linux-kernel - Re: [PATCH 1/1] libbpf: ensure F_DUPFD

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Message-ID: <CADvTj4r4UKTV1RSs-1v=QZT8hehLzqHhZ3zmwugkcwYQQxrfuA@mail.gmail.com>
Date:   Sat, 5 Mar 2022 02:57:03 -0700
From:   James Hilliard <james.hilliard1@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] libbpf: ensure F_DUPFD_CLOEXEC is defined

On Fri, Mar 4, 2022 at 12:01 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Mon, Feb 28, 2022 at 7:00 AM Daniel Borkmann <daniel@...earbox.net> wrote:
> >
> > Hi James,
> >
> > On 2/27/22 3:25 PM, James Hilliard wrote:
> > > This definition seems to be missing from some older toolchains.
> > >
> > > Note that the fcntl.h in libbpf_internal.h is not a kernel header
> > > but rather a toolchain libc header.
> > >
> > > Fixes:
> > > libbpf_internal.h:521:18: error: 'F_DUPFD_CLOEXEC' undeclared (first use in this function); did you mean 'FD_CLOEXEC'?
> > >     fd = fcntl(fd, F_DUPFD_CLOEXEC, 3);
> > >                    ^~~~~~~~~~~~~~~
> > >                    FD_CLOEXEC
> > >
> > > Signed-off-by: James Hilliard <james.hilliard1@...il.com>
> >
> > Do you have some more info on your env (e.g. libc)? Looks like F_DUPFD_CLOEXEC
> > was added back in 2.6.24 kernel. When did libc add it?
>
> It seems like it's guarded by __USE_XOPEN2K8 in glibc (from a quick
> glance at glibc code). But it's been there since 2010 or so, at the
> very least.

The toolchain that hit this issue appears to be uclibc based which seems to have
had some bugs with the F_DUPFD_CLOEXEC definition.

>
> >
> > Should we instead just add an include for <linux/fcntl.h> to libbpf_internal.h
> > (given it defines F_DUPFD_CLOEXEC as well)?
>
> yep, this is UAPI header so we can use it easily (we'll need to sync
> it into Github repo, but that's not a problem)
>
>
> >
> > > ---
> > >   tools/lib/bpf/libbpf_internal.h | 4 ++++
> > >   1 file changed, 4 insertions(+)
> > >
> > > diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> > > index 4fda8bdf0a0d..d2a86b5a457a 100644
> > > --- a/tools/lib/bpf/libbpf_internal.h
> > > +++ b/tools/lib/bpf/libbpf_internal.h
> > > @@ -31,6 +31,10 @@
> > >   #define EM_BPF 247
> > >   #endif
> > >
> > > +#ifndef F_DUPFD_CLOEXEC
> > > +#define F_DUPFD_CLOEXEC 1030
> > > +#endif
> > > +
> > >   #ifndef R_BPF_64_64
> > >   #define R_BPF_64_64 1
> > >   #endif
> > >
> >
> > Thanks,
> > Daniel