linux-kernel - Re: [PATCH bpf-next] libbpf: fix errno is overwritten after being closed.

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Message-ID: <CAADnVQ+FbGD9yMpRYtCbzC-snOG1tGLfvEXQvkJYcMRjbYjTyw@mail.gmail.com>
Date:   Thu, 29 Dec 2022 13:49:03 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Xin Liu <liuxin350@...wei.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        yanan@...wei.com, wuchangye@...wei.com, xiesongyang@...wei.com,
        kongweibin2@...wei.com, zhangmingyi5@...wei.com
Subject: Re: [PATCH bpf-next] libbpf: fix errno is overwritten after being closed.

On Thu, Dec 29, 2022 at 1:44 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Fri, Dec 23, 2022 at 5:36 AM Xin Liu <liuxin350@...wei.com> wrote:
> >
> > In the ensure_good_fd function, if the fcntl function succeeds but
> > the close function fails, ensure_good_fd returns a normal fd and
> > sets errno, which may cause users to misunderstand. The close
> > failure is not a serious problem, and the correct FD has been
> > handed over to the upper-layer application. Let's restore errno here.
> >
> > Signed-off-by: Xin Liu <liuxin350@...wei.com>
> > ---
> >  tools/lib/bpf/libbpf_internal.h | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> > index 377642ff51fc..98333a6c38e9 100644
> > --- a/tools/lib/bpf/libbpf_internal.h
> > +++ b/tools/lib/bpf/libbpf_internal.h
> > @@ -543,10 +543,9 @@ static inline int ensure_good_fd(int fd)
> >                 fd = fcntl(fd, F_DUPFD_CLOEXEC, 3);
> >                 saved_errno = errno;
> >                 close(old_fd);
> > -               if (fd < 0) {
> > +               errno = saved_errno;
> > +               if (fd < 0)
> >                         pr_warn("failed to dup FD %d to FD > 2: %d\n", old_fd, -saved_errno);
> > -                       errno = saved_errno;
>
> pr_warn calls into user-provided callback, which can clobber errno, so
> `errno = saved_errno` should happen after pr_warn. With your change
> there is even higher chance of errno clobbering.
>
> Please send a follow up fix to unconditionally restore errno *after*
> pr_warn, thanks.

Good point. I can follow up with one line fix too.