netdev - Re: [PATCH bpf-next v5 4/8] libbpf: Ensure that BPF syscall fds are never 0, 1, or 2

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Message-ID: <CAEf4Bza=PU3tKLs22_g-tn=-S12mZLLXEyPOzr1TRMC9mRDnZQ@mail.gmail.com>
Date:   Thu, 28 Oct 2021 11:00:18 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Song Liu <songliubraving@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...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 v5 4/8] libbpf: Ensure that BPF syscall fds are
 never 0, 1, or 2

On Wed, Oct 27, 2021 at 11:35 PM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> Add a simple wrapper for passing an fd and getting a new one >= 3 if it
> is one of 0, 1, or 2. There are two primary reasons to make this change:
> First, libbpf relies on the assumption a certain BPF fd is never 0 (e.g.
> most recently noticed in [0]). Second, Alexei pointed out in [1] that
> some environments reset stdin, stdout, and stderr if they notice an
> invalid fd at these numbers. To protect against both these cases, switch
> all internal BPF syscall wrappers in libbpf to always return an fd >= 3.
> We only need to modify the syscall wrappers and not other code that
> assumes a valid fd by doing >= 0, to avoid pointless churn, and because
> it is still a valid assumption. The cost paid is two additional syscalls
> if fd is in range [0, 2].
>
>   [0]: e31eec77e4ab ("bpf: selftests: Fix fd cleanup in get_branch_snapshot")
>   [1]: https://lore.kernel.org/bpf/CAADnVQKVKY8o_3aU8Gzke443+uHa-eGoM0h7W4srChMXU1S4Bg@mail.gmail.com
>
> Acked-by: Song Liu <songliubraving@...com>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---

LGTM, thanks.

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>  tools/lib/bpf/bpf.c             | 35 +++++++++++++++++++++------------
>  tools/lib/bpf/libbpf_internal.h | 24 ++++++++++++++++++++++
>  2 files changed, 46 insertions(+), 13 deletions(-)
>

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