bpf: Fix partial copy of non-linear test_run output

For non-linear test_run output, bpf_test_finish() derives the linear
data copy length from copy_size - frag_size. This only matches the
linear data length when copy_size is the full packet size.

When userspace provides a short data_out buffer, copy_size is clamped to
that buffer size. If copy_size is smaller than frag_size, the computed
length becomes negative and bpf_test_finish() returns -ENOSPC before
copying the packet prefix or updating data_size_out.

Compute the linear data length from the packet layout instead, and clamp
the linear copy length to copy_size. This preserves the expected
partial-copy semantics: return -ENOSPC, copy the packet prefix that fits
in data_out, and report the full packet length through data_size_out.

Fixes: 7855e0db150ad ("bpf: test_run: add xdp_shared_info pointer in bpf_test_finish signature")
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/20260617093557.63880-2-sun.jian.kdev@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 2 insertions, 6 deletions

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 7fdee8f52ee2..5d51f6cb7d15 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -452,12 +452,8 @@ static int bpf_test_finish(const union bpf_attr *kattr,
 	}
 
 	if (data_out) {
-		int len = sinfo ? copy_size - frag_size : copy_size;
-
-		if (len < 0) {
-			err = -ENOSPC;
-			goto out;
-		}
+		u32 head_len = size - frag_size;
+		u32 len = min(copy_size, head_len);
 
 		if (copy_to_user(data_out, data, len))
 			goto out;