Re: [RFC bpf-next 1/2] bpf: Fail uprobe multi link with negative offset

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Dec 13, 2023 at 03:43:04PM -0800, Andrii Nakryiko wrote:
> On Wed, Dec 13, 2023 at 6:12 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Currently the __uprobe_register will return 0 (success) when called with
> > negative offset. The reason is that the call to register_for_each_vma and
> > then build_map_info won't return error for negative offset. They just won't
> > do anything - no matching vma is found so there's no registered breakpoint
> > for the uprobe.
> >
> > I don't think we can change the behaviour of __uprobe_register and fail
> > for negative uprobe offset, because apps might depend on that already.
> >
> > But I think we can still make the change and check for it on bpf multi
> > link syscall level.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/trace/bpf_trace.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 774cf476a892..0dbf8d9b3ace 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -3397,6 +3397,11 @@ int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
> >                         goto error_free;
> >                 }
> >
> > +               if (uprobes[i].offset < 0) {
> 
> offset in UAPI is defined as unsigned, so how can it be negative?

right, but then it's passed to uprobe_register_refctr as loff_t which is 'long long'

jirka

> 
> > +                       err = -EINVAL;
> > +                       goto error_free;
> > +               }
> > +
> >                 uprobes[i].link = link;
> >
> >                 if (flags & BPF_F_UPROBE_MULTI_RETURN)
> > --
> > 2.43.0
> >