bpf: respect size hint to BPF_PROG_TEST_RUN if present

Use data_size_out as a size hint when copying test output to user space.
ENOSPC is returned if the output buffer is too small.
Callers which so far did not set data_size_out are not affected.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -374,8 +374,11 @@ union bpf_attr {
 	struct { /* anonymous struct used by BPF_PROG_TEST_RUN command */
 		__u32		prog_fd;
 		__u32		retval;
-		__u32		data_size_in;
-		__u32		data_size_out;
+		__u32		data_size_in;	/* input: len of data_in */
+		__u32		data_size_out;	/* input/output: len of data_out
+						 *   returns ENOSPC if data_out
+						 *   is too small.
+						 */
 		__aligned_u64	data_in;
 		__aligned_u64	data_out;
 		__u32		repeat;

net/bpf/test_run.c

@@ -74,8 +74,18 @@ static int bpf_test_finish(const union bpf_attr *kattr,
 {
 	void __user *data_out = u64_to_user_ptr(kattr->test.data_out);
 	int err = -EFAULT;
+	u32 copy_size = size;
 
-	if (data_out && copy_to_user(data_out, data, size))
+	/* Clamp copy if the user has provided a size hint, but copy the full
+	 * buffer if not to retain old behaviour.
+	 */
+	if (kattr->test.data_size_out &&
+	    copy_size > kattr->test.data_size_out) {
+		copy_size = kattr->test.data_size_out;
+		err = -ENOSPC;
+	}
+
+	if (data_out && copy_to_user(data_out, data, copy_size))
 		goto out;
 	if (copy_to_user(&uattr->test.data_size_out, &size, sizeof(size)))
 		goto out;
@@ -83,6 +93,7 @@ static int bpf_test_finish(const union bpf_attr *kattr,
 		goto out;
 	if (copy_to_user(&uattr->test.duration, &duration, sizeof(duration)))
 		goto out;
+	if (err != -ENOSPC)
 		err = 0;
 out:
 	return err;