Re: [PATCH bpf] libbpf: add null pointer check in bpf_object__init_user_btf_maps()

[Michal Rostecki <mrostecki@opensuse.org>]

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@fb.com>
> Reported-by: Michal Rostecki <mrostecki@opensuse.org>
> Signed-off-by: Quentin Monnet <quentin@isovalent.com>
> ---
>  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");
> +		return -EINVAL;
> +	}
>  	nr_types = btf__get_nr_types(obj->btf);
>  	for (i = 1; i <= nr_types; i++) {
>  		t = btf__type_by_id(obj->btf, i);
> 

Tested-by: Michal Rostecki <mrostecki@opensuse.org>

Thanks!