selftests/bpf: Cover oversized stream read buffers

Exercise BPF_PROG_STREAM_READ_BY_FD with a UINT_MAX buffer length and
require EINVAL. Continue with the existing two-part read of the queued
payload to verify that rejecting the oversized request does not consume
or otherwise disturb stream data.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20260823193202.2389715-3-memxor@gmail.com

Author: kkdwvd

tools/testing/selftests/bpf/prog_tests/stream.c

@@ -46,6 +46,10 @@ void test_stream_syscall(void)
 	ret = -errno;
 	ASSERT_EQ(ret, -EFAULT, "bad stream buf");
 
+	ASSERT_LT(bpf_prog_stream_read(prog_fd, BPF_STREAM_STDOUT, buf, UINT_MAX, NULL), 0, "error");
+	ret = -errno;
+	ASSERT_EQ(ret, -EINVAL, "large stream buf");
+
 	ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDOUT, buf, 2, NULL);
 	ASSERT_EQ(ret, 2, "bytes");
 	ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDOUT, buf, 2, NULL);