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

[Alexei Starovoitov]

On Thu, Dec 29, 2022 at 3:21 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Dec 29, 2022 at 1:49 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Thu, Dec 29, 2022 at 1:44 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > On Fri, Dec 23, 2022 at 5:36 AM Xin Liu <liuxin350@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >  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.
>
> that would be simplest, probably, thanks!

Pushed trivial fix to bpf-next.