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

[Jiri Olsa <olsajiri@gmail.com>]

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))
                return 0;

jirka