netdev - Re: [PATCH bpf] libbpf: Prevent overriding errno when logging errors

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Message-ID: <CAEf4BzZ6yM_QWu0x4b51NAVzN6-EAoQN4ff4BNiof5CJ5ukhpg@mail.gmail.com>
Date:   Thu, 13 Aug 2020 09:41:22 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] libbpf: Prevent overriding errno when logging errors

On Thu, Aug 13, 2020 at 7:29 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Turns out there were a few more instances where libbpf didn't save the
> errno before writing an error message, causing errno to be overridden by
> the printf() return and the error disappearing if logging is enabled.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---

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

>  tools/lib/bpf/libbpf.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 0a06124f7999..fd256440e233 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3478,10 +3478,11 @@ bpf_object__probe_global_data(struct bpf_object *obj)
>
>         map = bpf_create_map_xattr(&map_attr);
>         if (map < 0) {
> -               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
> +               ret = -errno;
> +               cp = libbpf_strerror_r(-ret, errmsg, sizeof(errmsg));

fyi, libbpf_strerror_r() is smart enough to work with both negative
and positive error numbers (it basically takes abs(err)), so no need
to ensure it's positive here and below.

>                 pr_warn("Error in %s():%s(%d). Couldn't create simple array map.\n",
> -                       __func__, cp, errno);
> -               return -errno;
> +                       __func__, cp, -ret);
> +               return ret;
>         }
>
>         insns[0].imm = map;
> @@ -6012,9 +6013,10 @@ int bpf_program__pin_instance(struct bpf_program *prog, const char *path,
>         }
>
>         if (bpf_obj_pin(prog->instances.fds[instance], path)) {
> -               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
> +               err = -errno;
> +               cp = libbpf_strerror_r(-err, errmsg, sizeof(errmsg));
>                 pr_warn("failed to pin program: %s\n", cp);
> -               return -errno;
> +               return err;
>         }
>         pr_debug("pinned program '%s'\n", path);
>
> --
> 2.28.0
>