libbpf: Fix segfault in static linker for objects without BTF

When a BPF object is compiled without BTF info (without -g),
trying to link such objects using bpftool causes a SIGSEGV due to
btf__get_nr_types accessing obj->btf which is NULL. Fix this by
checking for the NULL pointer, and return error.

Reproducer:
$ cat a.bpf.c
extern int foo(void);
int bar(void) { return foo(); }
$ cat b.bpf.c
int foo(void) { return 0; }
$ clang -O2 -target bpf -c a.bpf.c
$ clang -O2 -target bpf -c b.bpf.c
$ bpftool gen obj out a.bpf.o b.bpf.o
Segmentation fault (core dumped)

After fix:
$ bpftool gen obj out a.bpf.o b.bpf.o
libbpf: failed to find BTF info for object 'a.bpf.o'
Error: failed to link 'a.bpf.o': Unknown error -22 (-22)

Fixes: f609280ec255 (libbpf: Add linker extern resolution support for functions and global variables)
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210924023725.70228-1-memxor@gmail.com

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 10911a8cad0f2b7e64abd5ed45f9954971004d99..2df880cefdaeea1ff93b00134a7fcc6971939656 100644 (file)
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -1649,11 +1649,17 @@ static bool btf_is_non_static(const struct btf_type *t)
 static int find_glob_sym_btf(struct src_obj *obj, Elf64_Sym *sym, const char *sym_name,
                             int *out_btf_sec_id, int *out_btf_id)
 {
-       int i, j, n = btf__get_nr_types(obj->btf), m, btf_id = 0;
+       int i, j, n, m, btf_id = 0;
        const struct btf_type *t;
        const struct btf_var_secinfo *vi;
        const char *name;
 
+       if (!obj->btf) {
+               pr_warn("failed to find BTF info for object '%s'\n", obj->filename);
+               return -EINVAL;
+       }
+
+       n = btf__get_nr_types(obj->btf);
        for (i = 1; i <= n; i++) {
                t = btf__type_by_id(obj->btf, i);