bpf: Silence Coverity warning for find_kfunc_desc_btf

The helper function returns a pointer that in the failure case encodes
an error in the struct btf pointer. The current code lead to Coverity
warning about the use of the invalid pointer:

 *** CID 1507963:  Memory - illegal accesses  (USE_AFTER_FREE)
 /kernel/bpf/verifier.c: 1788 in find_kfunc_desc_btf()
 1782                          return ERR_PTR(-EINVAL);
 1783                  }
 1784
 1785                  kfunc_btf = __find_kfunc_desc_btf(env, offset, btf_modp);
 1786                  if (IS_ERR_OR_NULL(kfunc_btf)) {
 1787                          verbose(env, "cannot find module BTF for func_id %u\n", func_id);
 >>>      CID 1507963:  Memory - illegal accesses  (USE_AFTER_FREE)
 >>>      Using freed pointer "kfunc_btf".
 1788                          return kfunc_btf ?: ERR_PTR(-ENOENT);
 1789                  }
 1790                  return kfunc_btf;
 1791          }
 1792          return btf_vmlinux ?: ERR_PTR(-ENOENT);
 1793     }

Daniel suggested the use of ERR_CAST so that the intended use is clear
to Coverity, but on closer look it seems that we never return NULL from
the helper. Andrii noted that since __find_kfunc_desc_btf already logs
errors for all cases except btf_get_by_fd, it is much easier to add
logging for that and remove the IS_ERR check altogether, returning
directly from it.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211009040900.803436-1-memxor@gmail.com

kernel/bpf/verifier.c

@@ -1727,8 +1727,10 @@ static struct btf *__find_kfunc_desc_btf(struct bpf_verifier_env *env,
 			return ERR_PTR(-EFAULT);
 
 		btf = btf_get_by_fd(btf_fd);
-		if (IS_ERR(btf))
+		if (IS_ERR(btf)) {
+			verbose(env, "invalid module BTF fd specified\n");
 			return btf;
+		}
 
 		if (!btf_is_module(btf)) {
 			verbose(env, "BTF fd for kfunc is not a module BTF\n");
@@ -1771,8 +1773,6 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env,
 				       u32 func_id, s16 offset,
 				       struct module **btf_modp)
 {
-	struct btf *kfunc_btf;
-
 	if (offset) {
 		if (offset < 0) {
 			/* In the future, this can be allowed to increase limit
@@ -1782,12 +1782,7 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env,
 			return ERR_PTR(-EINVAL);
 		}
 
-		kfunc_btf = __find_kfunc_desc_btf(env, offset, btf_modp);
-		if (IS_ERR_OR_NULL(kfunc_btf)) {
-			verbose(env, "cannot find module BTF for func_id %u\n", func_id);
-			return kfunc_btf ?: ERR_PTR(-ENOENT);
-		}
-		return kfunc_btf;
+		return __find_kfunc_desc_btf(env, offset, btf_modp);
 	}
 	return btf_vmlinux ?: ERR_PTR(-ENOENT);
 }