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

[Quentin Monnet <quentin@isovalent.com>]

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);
-- 
2.20.1