[PATCH bpf 1/2] bpf: Fix partial copy of non-linear skb test_run output

[Sun Jian <sun.jian.kdev@gmail.com>]

For non-linear skbs, bpf_test_finish() derives the linear head copy
length from copy_size - frag_size. This only matches the skb head 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 head length from the skb layout instead, and clamp the
head 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: 838baa351cee ("bpf: Craft non-linear skbs in BPF_PROG_TEST_RUN")
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 net/bpf/test_run.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 2bc04feadfab..976e8fa31bc9 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -453,19 +453,16 @@ 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;
 
 		if (sinfo) {
-			int i, offset = len;
+			u32 offset = len;
 			u32 data_len;
+			int i;
 
 			for (i = 0; i < sinfo->nr_frags; i++) {
 				skb_frag_t *frag = &sinfo->frags[i];
-- 
2.43.0