rbd: remove VLA usage

In preparation to enabling -Wvla, remove the use of stack VLA.

In this particular case, variable buf_size is replaced with a constant expression that can be computed at preprocessing time. This avoids two VLA warnings. Also, as a consequence of the mentioned change, some code was slightly refactored.

The use of stack Variable Length Arrays needs to be avoided, as they can be a vector for stack exhaustion, which can be both a runtime bug or a security flaw. Also, in general, as code evolves it is easy to lose track of how big a VLA can get. Thus, we can end up having runtime failures that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 drivers/block/rbd.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

drivers/block/rbd.c +6 -6

index 1e03b04..5133122 100644
@@ -3091,20 +3091,20 @@ static int __rbd_notify_op_lock(struct rbd_device *rbd_dev,
 {
        struct ceph_osd_client *osdc = &rbd_dev->rbd_client->client->osdc;
        struct rbd_client_id cid = rbd_get_cid(rbd_dev);
-       int buf_size = 4 + 8 + 8 + CEPH_ENCODING_START_BLK_LEN;
-       char buf[buf_size];
+       char buf[4 + 8 + 8 + CEPH_ENCODING_START_BLK_LEN];
        void *p = buf;
 
        dout("%s rbd_dev %p notify_op %d\n", __func__, rbd_dev, notify_op);
 
        /* encode *LockPayload NotifyMessage (op + ClientId) */
-       ceph_start_encoding(&p, 2, 1, buf_size - CEPH_ENCODING_START_BLK_LEN);
+       ceph_start_encoding(&p, 2, 1,
+                           sizeof(buf) - CEPH_ENCODING_START_BLK_LEN);
        ceph_encode_32(&p, notify_op);
        ceph_encode_64(&p, cid.gid);
        ceph_encode_64(&p, cid.handle);
 
        return ceph_osdc_notify(osdc, &rbd_dev->header_oid,
-                               &rbd_dev->header_oloc, buf, buf_size,
+                               &rbd_dev->header_oloc, buf, sizeof(buf),
                                RBD_NOTIFY_TIMEOUT, preply_pages, preply_len);
 }
 

@@ -3610,19 +3610,18 @@ static void __rbd_acknowledge_notify(struct rbd_device *rbd_dev,
                                     u64 notify_id, u64 cookie, s32 *result)
 {
        struct ceph_osd_client *osdc = &rbd_dev->rbd_client->client->osdc;
-       int buf_size = 4 + CEPH_ENCODING_START_BLK_LEN;
-       char buf[buf_size];
+       char buf[4 + CEPH_ENCODING_START_BLK_LEN];
+       int buf_size = 0;
        int ret;
 
        if (result) {
                void *p = buf;
 

Quoted 72 lines · Gustavo A. R. Silva

In preparation to enabling -Wvla, remove the use of stack VLA.

In this particular case, variable buf_size is replaced with a constant expression that can be computed at preprocessing time. This avoids two VLA warnings. Also, as a consequence of the mentioned change, some code was slightly refactored.

The use of stack Variable Length Arrays needs to be avoided, as they can be a vector for stack exhaustion, which can be both a runtime bug or a security flaw. Also, in general, as code evolves it is easy to lose track of how big a VLA can get. Thus, we can end up having runtime failures that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
  drivers/block/rbd.c | 15 +++++++--------
  1 file changed, 7 insertions(+), 8 deletions(-)

drivers/block/rbd.c +7 -8

index 1e03b04..5133122 100644
@@ -3091,20 +3091,20 @@ static int __rbd_notify_op_lock(struct rbd_device *rbd_dev,
  {
         struct ceph_osd_client *osdc = &rbd_dev->rbd_client->client->osdc;
         struct rbd_client_id cid = rbd_get_cid(rbd_dev);
-       int buf_size = 4 + 8 + 8 + CEPH_ENCODING_START_BLK_LEN;
-       char buf[buf_size];
+       char buf[4 + 8 + 8 + CEPH_ENCODING_START_BLK_LEN];
         void *p = buf;
 
         dout("%s rbd_dev %p notify_op %d\n", __func__, rbd_dev, notify_op);
 
         /* encode *LockPayload NotifyMessage (op + ClientId) */
-       ceph_start_encoding(&p, 2, 1, buf_size - CEPH_ENCODING_START_BLK_LEN);
+       ceph_start_encoding(&p, 2, 1,
+                           sizeof(buf) - CEPH_ENCODING_START_BLK_LEN);
         ceph_encode_32(&p, notify_op);
         ceph_encode_64(&p, cid.gid);
         ceph_encode_64(&p, cid.handle);
 
         return ceph_osdc_notify(osdc, &rbd_dev->header_oid,
-                               &rbd_dev->header_oloc, buf, buf_size,
+                               &rbd_dev->header_oloc, buf, sizeof(buf),
                                 RBD_NOTIFY_TIMEOUT, preply_pages, preply_len);
  }
 

@@ -3610,19 +3610,18 @@ static void __rbd_acknowledge_notify(struct rbd_device *rbd_dev,
                                      u64 notify_id, u64 cookie, s32 *result)
  {
         struct ceph_osd_client *osdc = &rbd_dev->rbd_client->client->osdc;
-       int buf_size = 4 + CEPH_ENCODING_START_BLK_LEN;
-       char buf[buf_size];
+       char buf[4 + CEPH_ENCODING_START_BLK_LEN];
+       int buf_size = 0;
         int ret;
 
         if (result) {
                 void *p = buf;
 
+               buf_size = sizeof(buf);
                 /* encode ResponseMessage */
                 ceph_start_encoding(&p, 1, 1,
                                     buf_size - CEPH_ENCODING_START_BLK_LEN);
                 ceph_encode_32(&p, *result);
-       } else {
-               buf_size = 0;
         }
 

drivers/block/rbd.c static void __rbd_acknowledge_notify(struct rbd_device *rbd_dev, · line 3627

Quoted 1 line

drivers/block/rbd.c +0 -0

         ret = ceph_osdc_notify_ack(osdc, &rbd_dev->header_oid,

Already fixed:

   https://github.com/ceph/ceph-client/commit/2474e77e75215a3cad3b652d2f55ba5b123cb119