[PATCH bpf-next v3 1/5] bpf: Skip zero-length stream writes

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

A stream write whose formatted output is empty still allocates a stream
element and links it into the stream log. Capacity accounting only counts
payload bytes, so such elements never count against the stream limit, and a
program that keeps producing empty output allocates elements without bound
until a reader drains them.

Return success without allocating anything when there is nothing to write,
for both bpf_stream_vprintk() and staged writes. Every element in a stream
log then carries at least one byte, which later changes rely on when they
derive read readiness from published byte counts.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Suggested-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/stream.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index 2b80a0599865..9829d1ebce70 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -75,8 +75,13 @@ static void bpf_stream_release_capacity(struct bpf_stream *stream, int len)
 
 static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int len)
 {
-	int ret = bpf_stream_consume_capacity(stream, len);
+	int ret;
+
+	/* Nothing to publish; do not allocate an element for it. */
+	if (!len)
+		return 0;
 
+	ret = bpf_stream_consume_capacity(stream, len);
 	if (ret)
 		return ret;
 
@@ -333,8 +338,8 @@ int bpf_stream_stage_printk(struct bpf_stream_stage *ss, const char *fmt, ...)
 	va_start(args, fmt);
 	len = vscnprintf(buf->buf, ARRAY_SIZE(buf->buf), fmt, args);
 	va_end(args);
-	/* Exclude NULL byte during push. */
-	ret = __bpf_stream_push_str(&ss->log, buf->buf, len);
+	/* Exclude NULL byte during push; skip empty output entirely. */
+	ret = len ? __bpf_stream_push_str(&ss->log, buf->buf, len) : 0;
 	if (!ret)
 		ss->len += len;
 	bpf_put_buffers();
-- 
2.53.0