Re: [PATCH v2 bpf-next 01/11] bpf: Support ->fill_link_info for kprobe_multi

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Jun 9, 2023 at 7:05 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Jun 8, 2023 at 3:35 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > With the addition of support for fill_link_info to the kprobe_multi link,
> > users will gain the ability to inspect it conveniently using the
> > `bpftool link show` command. This enhancement provides valuable information
> > to the user, including the count of probed functions and their respective
> > addresses. It's important to note that if the kptr_restrict setting is set
> > to 2, the probed addresses will not be exposed, ensuring security.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> >  include/uapi/linux/bpf.h       |  5 +++++
> >  kernel/trace/bpf_trace.c       | 30 ++++++++++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h |  5 +++++
> >  3 files changed, 40 insertions(+)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index a7b5e91..d99cc16 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -6438,6 +6438,11 @@ struct bpf_link_info {
> >                         __s32 priority;
> >                         __u32 flags;
> >                 } netfilter;
> > +               struct {
> > +                       __aligned_u64 addrs; /* in/out: addresses buffer ptr */
> > +                       __u32 count;
> > +                       __u8  retprobe;
>
> from kernel API side it's probably better to just expose flags?

Agreed. The flags will be extensible.

> retprobe is determined by BPF_F_KPROBE_MULTI_RETURN flag

Should we print 'flags' in `bpftool link show` directly? As we print
it in `bpftool map show`.

>
> > +               } kprobe_multi;
> >         };
> >  } __attribute__((aligned(8)));
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 2bc41e6..738efcf 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2548,9 +2548,39 @@ static void bpf_kprobe_multi_link_dealloc(struct bpf_link *link)
> >         kfree(kmulti_link);
> >  }
> >
> > +static int bpf_kprobe_multi_link_fill_link_info(const struct bpf_link *link,
> > +                                               struct bpf_link_info *info)
> > +{
> > +       u64 __user *uaddrs = u64_to_user_ptr(info->kprobe_multi.addrs);
> > +       struct bpf_kprobe_multi_link *kmulti_link;
> > +       u32 ucount = info->kprobe_multi.count;
> > +
> > +       if (!uaddrs ^ !ucount)
> > +               return -EINVAL;
> > +
> > +       kmulti_link = container_of(link, struct bpf_kprobe_multi_link, link);
> > +       if (!uaddrs) {
> > +               info->kprobe_multi.count = kmulti_link->cnt;
> > +               return 0;
> > +       }
> > +
> > +       if (!ucount)
> > +               return 0;
> > +       if (ucount != kmulti_link->cnt)
> > +               return -EINVAL;
>
> should this just check that kmulti_link->cnt is <= ucount?...

Agreed.

>
> > +       info->kprobe_multi.retprobe = kmulti_link->fp.exit_handler ?
> > +                                     true : false;
> > +       if (kptr_restrict == 2)
> > +               return 0;
>
> use kallsyms_show_value() instead of hard-coding this?

Good point. Will use it.

-- 
Regards
Yafang