Re: bpf_link_info: perf_event link info name_len field returning zero

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> The reason name_len is 0 is that the user did not set both the buffer
> and the length. IOW, this happens when the user buffer is NULL and the
> input length is 0.

Thank you both for the follow-up.

> However, we should make this behavior consistent by
> returning the actual size to the user if both the buffer and length
> are unset.
>
> I will submit a fix.

I actually made a small patch for this when I was initially exploring
this behavior. If it's alright, I can submit this after some clean-up.
:)


On Sat, Sep 28, 2024 at 10:36 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> On Sat, Sep 28, 2024 at 7:14 AM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Sun, Sep 22, 2024 at 12:59 PM Tyrone Wu <wudevelops@xxxxxxxxx> wrote:
> > >
> > > Hello,
> > >
> > > When retrieving bpf_link_info.perf_event kprobe/uprobe/tracepoint
> > > data, I noticed that the name_len field always returns 0. After some
> > > digging, I see that name_len is never actually populated, which
> > > explains the 0 value.
> > >
> > > I expected it to function similarly to
> > > bpf_link_info.raw_tracepoint.tp_name_len, where that field is filled
> > > with the length of tp_name. However, I noticed that the selftest
> > > explicitly asserts that name_len should be 0. I was wondering if
> > > someone could clarify whether it is intended for the
> > > bpf_link_info.perf_event name_len field to not be populated.
> >
> > This sounds like a bug. It should behave consistently with the other
> > users of input/output string buffer size fields: on input we get
> > maximum buffer size, on output we should put an actual size of the
> > string (especially if it was truncated).
> >
> > Yafang, Jiri, WDYT?
>
> The reason name_len is 0 is that the user did not set both the buffer
> and the length. IOW, this happens when the user buffer is NULL and the
> input length is 0. However, we should make this behavior consistent by
> returning the actual size to the user if both the buffer and length
> are unset.
>
> I will submit a fix.
>
>
> --
> Regards
> Yafang





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