libbpf: Prevent null-pointer dereference when prog to load has no BTF

In bpf_objec_load_prog(), there's no guarantee that obj->btf is non-NULL
when passing it to btf__fd(), and this function does not perform any
check before dereferencing its argument (as bpf_object__btf_fd() used to
do). As a consequence, we get segmentation fault errors in bpftool (for
example) when trying to load programs that come without BTF information.

v2: Keep btf__fd() in the fix instead of reverting to bpf_object__btf_fd().

Fixes: df7c3f7d3a3d ("libbpf: make uniform use of btf__fd() accessor inside libbpf")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Quentin Monnet <qmo@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240314150438.232462-1-qmo@kernel.org

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index efab29b8935bd9f7027859efee6a1383046105f4..124883eaa0cfdb6bf0665c7c1e4f73cc895bfc51 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -7317,9 +7317,9 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
        char *cp, errmsg[STRERR_BUFSIZE];
        size_t log_buf_size = 0;
        char *log_buf = NULL, *tmp;
-       int btf_fd, ret, err;
        bool own_log_buf = true;
        __u32 log_level = prog->log_level;
+       int ret, err;
 
        if (prog->type == BPF_PROG_TYPE_UNSPEC) {
                /*
@@ -7343,9 +7343,8 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
        load_attr.prog_ifindex = prog->prog_ifindex;
 
        /* specify func_info/line_info only if kernel supports them */
-       btf_fd = btf__fd(obj->btf);
-       if (btf_fd >= 0 && kernel_supports(obj, FEAT_BTF_FUNC)) {
-               load_attr.prog_btf_fd = btf_fd;
+       if (obj->btf && btf__fd(obj->btf) >= 0 && kernel_supports(obj, FEAT_BTF_FUNC)) {
+               load_attr.prog_btf_fd = btf__fd(obj->btf);
                load_attr.func_info = prog->func_info;
                load_attr.func_info_rec_size = prog->func_info_rec_size;
                load_attr.func_info_cnt = prog->func_info_cnt;