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

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Apr 22, 2025 at 05:04:03PM -0700, Andrii Nakryiko wrote:

SNIP

> >  arch/x86/include/asm/uprobes.h |   7 +
> >  arch/x86/kernel/uprobes.c      | 281 ++++++++++++++++++++++++++++++++-
> >  include/linux/uprobes.h        |   6 +-
> >  kernel/events/uprobes.c        |  15 +-
> >  4 files changed, 301 insertions(+), 8 deletions(-)
> >
> 
> just minor nits, LGTM
> 
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> 
> > +int set_swbp(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> > +            unsigned long vaddr)
> > +{
> > +       if (should_optimize(auprobe)) {
> > +               bool optimized = false;
> > +               int err;
> > +
> > +               /*
> > +                * We could race with another thread that already optimized the probe,
> > +                * so let's not overwrite it with int3 again in this case.
> > +                */
> > +               err = is_optimized(vma->vm_mm, vaddr, &optimized);
> > +               if (err || optimized)
> > +                       return err;
> 
> IMO, this is a bit too clever, I'd go with plain
> 
> if (err)
>     return err;
> if (optimized)
>     return 0; /* we are done */
> 

ok

> (and mirror set_orig_insn() structure, consistently)

set_orig_insn does that already, right?

> 
> 
> > +       }
> > +       return uprobe_write_opcode(vma, vaddr, UPROBE_SWBP_INSN, true);
> > +}
> > +
> > +int set_orig_insn(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> > +                 unsigned long vaddr)
> > +{
> > +       if (test_bit(ARCH_UPROBE_FLAG_CAN_OPTIMIZE, &auprobe->flags)) {
> > +               struct mm_struct *mm = vma->vm_mm;
> > +               bool optimized = false;
> > +               int err;
> > +
> > +               err = is_optimized(mm, vaddr, &optimized);
> > +               if (err)
> > +                       return err;
> > +               if (optimized)
> > +                       WARN_ON_ONCE(swbp_unoptimize(auprobe, vma, vaddr));
> > +       }
> > +       return uprobe_write_opcode(vma, vaddr, *(uprobe_opcode_t *)&auprobe->insn, false);
> > +}
> > +
> > +static int __arch_uprobe_optimize(struct mm_struct *mm, unsigned long vaddr)
> > +{
> > +       struct uprobe_trampoline *tramp;
> > +       struct vm_area_struct *vma;
> > +       int err = 0;
> > +
> > +       vma = find_vma(mm, vaddr);
> > +       if (!vma)
> > +               return -1;
> 
> this is EPERM, will be confusing to debug... why not -EINVAL?
> 
> > +       tramp = uprobe_trampoline_get(vaddr);
> > +       if (!tramp)
> > +               return -1;
> 
> ditto

so the error value is not exposed to user space in this case,
we try to optimize in the first hit with:

	handle_swbp()
	{
		arch_uprobe_optimize()
		{

			if (__arch_uprobe_optimize(mm, vaddr))
				set_bit(ARCH_UPROBE_FLAG_OPTIMIZE_FAIL, &auprobe->flags);

		}
	}

and set ARCH_UPROBE_FLAG_OPTIMIZE_FAIL flags bit in case of error,
plus there's WARN for swbp_optimize which should pass in case we
get that far

thanks,
jirka

> 
> > +       err = swbp_optimize(vma, vaddr, tramp->vaddr);
> > +       if (WARN_ON_ONCE(err))
> > +               uprobe_trampoline_put(tramp);
> > +       return err;
> > +}
> > +
> 
> [...]