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

[Andrii Nakryiko]

On Wed, May 29, 2019 at 1:30 AM Michal Rostecki <mrostecki@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
>  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;

>                 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.

>         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
>