Re: [PATCHv5 bpf-next 03/13] bpf: Add support for uprobe multi session attach

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Oct 01, 2024 at 10:11:13AM -0700, Andrii Nakryiko wrote:
> On Tue, Oct 1, 2024 at 6:17 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Mon, Sep 30, 2024 at 02:36:08PM -0700, Andrii Nakryiko wrote:
> >
> > SNIP
> >
> > > >  struct bpf_uprobe_multi_link {
> > > > @@ -3248,9 +3260,13 @@ uprobe_multi_link_handler(struct uprobe_consumer *con, struct pt_regs *regs,
> > > >                           __u64 *data)
> > > >  {
> > > >         struct bpf_uprobe *uprobe;
> > > > +       int ret;
> > > >
> > > >         uprobe = container_of(con, struct bpf_uprobe, consumer);
> > > > -       return uprobe_prog_run(uprobe, instruction_pointer(regs), regs);
> > > > +       ret = uprobe_prog_run(uprobe, instruction_pointer(regs), regs);
> > > > +       if (uprobe->session)
> > > > +               return ret ? UPROBE_HANDLER_IGNORE : 0;
> > > > +       return ret;
> > >
> > > isn't this a bug that BPF program can return arbitrary value here and,
> > > e.g., request uprobe unregistration?
> > >
> > > Let's return 0, unless uprobe->session? (it would be good to move that
> > > into a separate patch with Fixes)
> >
> > yea there's no use case for uprobe multi user, so let's return
> > 0 as you suggest
> >
> > >
> > > >  }
> > > >
> > > >  static int
> > > > @@ -3260,6 +3276,12 @@ uprobe_multi_link_ret_handler(struct uprobe_consumer *con, unsigned long func, s
> > > >         struct bpf_uprobe *uprobe;
> > > >
> > > >         uprobe = container_of(con, struct bpf_uprobe, consumer);
> > > > +       /*
> > > > +        * There's chance we could get called with NULL data if we registered uprobe
> > > > +        * after it hit entry but before it hit return probe, just ignore it.
> > > > +        */
> > > > +       if (uprobe->session && !data)
> > > > +               return 0;
> > >
> > > why can't handle_uretprobe_chain() do this check instead? We know when
> > > we are dealing with session uprobe/uretprobe, so we can filter out
> > > these spurious calls, no?
> >
> > right, now that we decide session based on presence of both callbacks
> > we have that info in here handle_uretprobe_chain.. but let's still check
> > it for sanity and warn? like
> >
> >         if (WARN_ON_ONCE(uprobe->session && !data))
> 
> You mean to check this *additionally* in uprobe_multi_link_handler(),
> after core uprobe code already filtered that condition out? It won't
> hurt, but I'm not sure I see the point?

yes, it's cross subsytem call so just to be on safe side for future,
but I don't insist

jirka