netdev - Re: [PATCH bpf] tools/bpftool: fix PID fetching with a lot of results

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Date:   Mon, 7 Dec 2020 18:55:54 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf] tools/bpftool: fix PID fetching with a lot of results

On Sat, Dec 5, 2020 at 11:11 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 12/4/20 3:20 PM, Andrii Nakryiko wrote:
> > In case of having so many PID results that they don't fit into a singe page
> > (4096) bytes, bpftool will erroneously conclude that it got corrupted data due
> > to 4096 not being a multiple of struct pid_iter_entry, so the last entry will
> > be partially truncated. Fix this by sizing the buffer to fit exactly N entries
> > with no truncation in the middle of record.
> >
> > Fixes: d53dee3fe013 ("tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs")
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
>
> Ack with one nit below.
>
> Acked-by: Yonghong Song <yhs@...com>
>
> > ---
> >   tools/bpf/bpftool/pids.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/bpf/bpftool/pids.c b/tools/bpf/bpftool/pids.c
> > index df7d8ec76036..477e55d59c34 100644
> > --- a/tools/bpf/bpftool/pids.c
> > +++ b/tools/bpf/bpftool/pids.c
> > @@ -89,9 +89,9 @@ libbpf_print_none(__maybe_unused enum libbpf_print_level level,
> >
> >   int build_obj_refs_table(struct obj_refs_table *table, enum bpf_obj_type type)
> >   {
> > -     char buf[4096];
> > -     struct pid_iter_bpf *skel;
> >       struct pid_iter_entry *e;
> > +     char buf[4096 / sizeof(*e) * sizeof(*e)];
> > +     struct pid_iter_bpf *skel;
>
> No need to move "struct pid_iter_bpf *skel", right?

It's actually a move of `struct pid_iter_entry *e;` in from of char
buf[], to be able to use sizeof(*e) instead of sizeof(struct
pid_iter_bpf). It's just that diff tool didn't catch this properly :)

>
> >       int err, ret, fd = -1, i;
> >       libbpf_print_fn_t default_print;
> >
> >