qemu-nbd: Use raw block driver for --offset

Instead of implementing qemu-nbd --offset in the NBD code, just put a
raw block node with the requested offset on top of the user image and
rely on that doing the job.

This does not only simplify the nbd_export_new() interface and bring it
closer to the set of options that the nbd-server-add QMP command offers,
but in fact it also eliminates a potential source for bugs in the NBD
code which previously had to add the offset manually in all relevant
places.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200924152717.287415-7-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 16 insertions, 17 deletions

diff --git a/nbd/server.c b/nbd/server.c
index f5af93c253..33aaca918c 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -89,7 +89,6 @@ struct NBDExport {
     BlockBackend *blk;
     char *name;
     char *description;
-    uint64_t dev_offset;
     uint64_t size;
     uint16_t nbdflags;
     QTAILQ_HEAD(, NBDClient) clients;
@@ -1507,8 +1506,8 @@ static void nbd_eject_notifier(Notifier *n, void *data)
     aio_context_release(aio_context);
 }
 
-NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
-                          uint64_t size, const char *name, const char *desc,
+NBDExport *nbd_export_new(BlockDriverState *bs,
+                          const char *name, const char *desc,
                           const char *bitmap, bool readonly, bool shared,
                           void (*close)(NBDExport *), bool writethrough,
                           BlockBackend *on_eject_blk, Error **errp)
@@ -1516,9 +1515,17 @@ NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
     AioContext *ctx;
     BlockBackend *blk;
     NBDExport *exp;
+    int64_t size;
     uint64_t perm;
     int ret;
 
+    size = bdrv_getlength(bs);
+    if (size < 0) {
+        error_setg_errno(errp, -size,
+                         "Failed to determine the NBD export's length");
+        return NULL;
+    }
+
     exp = g_new0(NBDExport, 1);
     exp->common = (BlockExport) {
         .drv = &blk_exp_nbd,
@@ -1553,8 +1560,6 @@ NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
     exp->refcount = 1;
     QTAILQ_INIT(&exp->clients);
     exp->blk = blk;
-    assert(dev_offset <= INT64_MAX);
-    exp->dev_offset = dev_offset;
     exp->name = g_strdup(name);
     assert(!desc || strlen(desc) <= NBD_MAX_STRING_SIZE);
     exp->description = g_strdup(desc);
@@ -1569,7 +1574,6 @@ NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
         exp->nbdflags |= (NBD_FLAG_SEND_TRIM | NBD_FLAG_SEND_WRITE_ZEROES |
                           NBD_FLAG_SEND_FAST_ZERO);
     }
-    assert(size <= INT64_MAX - dev_offset);
     exp->size = QEMU_ALIGN_DOWN(size, BDRV_SECTOR_SIZE);
 
     if (bitmap) {
@@ -1928,8 +1932,7 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client,
             stl_be_p(&chunk.length, pnum);
             ret = nbd_co_send_iov(client, iov, 1, errp);
         } else {
-            ret = blk_pread(exp->blk, offset + progress + exp->dev_offset,
-                            data + progress, pnum);
+            ret = blk_pread(exp->blk, offset + progress, data + progress, pnum);
             if (ret < 0) {
                 error_setg_errno(errp, -ret, "reading from file failed");
                 break;
@@ -2303,8 +2306,7 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,
                                        data, request->len, errp);
     }
 
-    ret = blk_pread(exp->blk, request->from + exp->dev_offset, data,
-                    request->len);
+    ret = blk_pread(exp->blk, request->from, data, request->len);
     if (ret < 0) {
         return nbd_send_generic_reply(client, request->handle, ret,
                                       "reading from file failed", errp);
@@ -2339,7 +2341,7 @@ static coroutine_fn int nbd_do_cmd_cache(NBDClient *client, NBDRequest *request,
 
     assert(request->type == NBD_CMD_CACHE);
 
-    ret = blk_co_preadv(exp->blk, request->from + exp->dev_offset, request->len,
+    ret = blk_co_preadv(exp->blk, request->from, request->len,
                         NULL, BDRV_REQ_COPY_ON_READ | BDRV_REQ_PREFETCH);
 
     return nbd_send_generic_reply(client, request->handle, ret,
@@ -2370,8 +2372,7 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
         if (request->flags & NBD_CMD_FLAG_FUA) {
             flags |= BDRV_REQ_FUA;
         }
-        ret = blk_pwrite(exp->blk, request->from + exp->dev_offset,
-                         data, request->len, flags);
+        ret = blk_pwrite(exp->blk, request->from, data, request->len, flags);
         return nbd_send_generic_reply(client, request->handle, ret,
                                       "writing to file failed", errp);
 
@@ -2392,8 +2393,7 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
             int align = client->check_align ?: 1;
             int len = MIN(request->len, QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES,
                                                         align));
-            ret = blk_pwrite_zeroes(exp->blk, request->from + exp->dev_offset,
-                                    len, flags);
+            ret = blk_pwrite_zeroes(exp->blk, request->from, len, flags);
             request->len -= len;
             request->from += len;
         }
@@ -2416,8 +2416,7 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
             int align = client->check_align ?: 1;
             int len = MIN(request->len, QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES,
                                                         align));
-            ret = blk_co_pdiscard(exp->blk, request->from + exp->dev_offset,
-                                  len);
+            ret = blk_co_pdiscard(exp->blk, request->from, len);
             request->len -= len;
             request->from += len;
         }