Re: [PATCH] media: dvb_ringbuffer : Fix a bug in dvb_ringbuffer.c

[Hans Petter Selasky]

Hi Kieran,

The consumed variable should not be negative. This bug has been there 
since the beginning of the GIT at Linux from what I can see.

+1

--HPS

From 108c6acb2cc4bc4314b96f6f254a04b2873a096c Mon Sep 17 00:00:00 2001
From: YongSu Yoo <yongsuyoo0215@xxxxxxxxx>
Date: Sun, 22 May 2022 04:53:12 +0000
Subject: [PATCH] media: dvb_ringbuffer : Fix a bug in dvb_ringbuffer.c

Signed-off-by:Yongsu Yoo <yongsuyoo0215@xxxxxxxxx>

The function dvb_ringbuffer_pkt_next in
/linux-next/drviers/media/dvb-core/dvb_ringbuffer.c,
which searches the idx of the next valid packet in the ring
buffer of the ca->slot_info[slot].rx_buffer at
/linux-next/drivers/media/dvb-core/dvb_ca_en50221.c,
has the following problem.
In calculating the amounts of the consumed address of the ring
buffer, if the read address(rbuf->pread) of the ring buffer is
smaller than the idx, the amounts of the searched address
should be (idx - rbuf->pread),
whereas if the read address(rbuf->pread) of the ring buffer is
larger than the idx, the amounts of the consumed address should
be (idx - rbuf->pread + rbug->size). But there exists an
incorrect logic that the rbug-size was not properly added on
(idx - rbug->pread) in the later case. With this commit, we
fixed this bug.
---
 drivers/media/dvb-core/dvb_ringbuffer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/media/dvb-core/dvb_ringbuffer.c 
b/drivers/media/dvb-core/dvb_ringbuffer.c
index d1d471af0636..7d4558de8e83 100644
--- a/drivers/media/dvb-core/dvb_ringbuffer.c
+++ b/drivers/media/dvb-core/dvb_ringbuffer.c
@@ -335,7 +335,9 @@ ssize_t dvb_ringbuffer_pkt_next(struct 
dvb_ringbuffer *rbuf, size_t idx, size_t*
 		idx = (idx + curpktlen + DVB_RINGBUFFER_PKTHDRSIZE) % rbuf->size;
 	}

-	consumed = (idx - rbuf->pread) % rbuf->size;
+	consumed = (idx - rbuf->pread);
+	if (consumed < 0)
+		consumed += rbuf->size;

 	while((dvb_ringbuffer_avail(rbuf) - consumed) > 
DVB_RINGBUFFER_PKTHDRSIZE) {

--
2.17.1