netdev - Re: [PATCH bpf] libbpf: add null pointer check in bpf_object__init_user_btf

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Date:   Thu, 12 Mar 2020 16:37:59 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Andrii Nakryiko <andriin@...com>,
        Michal Rostecki <mrostecki@...nsuse.org>
Subject: Re: [PATCH bpf] libbpf: add null pointer check in
 bpf_object__init_user_btf_maps()

On 3/12/20 3:03 PM, Quentin Monnet wrote:
> When compiling bpftool with clang 7, after the addition of its recent
> "bpftool prog profile" feature, Michal reported a segfault. This
> occurred while the build process was attempting to generate the
> skeleton needed for the profiling program, with the following command:
> 
>      ./_bpftool gen skeleton skeleton/profiler.bpf.o > profiler.skel.h
> 
> Tracing the error showed that bpf_object__init_user_btf_maps() does no
> verification on obj->btf before passing it to btf__get_nr_types(), where
> btf is dereferenced. Libbpf considers BTF information should be here
> because of the presence of a ".maps" section in the object file (hence
> the check on "obj->efile.btf_maps_shndx < 0" fails and we do not exit
> from the function early), but it was unable to load BTF info as there is
> no .BTF section.
> 
> Add a null pointer check and error out if the pointer is null. The final
> bpftool executable still fails to build, but at least we have a proper
> error and no more segfault.
> 
> Fixes: abd29c931459 ("libbpf: allow specifying map definitions using BTF")
> Cc: Andrii Nakryiko <andriin@...com>
> Reported-by: Michal Rostecki <mrostecki@...nsuse.org>
> Signed-off-by: Quentin Monnet <quentin@...valent.com>

Applied to bpf-next, thanks! Note ...

> ---
>   tools/lib/bpf/libbpf.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 223be01dc466..19c0c40e8a80 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2140,6 +2140,10 @@ static int bpf_object__init_user_btf_maps(struct bpf_object *obj, bool strict,
>   		return -EINVAL;
>   	}
>   
> +	if (!obj->btf) {
> +		pr_warn("failed to retrieve BTF for map");

I've added a '\n' here, otherwise it looks like:

[...]
   LINK     _bpftool
   CLANG    skeleton/profiler.bpf.o
   GEN      profiler.skel.h
libbpf: failed to retrieve BTF for mapError: failed to open BPF object file: 0
Makefile:129: recipe for target 'profiler.skel.h' failed

Fixed version:

   LINK     _bpftool
   GEN      profiler.skel.h
libbpf: failed to retrieve BTF for map
Error: failed to open BPF object file: 0
Makefile:129: recipe for target 'profiler.skel.h' failed

Thanks,
Daniel