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

[Alexei Starovoitov]

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.