Re: [PATCHv6 perf/core 10/22] uprobes/x86: Add support to optimize uprobes

[Peter Zijlstra]

On Sun, Jul 20, 2025 at 01:21:20PM +0200, Jiri Olsa wrote:

> +static bool __is_optimized(uprobe_opcode_t *insn, unsigned long vaddr)
> +{
> +	struct __packed __arch_relative_insn {
> +		u8 op;
> +		s32 raddr;
> +	} *call = (struct __arch_relative_insn *) insn;

Not something you need to clean up now I suppose, but we could do with
unifying this thing. we have a bunch of instances around.

> +
> +	if (!is_call_insn(insn))
> +		return false;
> +	return __in_uprobe_trampoline(vaddr + 5 + call->raddr);
> +}

> +void arch_uprobe_optimize(struct arch_uprobe *auprobe, unsigned long vaddr)
> +{
> +	struct mm_struct *mm = current->mm;
> +	uprobe_opcode_t insn[5];
> +
> +	/*
> +	 * Do not optimize if shadow stack is enabled, the return address hijack
> +	 * code in arch_uretprobe_hijack_return_addr updates wrong frame when
> +	 * the entry uprobe is optimized and the shadow stack crashes the app.
> +	 */
> +	if (shstk_is_enabled())
> +		return;

Kernel should be able to fix up userspace shadow stack just fine.

> +	if (!should_optimize(auprobe))
> +		return;
> +
> +	mmap_write_lock(mm);
> +
> +	/*
> +	 * Check if some other thread already optimized the uprobe for us,
> +	 * if it's the case just go away silently.
> +	 */
> +	if (copy_from_vaddr(mm, vaddr, &insn, 5))
> +		goto unlock;
> +	if (!is_swbp_insn((uprobe_opcode_t*) &insn))
> +		goto unlock;
> +
> +	/*
> +	 * If we fail to optimize the uprobe we set the fail bit so the
> +	 * above should_optimize will fail from now on.
> +	 */
> +	if (__arch_uprobe_optimize(auprobe, mm, vaddr))
> +		set_bit(ARCH_UPROBE_FLAG_OPTIMIZE_FAIL, &auprobe->flags);
> +
> +unlock:
> +	mmap_write_unlock(mm);
> +}
> +
> +static bool can_optimize(struct arch_uprobe *auprobe, unsigned long vaddr)
> +{
> +	if (memcmp(&auprobe->insn, x86_nops[5], 5))
> +		return false;
> +	/* We can't do cross page atomic writes yet. */
> +	return PAGE_SIZE - (vaddr & ~PAGE_MASK) >= 5;
> +}

This seems needlessly restrictive. Something like:

is_nop5(const char *buf)
{
	struct insn insn;

	ret = insn_decode_kernel(&insn, buf)
	if (ret < 0)
		return false;

	if (insn.length != 5)
		return false;

	if (insn.opcode[0] != 0x0f ||
	    insn.opcode[1] != 0x1f)
	    	return false;

	return true;
}

Should do I suppose. Anyway, I think something like:

  f0 0f 1f 44 00 00	lock nopl 0(%eax, %eax, 1)

is a valid NOP5 at +1 and will 'optimize' and result in:

  f0 e8 disp32		lock call disp32

which will #UD.

But this is nearly unfixable. Just doing my best to find weirdo cases
;-)