linux-kernel - [PATCH] kernel/relay.c: fix read_pos error when multiple readers

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Date:   Tue, 21 Jan 2020 19:16:40 +0800
From:   Pengcheng Yang <yangpc@...gsu.com>
To:     gregkh@...uxfoundation.org, viro@...iv.linux.org.uk
Cc:     linux-kernel@...r.kernel.org, Pengcheng Yang <yangpc@...gsu.com>
Subject: [PATCH] kernel/relay.c: fix read_pos error when multiple readers

When reading, read_pos should start with bytes_consumed,
not file->f_pos. Because when there is more than one reader,
the read_pos corresponding to file->f_pos may have been consumed,
which will cause the data that has been consumed to be read
and the bytes_consumed update error.

Signed-off-by: Pengcheng Yang <yangpc@...gsu.com>
---
 kernel/relay.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index ade14fb..07ee1a7 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -991,14 +991,14 @@ static void relay_file_read_consume(struct rchan_buf *buf,
 /*
  *	relay_file_read_avail - boolean, are there unconsumed bytes available?
  */
-static int relay_file_read_avail(struct rchan_buf *buf, size_t read_pos)
+static int relay_file_read_avail(struct rchan_buf *buf)
 {
 	size_t subbuf_size = buf->chan->subbuf_size;
 	size_t n_subbufs = buf->chan->n_subbufs;
 	size_t produced = buf->subbufs_produced;
 	size_t consumed = buf->subbufs_consumed;
 
-	relay_file_read_consume(buf, read_pos, 0);
+	relay_file_read_consume(buf, 0, 0);
 
 	consumed = buf->subbufs_consumed;
 
@@ -1059,23 +1059,20 @@ static size_t relay_file_read_subbuf_avail(size_t read_pos,
 
 /**
  *	relay_file_read_start_pos - find the first available byte to read
- *	@read_pos: file read position
  *	@buf: relay channel buffer
  *
- *	If the @read_pos is in the middle of padding, return the
+ *	If the read_pos is in the middle of padding, return the
  *	position of the first actually available byte, otherwise
  *	return the original value.
  */
-static size_t relay_file_read_start_pos(size_t read_pos,
-					struct rchan_buf *buf)
+static size_t relay_file_read_start_pos(struct rchan_buf *buf)
 {
 	size_t read_subbuf, padding, padding_start, padding_end;
 	size_t subbuf_size = buf->chan->subbuf_size;
 	size_t n_subbufs = buf->chan->n_subbufs;
 	size_t consumed = buf->subbufs_consumed % n_subbufs;
+	size_t read_pos = consumed * subbuf_size + buf->bytes_consumed;
 
-	if (!read_pos)
-		read_pos = consumed * subbuf_size + buf->bytes_consumed;
 	read_subbuf = read_pos / subbuf_size;
 	padding = buf->padding[read_subbuf];
 	padding_start = (read_subbuf + 1) * subbuf_size - padding;
@@ -1131,10 +1128,10 @@ static ssize_t relay_file_read(struct file *filp,
 	do {
 		void *from;
 
-		if (!relay_file_read_avail(buf, *ppos))
+		if (!relay_file_read_avail(buf))
 			break;
 
-		read_start = relay_file_read_start_pos(*ppos, buf);
+		read_start = relay_file_read_start_pos(buf);
 		avail = relay_file_read_subbuf_avail(read_start, buf);
 		if (!avail)
 			break;
-- 
1.8.3.1