netdev - Re: [PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is never 0

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Date:   Wed, 6 Oct 2021 12:38:58 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is
 never 0

On Wed, Oct 6, 2021 at 12:09 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Oct 6, 2021 at 9:43 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Tue, Oct 5, 2021 at 10:24 PM Kumar Kartikeya Dwivedi
> > <memxor@...il.com> wrote:
> > >
> > > On Wed, Oct 06, 2021 at 10:11:29AM IST, Andrii Nakryiko wrote:
> > > > On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
> > > > >
> > > > > Since the code assumes in various places that BTF fd for modules is
> > > > > never 0, if we end up getting fd as 0, obtain a new fd > 0. Even though
> > > > > fd 0 being free for allocation is usually an application error, it is
> > > > > still possible that we end up getting fd 0 if the application explicitly
> > > > > closes its stdin. Deal with this by getting a new fd using dup and
> > > > > closing fd 0.
> > > > >
> > > > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > > > > ---
> > > > >  tools/lib/bpf/libbpf.c | 14 ++++++++++++++
> > > > >  1 file changed, 14 insertions(+)
> > > > >
> > > > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > > > > index d286dec73b5f..3e5e460fe63e 100644
> > > > > --- a/tools/lib/bpf/libbpf.c
> > > > > +++ b/tools/lib/bpf/libbpf.c
> > > > > @@ -4975,6 +4975,20 @@ static int load_module_btfs(struct bpf_object *obj)
> > > > >                         pr_warn("failed to get BTF object #%d FD: %d\n", id, err);
> > > > >                         return err;
> > > > >                 }
> > > > > +               /* Make sure module BTF fd is never 0, as kernel depends on it
> > > > > +                * being > 0 to distinguish between vmlinux and module BTFs,
> > > > > +                * e.g. for BPF_PSEUDO_BTF_ID ld_imm64 insns (ksyms).
> > > > > +                */
> > > > > +               if (!fd) {
> > > > > +                       fd = dup(0);
> > > >
> > > > This is not the only place where we make assumptions that fd > 0 but
> > > > technically can get fd == 0. Instead of doing such a check in every
> > > > such place, would it be possible to open (cheaply) some FD (/dev/null
> > > > or whatever, don't know what's the best file to open), if we detect
> > > > that FD == 0 is not allocated? Can we detect that fd 0 is not
> > > > allocated?
> > > >
> > >
> > > We can, e.g. using access("/proc/self/fd/0", F_OK), but I think just calling
> > > open unconditonally and doing if (ret > 0) close(ret) is better. Also, do I
> >
> > yeah, I like this idea, let's go with it
>
> FYI some production environments may detect that FDs 0,1,2 are not
> pointing to stdin, stdout, stderr and will force close whatever files are there
> and open 0,1,2 with canonical files.
>
> libbpf doesn't have to resort to such measures, but it would be prudent to
> make libbpf operate on FDs > 2 for all bpf objects to make sure other
> frameworks don't ruin libbpf's view of FDs.

oh well, even without those production complications this would be a
bit fragile, e.g., if the application temporarily opened FD 0 and then
closed it.

Ok, Kumar, can you please do it as a simple helper that would
dup()'ing until we have FD>2, and use it in as few places as possible
to make sure that all FDs (not just module BTF) are covered. I'd
suggest doing that only in low-level helpers in btf.c, I think
libbpf's logic always goes through those anyways (but please
double-check that we don't call bpf syscall directly anywhere else).