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

Oleg Nesterov
2025-04-27T17:12:38Z

I didn't actually read this patch yet, but let me ask anyway...

On 04/21, Jiri Olsa wrote:
>
> +static int swbp_optimize(struct vm_area_struct *vma, unsigned long vaddr, unsigned long tramp)
> +{
> + struct write_opcode_ctx ctx = {
> + .base = vaddr,
> + };
> + char call[5];
> + int err;
> +
> + relative_call(call, vaddr, tramp);
> +
> + /*
> + * We are in state where breakpoint (int3) is installed on top of first
> + * byte of the nop5 instruction. We will do following steps to overwrite
> + * this to call instruction:
> + *
> + * - sync cores
> + * - write last 4 bytes of the call instruction
> + * - sync cores
> + * - update the call instruction opcode
> + */
> +
> + text_poke_sync();

Hmm. I would like to understand why exactly we need at least this first
text_poke_sync() before "write last 4 bytes of the call instruction".

And... I don't suggest to do this right now, but I am wondering if we can
use mm_cpumask(vma->vm_mm) later, I guess we don't care if we race with
switch_mm_irqs_off() which can add another CPU to this mask...

> +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;

Not sure I fully understand the comment/problem, but what if
prctl(ARCH_SHSTK_ENABLE) is called after arch_uprobe_optimize() succeeds?

Oleg.