Re: [PATCH bpf] libbpf: Return btf_fd in libbpf__probe_raw_btf

[Michal Rostecki <mrostecki@suse.de>]

On Wed, May 29, 2019 at 08:35:25AM -0700, Andrii Nakryiko wrote:
> On Wed, May 29, 2019 at 1:30 AM Michal Rostecki <mrostecki@opensuse.org> wrote:
> >
> > Function load_sk_storage_btf expects that libbpf__probe_raw_btf is
> > returning a btf descriptor, but before this change it was returning
> > an information about whether the probe was successful (0 or 1).
> > load_sk_storage_btf was using that value as an argument to the close
> > function, which was resulting in closing stdout and thus terminating the
> > process which used that dunction.
> >
> > That bug was visible in bpftool. `bpftool feature` subcommand was always
> > exiting too early (because of closed stdout) and it didn't display all
> > requested probes. `bpftool -j feature` or `bpftool -p feature` were not
> > returning a valid json object.
> >
> 
> Thanks for the fix!
> 
> > Fixes: d7c4b3980c18 ("libbpf: detect supported kernel BTF features and sanitize BTF")
> > Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>
> > ---
> >  tools/lib/bpf/libbpf.c        | 36 +++++++++++++++++++++--------------
> >  tools/lib/bpf/libbpf_probes.c |  7 +------
> >  2 files changed, 23 insertions(+), 20 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 197b574406b3..bc2dca36bced 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -1645,15 +1645,19 @@ static int bpf_object__probe_btf_func(struct bpf_object *obj)
> >                 /* FUNC x */                                    /* [3] */
> >                 BTF_TYPE_ENC(5, BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0), 2),
> >         };
> > -       int res;
> > +       int btf_fd;
> > +       int ret;
> >
> > -       res = libbpf__probe_raw_btf((char *)types, sizeof(types),
> > -                                   strs, sizeof(strs));
> > -       if (res < 0)
> > -               return res;
> > -       if (res > 0)
> > +       btf_fd = libbpf__probe_raw_btf((char *)types, sizeof(types),
> > +                                      strs, sizeof(strs));
> > +       if (btf_fd < 0)
> > +               ret = 0;
> > +       else {
> > +               ret = 1;
> 
> This whole ret variable seems unnecessary. Also if btf_fd is invalid,
> we probably shouldn't close it. So just this should work:
> 
> btf_fd = libbpf__probe_raw_btf(...);
> if (btf_fd >= 0) {
>     obj->caps.btf_func = 1;
>     close(btf_fd);
> }
> return btf_fd >= 0;
> 

Makes sense, I will do it in v3.

> >                 obj->caps.btf_func = 1;
> > -       return 0;
> > +       }
> > +       close(btf_fd);
> > +       return ret;
> >  }
> >
> >  static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
> > @@ -1670,15 +1674,19 @@ static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
> >                 BTF_TYPE_ENC(3, BTF_INFO_ENC(BTF_KIND_DATASEC, 0, 1), 4),
> >                 BTF_VAR_SECINFO_ENC(2, 0, 4),
> >         };
> > -       int res;
> > +       int btf_fd;
> > +       int ret;
> >
> > -       res = libbpf__probe_raw_btf((char *)types, sizeof(types),
> > -                                   strs, sizeof(strs));
> > -       if (res < 0)
> > -               return res;
> > -       if (res > 0)
> > +       btf_fd = libbpf__probe_raw_btf((char *)types, sizeof(types),
> > +                                      strs, sizeof(strs));
> > +       if (btf_fd < 0)
> > +               ret = 0;
> > +       else {
> > +               ret = 1;
> >                 obj->caps.btf_datasec = 1;
> > -       return 0;
> > +       }
> > +       close(btf_fd);
> 
> Same as above.
> 
> > +       return ret;
> >  }
> >
> >  static int
> > diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
> > index 5e2aa83f637a..2c2828345514 100644
> > --- a/tools/lib/bpf/libbpf_probes.c
> > +++ b/tools/lib/bpf/libbpf_probes.c
> > @@ -157,14 +157,9 @@ int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
> 
> I'm wondering if it's better to rename this function to something like
> libbpf__load_raw_btf? probe (at least to me) implies true/false
> result, so feels like it might be easily misused.
> 

Good idea.

> >         memcpy(raw_btf + hdr.hdr_len + hdr.type_len, str_sec, hdr.str_len);
> >
> >         btf_fd = bpf_load_btf(raw_btf, btf_len, NULL, 0, false);
> > -       if (btf_fd < 0) {
> > -               free(raw_btf);
> > -               return 0;
> > -       }
> >
> > -       close(btf_fd);
> >         free(raw_btf);
> > -       return 1;
> > +       return btf_fd;
> >  }
> >
> >  static int load_sk_storage_btf(void)
> > --
> > 2.21.0
> >