct insn *insn, int *next, int *prev, int *target)
-{
- int i = *next - insn->length;
-
- switch (insn->opcode.bytes[0]) {
- case JMP8_INSN_OPCODE:
- case JMP32_INSN_OPCODE:
- *prev = i;
- *target = *next + insn->immediate.value;
- return false;
- }
-
- if (insn_is_nop(insn)) {
- int nop = i;
-
- *next = skip_nops(buf, *next, len);
- if (*target && *next == *target)
- nop = *prev;
-
- add_nop(buf + nop, *next - nop);
- DUMP_BYTES(ALT, buf, len, "%px: [%d:%d) optimized NOPs: ", instr, nop, *next);
- return true;
- }
-
- *target = 0;
- return false;
-}
-
/*
* "noinline" to cause control flow change and thus invalidate I$ and
* cause refetch after modification.
*/
static void __init_or_module noinline optimize_nops(const u8 * const instr, u8 *buf, size_t len)
{
- int prev, target = 0;
-
for (int next, i = 0; i < len; i = next) {
struct insn insn;

@@ -265,7 +230,14 @@ static void __init_or_module noinline optimize_nops(const u8 * const instr, u8 *

next = i + insn.length;

- __optimize_nops(instr, buf, len, &insn, &next, &prev, &target);
+ if (insn_is_nop(&insn)) {
+ int nop = i;
+
+ next = skip_nops(buf, next, len);
+
+ add_nop(buf + nop, next - nop);
+ DUMP_BYTES(ALT, buf, len, "%px: [%d:%d) optimized NOPs: ", instr, nop, next);
+ }
}
}

@@ -342,8 +314,6 @@ bool need_reloc(unsigned long offset, u8 *src, size_t src_len)
static void __init_or_module noinline
apply_relocation(const u8 * const instr, u8 *buf, size_t len, u8 *src, size_t src_len)
{
- int prev, target = 0;
-
for (int next, i = 0; i < len; i = next) {
struct insn insn;

@@ -352,9 +322,6 @@ apply_relocation(const u8 * const instr, u8 *buf, size_t len, u8 *src, size_t sr

next = i + insn.length;

- if (__optimize_nops(instr, buf, len, &insn, &next, &prev, &target))
- continue;
-
switch (insn.opcode.bytes[0]) {
case 0x0f:
if (insn.opcode.bytes[1] < 0x80 ||
@@ -533,7 +500,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
insn_buff[insn_buff_sz] = 0x90;

- apply_relocation(instr, insn_buff, a->instrlen, replacement, a->replacementlen);
+ apply_relocation(instr, insn_buff, a->instrlen, replacement, insn_buff_sz);
+ optimize_nops(instr, insn_buff, insn_buff_sz);

DUMP_BYTES(ALT, instr, a->instrlen, "%px: old_insn: ", instr);
DUMP_BYTES(ALT, replacement, a->replacementlen, "%px: rpl_insn: ", replacement);
--
2.43.0