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@wangsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jann Horn <jannh@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>e
Link: http://lkml.kernel.org/r/1579691175-28949-1-git-send-email-yangpc@wangsu.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/relay.c b/kernel/relay.c
index dc82705e1cff5478eef63815ad0822ddadf836ef..204867220f8aeffbc38f0afc57d73f63a5caea10 100644 (file)
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -996,14 +996,14 @@ 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;
 
@@ -1064,23 +1064,20 @@ 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;
@@ -1136,10 +1133,10 @@ 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;