bpf: Fix calling global functions from BPF_PROG_TYPE_EXT programs

The verifier allows programs to call global functions as long as their
argument types match, using BTF to check the function arguments. One of the
allowed argument types to such global functions is PTR_TO_CTX; however the
check for this fails on BPF_PROG_TYPE_EXT functions because the verifier
uses the wrong type to fetch the vmlinux BTF ID for the program context
type. This failure is seen when an XDP program is loaded using
libxdp (which loads it as BPF_PROG_TYPE_EXT and attaches it to a global XDP
type program).

Fix the issue by passing in the target program type instead of the
BPF_PROG_TYPE_EXT type to bpf_prog_get_ctx() when checking function
argument compatibility.

The first Fixes tag refers to the latest commit that touched the code in
question, while the second one points to the code that first introduced
the global function call verification.

v2:
- Use resolve_prog_type()

Fixes: 3363bd0cfb ("bpf: Extend kfunc with PTR_TO_CTX, PTR_TO_MEM argument support")
Fixes: 51c39bb1d5 ("bpf: Introduce function-by-function verification")
Reported-by: Simon Sundberg <simon.sundberg@kau.se>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20220606075253.28422-1-toke@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -6054,6 +6054,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				    struct bpf_reg_state *regs,
 				    bool ptr_to_mem_ok)
 {
+	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
 	struct bpf_verifier_log *log = &env->log;
 	u32 i, nargs, ref_id, ref_obj_id = 0;
 	bool is_kfunc = btf_is_kernel(btf);
@@ -6171,7 +6172,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				return -EINVAL;
 			}
 			/* rest of the arguments can be anything, like normal kfunc */
-		} else if (btf_get_prog_ctx_type(log, btf, t, env->prog->type, i)) {
+		} else if (btf_get_prog_ctx_type(log, btf, t, prog_type, i)) {
 			/* If function expects ctx type in BTF check that caller
 			 * is passing PTR_TO_CTX.
 			 */