bpf: Add BPF token support to BPF_BTF_LOAD command

Accept BPF token FD in BPF_BTF_LOAD command to allow BTF data loading
through delegated BPF token. BPF_F_TOKEN_FD flag has to be specified
when passing BPF token FD. Given BPF_BTF_LOAD command didn't have flags
field before, we also add btf_flags field.

BTF loading is a pretty straightforward operation, so as long as BPF
token is created with allow_cmds granting BPF_BTF_LOAD command, kernel
proceeds to parsing BTF data and creating BTF object.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20240124022127.2379740-6-andrii@kernel.org

include/uapi/linux/bpf.h

@@ -1632,6 +1632,11 @@ union bpf_attr {
 		 * truncated), or smaller (if log buffer wasn't filled completely).
 		 */
 		__u32		btf_log_true_size;
+		__u32		btf_flags;
+		/* BPF token FD to use with BPF_BTF_LOAD operation.
+		 * If provided, btf_flags should have BPF_F_TOKEN_FD flag set.
+		 */
+		__s32		btf_token_fd;
 	};
 
 	struct {

kernel/bpf/syscall.c

@@ -4831,15 +4831,34 @@ static int bpf_obj_get_info_by_fd(const union bpf_attr *attr,
 	return err;
 }
 
-#define BPF_BTF_LOAD_LAST_FIELD btf_log_true_size
+#define BPF_BTF_LOAD_LAST_FIELD btf_token_fd
 
 static int bpf_btf_load(const union bpf_attr *attr, bpfptr_t uattr, __u32 uattr_size)
 {
+	struct bpf_token *token = NULL;
+
 	if (CHECK_ATTR(BPF_BTF_LOAD))
 		return -EINVAL;
 
-	if (!bpf_capable())
+	if (attr->btf_flags & ~BPF_F_TOKEN_FD)
+		return -EINVAL;
+
+	if (attr->btf_flags & BPF_F_TOKEN_FD) {
+		token = bpf_token_get_from_fd(attr->btf_token_fd);
+		if (IS_ERR(token))
+			return PTR_ERR(token);
+		if (!bpf_token_allow_cmd(token, BPF_BTF_LOAD)) {
+			bpf_token_put(token);
+			token = NULL;
+		}
+	}
+
+	if (!bpf_token_capable(token, CAP_BPF)) {
+		bpf_token_put(token);
 		return -EPERM;
+	}
+
+	bpf_token_put(token);
 
 	return btf_new_fd(attr, uattr, uattr_size);
 }

tools/include/uapi/linux/bpf.h

@@ -1632,6 +1632,11 @@ union bpf_attr {
 		 * truncated), or smaller (if log buffer wasn't filled completely).
 		 */
 		__u32		btf_log_true_size;
+		__u32		btf_flags;
+		/* BPF token FD to use with BPF_BTF_LOAD operation.
+		 * If provided, btf_flags should have BPF_F_TOKEN_FD flag set.
+		 */
+		__s32		btf_token_fd;
 	};
 
 	struct {