linux-kernel - Re: [PATCH bpf-next] libbpf: handle ENOTSUPP errno in libbpf

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Date:   Fri, 30 Apr 2021 11:16:16 -0300
From:   Pedro Tammela <pctammela@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     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>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH bpf-next] libbpf: handle ENOTSUPP errno in libbpf_strerror()

Em ter., 27 de abr. de 2021 às 13:18, Daniel Borkmann
<daniel@...earbox.net> escreveu:
>
> On 4/25/21 12:16 AM, Pedro Tammela wrote:
> > The 'bpf()' syscall is leaking the ENOTSUPP errno that is internal to the kernel[1].
> > More recent code is already using the correct EOPNOTSUPP, but changing
> > older return codes is not possible due to dependency concerns, so handle ENOTSUPP
> > in libbpf_strerror().
> >
> > [1] https://lore.kernel.org/netdev/20200511165319.2251678-1-kuba@kernel.org/
> >
> > Signed-off-by: Pedro Tammela <pctammela@...atatu.com>
> > ---
> >   tools/lib/bpf/libbpf_errno.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/tools/lib/bpf/libbpf_errno.c b/tools/lib/bpf/libbpf_errno.c
> > index 0afb51f7a919..7de8bbc34a37 100644
> > --- a/tools/lib/bpf/libbpf_errno.c
> > +++ b/tools/lib/bpf/libbpf_errno.c
> > @@ -13,6 +13,9 @@
> >
> >   #include "libbpf.h"
> >
> > +/* This errno is internal to the kernel but leaks in the bpf() syscall. */
> > +#define ENOTSUPP 524
> > +
> >   /* make sure libbpf doesn't use kernel-only integer typedefs */
> >   #pragma GCC poison u8 u16 u32 u64 s8 s16 s32 s64
> >
> > @@ -43,6 +46,12 @@ int libbpf_strerror(int err, char *buf, size_t size)
> >
> >       err = err > 0 ? err : -err;
> >
> > +     if (err == ENOTSUPP) {
> > +             snprintf(buf, size, "Operation not supported");
> > +             buf[size - 1] = '\0';
> > +             return 0;
> > +     }
> > +
> >       if (err < __LIBBPF_ERRNO__START) {
> >               int ret;
>
> Could you fold this into the __LIBBPF_ERRNO__START test body to denote that it
> belongs outside the libbpf error range? For example, could be simplified like this:
>
>          if (err < __LIBBPF_ERRNO__START) {
>                  int ret;
>
>                  /* Handle ENOTSUPP separate here given it's kernel internal,
>                   * but for sake of error string it has the same meaning as
>                   * the EOPNOTSUPP error.
>                   */
>                  if (err == ENOTSUPP)
>                          err = EOPNOTSUPP;
>                  ret = strerror_r(err, buf, size);
>                  buf[size - 1] = '\0';
>                  return ret;
>          }
>
> Thanks,
> Daniel

Sure, looks simpler indeed.

Pedro