netdev - Re: [PATCH bpf] libbpf: don't crash on object files with no symbol tables

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Date:   Tue, 31 Aug 2021 15:45:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] libbpf: don't crash on object files with no symbol tables

On Tue, Aug 31, 2021 at 9:58 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> If libbpf encounters an ELF file that has been stripped of its symbol
> table, it will crash in bpf_object__add_programs() when trying to
> dereference the obj->efile.symbols pointer. Add a check and return to avoid
> this.
>
> Fixes: 6245947c1b3c ("libbpf: Allow gaps in BPF program sections to support overriden weak functions")
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  tools/lib/bpf/libbpf.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 6f5e2757bb3c..4cd102affeef 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -668,6 +668,9 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
>         const char *name;
>         GElf_Sym sym;
>
> +       if (!symbols)
> +               return -ENOENT;
> +

The more logical place to do this check is in
bpf_object__elf_collect(). Can you add this there? We can also include
helpful error message.

But I'm also curious which Clang version is being used to cause no ELF
symbols being generated?

>         progs = obj->programs;
>         nr_progs = obj->nr_programs;
>         nr_syms = symbols->d_size / sizeof(GElf_Sym);
> --
> 2.33.0
>