[ This is post-2.6.22 material - it fixes a bug, but not one that I
think has been seen in the wild, plus an earlier version of this fix
caused file corruption ]
It is theoretically possible for a request to finish and be freed
between writing it to the I/O thread and updating the sector count.
In this case, the update will dereference a freed pointer.
To avoid this, I delay the update until processing the next sg
segment, when the request pointer is known to be good.
Signed-off-by: Jeff Dike <[email protected]>
--
arch/um/drivers/ubd_kern.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
Index: linux-2.6.21-mm/arch/um/drivers/ubd_kern.c
===================================================================
--- linux-2.6.21-mm.orig/arch/um/drivers/ubd_kern.c 2007-06-02 12:13:50.000000000 -0400
+++ linux-2.6.21-mm/arch/um/drivers/ubd_kern.c 2007-06-02 22:23:41.000000000 -0400
@@ -1082,7 +1082,7 @@ static void do_ubd_request(request_queue
{
struct io_thread_req *io_req;
struct request *req;
- int n;
+ int n, last_sectors;
while(1){
struct ubd *dev = q->queuedata;
@@ -1098,9 +1098,11 @@ static void do_ubd_request(request_queue
}
req = dev->request;
+ last_sectors = 0;
while(dev->start_sg < dev->end_sg){
struct scatterlist *sg = &dev->sg[dev->start_sg];
+ req->sector += last_sectors;
io_req = kmalloc(sizeof(struct io_thread_req),
GFP_ATOMIC);
if(io_req == NULL){
@@ -1112,6 +1114,7 @@ static void do_ubd_request(request_queue
(unsigned long long) req->sector << 9,
sg->offset, sg->length, sg->page);
+ last_sectors = sg->length >> 9;
n = os_write_file(thread_fd, &io_req,
sizeof(struct io_thread_req *));
if(n != sizeof(struct io_thread_req *)){
@@ -1123,7 +1126,6 @@ static void do_ubd_request(request_queue
return;
}
- req->sector += sg->length >> 9;
dev->start_sg++;
}
dev->end_sg = 0;