UML - Fix request->sector update

PATCH In review Jeff Dike 1 participant 1 message Jun 5, 2007 User-mode Linux (UML)

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Jeff Dike

author

2007-06-05 20:59 UTC #0 lore raw

PATCH 1/2 UML - Fix request->sector update

[ 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(-)

arch/um/drivers/ubd_kern.c +4 -2

Index: linux-2.6.21-mm/arch/um/drivers/ubd_kern.c
===================================================================
@@ -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;

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