netdev - Re: [PATCH bpf-next v5 10/12] libbpf: Fix skel_internal.h to set errno on loader retval

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Message-ID: <CAADnVQKBBswOOsdpNsaxtTprSkcAvL3ggVV8g0A-mUjs4_ucaQ@mail.gmail.com>
Date:   Wed, 29 Sep 2021 20:45:17 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 10/12] libbpf: Fix skel_internal.h to set
 errno on loader retval < 0

On Mon, Sep 27, 2021 at 8:00 AM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> When the loader indicates an internal error (result of a checked bpf
> system call), it returns the result in attr.test.retval. However, tests
> that rely on ASSERT_OK_PTR on NULL (returned from light skeleton) may
> miss that NULL denotes an error if errno is set to 0. This would result
> in skel pointer being NULL, while ASSERT_OK_PTR returning 1, leading to
> a SEGV on dereference of skel, because libbpf_get_error relies on the
> assumption that errno is always set in case of error for ptr == NULL.
>
> In particular, this was observed for the ksyms_module test. When
> executed using `./test_progs -t ksyms`, prior tests manipulated errno
> and the test didn't crash when it failed at ksyms_module load, while
> using `./test_progs -t ksyms_module` crashed due to errno being
> untouched.
>
> Fixes: 67234743736a (libbpf: Generate loader program out of BPF ELF file.)
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>  tools/lib/bpf/skel_internal.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/skel_internal.h b/tools/lib/bpf/skel_internal.h
> index b22b50c1b173..9cf66702fa8d 100644
> --- a/tools/lib/bpf/skel_internal.h
> +++ b/tools/lib/bpf/skel_internal.h
> @@ -105,10 +105,12 @@ static inline int bpf_load_and_run(struct bpf_load_and_run_opts *opts)
>         err = skel_sys_bpf(BPF_PROG_RUN, &attr, sizeof(attr));
>         if (err < 0 || (int)attr.test.retval < 0) {
>                 opts->errstr = "failed to execute loader prog";
> -               if (err < 0)
> +               if (err < 0) {
>                         err = -errno;
> -               else
> +               } else {
>                         err = (int)attr.test.retval;
> +                       errno = -err;
> +               }

Applied this fix as well, since I hit this bug too :)
Thanks!