Re: [PATCH RFC bpf-next 4/3] uprobe: ensure sys_uretprobe uses sysret

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Mar 19, 2024 at 09:25:57AM -0700, Andrii Nakryiko wrote:
> On Tue, Mar 19, 2024 at 4:08 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Tue, Mar 19, 2024 at 11:25:24AM +0100, Oleg Nesterov wrote:
> > > Obviously not for inclusion yet ;) untested, lacks the comments, and I am not
> > > sure it makes sense.
> > >
> > > But I am wondering if this change can speedup uretprobes a bit more. Any chance
> > > you can test it?
> > >
> > > With 1/3 sys_uretprobe() changes regs->r11/cx, this is correct but implies iret.
> > > See the /* SYSRET requires RCX == RIP and R11 == EFLAGS */ code in do_syscall_64().
> >
> > nice idea, looks like sysexit should be faster
> >
> > >
> > > With this patch uretprobe_syscall_entry restores rcx/r11 itself and does retq,
> > > sys_uretprobe() needs to hijack regs->ip after uprobe_handle_trampoline() to
> > > make it possible.
> > >
> > > Comments?
> > >
> > > Oleg.
> > > ---
> > >
> > > diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> > > index 069371e86180..b99f1d80a8c8 100644
> > > --- a/arch/x86/kernel/uprobes.c
> > > +++ b/arch/x86/kernel/uprobes.c
> > > @@ -319,6 +319,9 @@ asm (
> > >       "pushq %r11\n"
> > >       "movq $462, %rax\n"
> > >       "syscall\n"
> > > +     "popq %r11\n"
> > > +     "popq %rcx\n"
> > > +     "retq\n"
> >
> > using rax space on stack for return pointer, cool :)
> >
> > I'll run the test with this change
> >
> 
> I can do some benchmarking on my side as well, given I have everything

that'd be great, thanks

> set up for this anyways. Thanks for the help with speeding all this
> up!
> 
> BTW, Jiri, what are you plans regarding sys_uprobe (entry probe
> optimization through syscall), while we are on the topic?

I plan to work on that after this one is sorted out

jirka