[PATCH bpf-next v4 3/5] bpf: return partial progress from bpf_stream_read on fault

bpf_stream_read() pops and frees stream elements after a successful
copy_to_user(). If a later copy_to_user() fails, it currently restores
only the current element's consumed_len and returns -EFAULT, hiding
bytes already delivered to userspace and making the consumed data
unrecoverable on retry.

On a short copy, keep the successfully copied prefix of the current
element and return the number of bytes copied. Return -EFAULT only when
no bytes were copied for the call.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Signed-off-by: Jianlin Shi <[email protected]>
---
 kernel/bpf/stream.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index a36fc9338967..09471a605268 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -167,6 +167,7 @@ static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 
 	while (rem_len) {
 		int pos = len - rem_len;
+		int chunk, n;
 		bool cont;
 
 		node = bpf_stream_backlog_peek(stream);
@@ -180,13 +181,14 @@ static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 
 		cons_len = elem->consumed_len;
 		cont = bpf_stream_consume_elem(elem, &rem_len) == false;
-
-		ret = copy_to_user(buf + pos, elem->str + cons_len,
-				   elem->consumed_len - cons_len);
-		/* Restore in case of error. */
-		if (ret) {
-			ret = -EFAULT;
-			elem->consumed_len = cons_len;
+		chunk = elem->consumed_len - cons_len;
+
+		n = copy_to_user(buf + pos, elem->str + cons_len, chunk);
+		if (n) {
+			/* Keep any successfully copied bytes; -EFAULT only if none. */
+			elem->consumed_len -= n;
+			rem_len += n;
+			ret = (len == rem_len) ? -EFAULT : 0;
 			break;
 		}
 
-- 
2.43.0