From: "Toke Høiland-Jørgensen" <toke@redhat•com>
To: Andrii Nakryiko <andrii.nakryiko@gmail•com>,
Alexei Starovoitov <alexei.starovoitov@gmail•com>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail•com>,
bpf <bpf@vger•kernel.org>, Alexei Starovoitov <ast@kernel•org>,
Daniel Borkmann <daniel@iogearbox•net>,
Andrii Nakryiko <andrii@kernel•org>,
Martin KaFai Lau <kafai@fb•com>, Song Liu <songliubraving@fb•com>,
Yonghong Song <yhs@fb•com>,
Jesper Dangaard Brouer <brouer@redhat•com>,
Networking <netdev@vger•kernel.org>
Subject: Re: [PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is never 0
Date: Thu, 07 Oct 2021 12:24:34 +0200 [thread overview]
Message-ID: <87zgrlm8t9.fsf@toke.dk> (raw)
In-Reply-To: <CAEf4Bza186k8BeRG8XrUGaUb4_6hf0dCB4a1A5czcS69aBMffw@mail.gmail.com>
Andrii Nakryiko <andrii.nakryiko@gmail•com> writes:
> On Wed, Oct 6, 2021 at 12:09 PM Alexei Starovoitov
> <alexei.starovoitov@gmail•com> wrote:
>>
>> On Wed, Oct 6, 2021 at 9:43 AM Andrii Nakryiko
>> <andrii.nakryiko@gmail•com> wrote:
>> >
>> > On Tue, Oct 5, 2021 at 10:24 PM Kumar Kartikeya Dwivedi
>> > <memxor@gmail•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@gmail•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@gmail•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).
FYI, you can use fcntl() with F_DUPFD{,_CLOEXEC} and tell it the minimum
fd number you're interested in for the clone. We do that in libxdp to
protect against fd 0:
https://github.com/xdp-project/xdp-tools/blob/master/lib/libxdp/libxdp.c#L1184
Given Alexei's comments above, maybe we should be '3' for the last arg
instead of 1...
-Toke
next prev parent reply other threads:[~2021-10-07 10:24 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-06 0:28 [PATCH bpf-next v1 0/6] Typeless/weak ksym for gen_loader + misc fixups Kumar Kartikeya Dwivedi
2021-10-06 0:28 ` [PATCH bpf-next v1 1/6] bpf: Add bpf_kallsyms_lookup_name helper Kumar Kartikeya Dwivedi
2021-10-07 20:23 ` Song Liu
2021-10-06 0:28 ` [PATCH bpf-next v1 2/6] libbpf: Add typeless and weak ksym support to gen_loader Kumar Kartikeya Dwivedi
2021-10-07 21:45 ` Song Liu
2021-10-07 22:01 ` Kumar Kartikeya Dwivedi
2021-10-07 22:17 ` Song Liu
2021-10-06 0:28 ` [PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is never 0 Kumar Kartikeya Dwivedi
2021-10-06 4:41 ` Andrii Nakryiko
2021-10-06 5:24 ` Kumar Kartikeya Dwivedi
2021-10-06 16:43 ` Andrii Nakryiko
2021-10-06 19:09 ` Alexei Starovoitov
2021-10-06 19:38 ` Andrii Nakryiko
2021-10-07 10:24 ` Toke Høiland-Jørgensen [this message]
2021-10-07 18:44 ` Kumar Kartikeya Dwivedi
2021-10-07 21:29 ` Andrii Nakryiko
2021-10-06 0:28 ` [PATCH bpf-next v1 4/6] bpf: selftests: Move test_ksyms_weak test to lskel, add libbpf test Kumar Kartikeya Dwivedi
2021-10-07 20:33 ` Song Liu
2021-10-07 20:46 ` Kumar Kartikeya Dwivedi
2021-10-07 20:55 ` Kumar Kartikeya Dwivedi
2021-10-07 20:57 ` Song Liu
2021-10-06 0:28 ` [PATCH bpf-next v1 5/6] bpf: selftests: Fix fd cleanup in sk_lookup test Kumar Kartikeya Dwivedi
2021-10-06 6:49 ` Jakub Sitnicki
2021-10-07 21:48 ` Song Liu
2021-10-06 0:28 ` [PATCH bpf-next v1 6/6] bpf: selftests: Fix memory leak in test_ima Kumar Kartikeya Dwivedi
2021-10-06 4:44 ` Andrii Nakryiko
2021-10-07 21:48 ` Song Liu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87zgrlm8t9.fsf@toke.dk \
--to=toke@redhat$(echo .)com \
--cc=alexei.starovoitov@gmail$(echo .)com \
--cc=andrii.nakryiko@gmail$(echo .)com \
--cc=andrii@kernel$(echo .)org \
--cc=ast@kernel$(echo .)org \
--cc=bpf@vger$(echo .)kernel.org \
--cc=brouer@redhat$(echo .)com \
--cc=daniel@iogearbox$(echo .)net \
--cc=kafai@fb$(echo .)com \
--cc=memxor@gmail$(echo .)com \
--cc=netdev@vger$(echo .)kernel.org \
--cc=songliubraving@fb$(echo .)com \
--cc=yhs@fb$(echo .)com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox