[bpf,v3] libbpf: Return btf_fd for load_sk_storage_btf

Before this change, function load_sk_storage_btf expected that
libbpf__probe_raw_btf was returning a BTF descriptor, but in fact 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 of the close
function, which was resulting in closing stdout and thus terminating the
process which called that function.

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.

This change remnames the libbpf__probe_raw_btf function to
libbpf__load_raw_btf, which now returns a BTF descriptor, as expected in
load_sk_storage_btf.

v2:
- Fix typo in the commit message.

v3:
- Simplify BTF descriptor handling in bpf_object__probe_btf_* functions.
- Rename libbpf__probe_raw_btf function to libbpf__load_raw_btf and
return a BTF descriptor.

Fixes: d7c4b3980c18 ("libbpf: detect supported kernel BTF features and sanitize BTF")
Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>
---
 tools/lib/bpf/libbpf.c          | 28 ++++++++++++++++------------
 tools/lib/bpf/libbpf_internal.h |  4 ++--
 tools/lib/bpf/libbpf_probes.c   | 13 ++++---------
 3 files changed, 22 insertions(+), 23 deletions(-)

On Wed, May 29, 2019 at 9:35 AM Michal Rostecki <mrostecki@opensuse.org> wrote:
>
> Before this change, function load_sk_storage_btf expected that
> libbpf__probe_raw_btf was returning a BTF descriptor, but in fact 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 of the close
> function, which was resulting in closing stdout and thus terminating the
> process which called that function.
>
> 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.
>
> This change remnames the libbpf__probe_raw_btf function to

typo: remnames -> renames

> libbpf__load_raw_btf, which now returns a BTF descriptor, as expected in
> load_sk_storage_btf.
>
> v2:
> - Fix typo in the commit message.
>
> v3:
> - Simplify BTF descriptor handling in bpf_object__probe_btf_* functions.
> - Rename libbpf__probe_raw_btf function to libbpf__load_raw_btf and
> return a BTF descriptor.
>
> Fixes: d7c4b3980c18 ("libbpf: detect supported kernel BTF features and sanitize BTF")
> Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>
> ---

Thanks!

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  tools/lib/bpf/libbpf.c          | 28 ++++++++++++++++------------
>  tools/lib/bpf/libbpf_internal.h |  4 ++--
>  tools/lib/bpf/libbpf_probes.c   | 13 ++++---------
>  3 files changed, 22 insertions(+), 23 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 197b574406b3..5d046cc7b207 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1645,14 +1645,16 @@ 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;
>
> -       res = libbpf__probe_raw_btf((char *)types, sizeof(types),
> -                                   strs, sizeof(strs));
> -       if (res < 0)
> -               return res;
> -       if (res > 0)
> +       btf_fd = libbpf__load_raw_btf((char *)types, sizeof(types),
> +                                     strs, sizeof(strs));
> +       if (btf_fd >= 0) {
>                 obj->caps.btf_func = 1;
> +               close(btf_fd);
> +               return 1;
> +       }
> +
>         return 0;
>  }
>
> @@ -1670,14 +1672,16 @@ 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;
>
> -       res = libbpf__probe_raw_btf((char *)types, sizeof(types),
> -                                   strs, sizeof(strs));
> -       if (res < 0)
> -               return res;
> -       if (res > 0)
> +       btf_fd = libbpf__load_raw_btf((char *)types, sizeof(types),
> +                                     strs, sizeof(strs));
> +       if (btf_fd >= 0) {
>                 obj->caps.btf_datasec = 1;
> +               close(btf_fd);
> +               return 1;
> +       }
> +
>         return 0;
>  }
>
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index f3025b4d90e1..dfab8012185c 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -34,7 +34,7 @@ do {                          \
>  #define pr_info(fmt, ...)      __pr(LIBBPF_INFO, fmt, ##__VA_ARGS__)
>  #define pr_debug(fmt, ...)     __pr(LIBBPF_DEBUG, fmt, ##__VA_ARGS__)
>
> -int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
> -                         const char *str_sec, size_t str_len);
> +int libbpf__load_raw_btf(const char *raw_types, size_t types_len,
> +                        const char *str_sec, size_t str_len);
>
>  #endif /* __LIBBPF_LIBBPF_INTERNAL_H */
> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
> index 5e2aa83f637a..6635a31a7a16 100644
> --- a/tools/lib/bpf/libbpf_probes.c
> +++ b/tools/lib/bpf/libbpf_probes.c
> @@ -133,8 +133,8 @@ bool bpf_probe_prog_type(enum bpf_prog_type prog_type, __u32 ifindex)
>         return errno != EINVAL && errno != EOPNOTSUPP;
>  }
>
> -int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
> -                         const char *str_sec, size_t str_len)
> +int libbpf__load_raw_btf(const char *raw_types, size_t types_len,
> +                        const char *str_sec, size_t str_len)
>  {
>         struct btf_header hdr = {
>                 .magic = BTF_MAGIC,
> @@ -157,14 +157,9 @@ int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
>         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)
> @@ -190,7 +185,7 @@ static int load_sk_storage_btf(void)
>                 BTF_MEMBER_ENC(23, 2, 32),/* struct bpf_spin_lock l; */
>         };
>
> -       return libbpf__probe_raw_btf((char *)types, sizeof(types),
> +       return libbpf__load_raw_btf((char *)types, sizeof(types),
>                                      strs, sizeof(strs));
>  }
>
> --
> 2.21.0
>