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

Jiri Olsa
2025-04-28T13:24:14Z

On Sun, Apr 27, 2025 at 07:11:43PM +0200, Oleg Nesterov wrote:
> 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".

I followed David's comment in here:

https://lore.kernel.org/bpf/e206df95d98d4cbab77824cf7a32a80f@AcuMS.aculab.com/

> That might work provided there are IPI (to flush the decode pipeline)
> after the write of the 'int3' and one before the write of the 'call'.
> You'll need to ensure the I-cache gets invalidated as well.

swbp_optimize is called when there's already int3 in place

>
>
> 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...

hum, probably..

>
> > +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?

I'll address this in separate email

thanks,
jirka