Re: [PATCH 3/3] objtool/x86: Fix NOP decode

Alexandre Chartre
2025-09-24T17:34:14Z

On 9/24/25 15:45, Peter Zijlstra wrote:
> For x86_64 the kernel consistently uses 2 instructions for all NOPs:
>
> 90 - NOP
> 0f 1f /0 - NOPL
>
>
> Notably:
>
> - REP NOP is PAUSE, not a NOP instruction.
>
> - 0f {0c...0f} is reserved space,
> except for 0f 0d /1, which is PREFETCHW, not a NOP.
>
> - 0f {19,1c...1f} is reserved space,
> except for 0f 1f /0, which is NOPL.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> tools/objtool/arch/x86/decode.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -494,7 +494,8 @@ int arch_decode_instruction(struct objto
> break;
>
> case 0x90:
> - insn->type = INSN_NOP;
> + if (prefix != 0xf3) /* REP NOP := PAUSE */
> + insn->type = INSN_NOP;
> break;

So this covers NOP1 (0x90) and NOP2 (0x66 0x90), right?

>
> case 0x9c:
> @@ -547,13 +548,14 @@ int arch_decode_instruction(struct objto
>
> } else if (op2 == 0x0b || op2 == 0xb9) {
>
> - /* ud2 */
> + /* ud2, ud1 */
> insn->type = INSN_BUG;
>
> - } else if (op2 == 0x0d || op2 == 0x1f) {
> + } else if (op2 == 0x1f) {
>
> - /* nopl/nopw */
> - insn->type = INSN_NOP;
> + /* 0f 1f /0 := NOPL */
> + if (modrm_reg == 0)
> + insn->type = INSN_NOP;
>
> } else if (op2 == 0x1e) {
>

And this covers all other NOPs (0x0f 0x1f ...), including NOP6 which has
a 0x66 preifx (0x66 0xf 0x1f ...) ?

From arch/x86/include/asm/nops.h we have:

/*
* Generic 64bit nops from GAS:
*
* 1: nop
* 2: osp nop
* 3: nopl (%eax)
* 4: nopl 0x00(%eax)
* 5: nopl 0x00(%eax,%eax,1)
* 6: osp nopl 0x00(%eax,%eax,1)
* 7: nopl 0x00000000(%eax)
* 8: nopl 0x00000000(%eax,%eax,1)
*/
#define BYTES_NOP1 0x90
#define BYTES_NOP2 0x66,BYTES_NOP1
#define BYTES_NOP3 0x0f,0x1f,0x00
#define BYTES_NOP4 0x0f,0x1f,0x40,0x00
#define BYTES_NOP5 0x0f,0x1f,0x44,0x00,0x00
#define BYTES_NOP6 0x66,BYTES_NOP5
#define BYTES_NOP7 0x0f,0x1f,0x80,0x00,0x00,0x00,0x00
#define BYTES_NOP8 0x0f,0x1f,0x84,0x00,0x00,0x00,0x00,0x00

alex.