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>

blockdev-nbd.c

@@ -154,7 +154,6 @@ BlockExport *nbd_export_create(BlockExportOptions *exp_args, Error **errp)
     BlockDriverState *bs = NULL;
     BlockBackend *on_eject_blk;
     NBDExport *exp = NULL;
-    int64_t len;
     AioContext *aio_context;
 
     assert(exp_args->type == BLOCK_EXPORT_TYPE_NBD);
@@ -192,12 +191,6 @@ BlockExport *nbd_export_create(BlockExportOptions *exp_args, Error **errp)
 
     aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(aio_context);
-    len = bdrv_getlength(bs);
-    if (len < 0) {
-        error_setg_errno(errp, -len,
-                         "Failed to determine the NBD export's length");
-        goto out;
-    }
 
     if (!arg->has_writable) {
         arg->writable = false;
@@ -206,7 +199,7 @@ BlockExport *nbd_export_create(BlockExportOptions *exp_args, Error **errp)
         arg->writable = false;
     }
 
-    exp = nbd_export_new(bs, 0, len, arg->name, arg->description, arg->bitmap,
+    exp = nbd_export_new(bs, arg->name, arg->description, arg->bitmap,
                          !arg->writable, !arg->writable,
                          NULL, false, on_eject_blk, errp);
     if (!exp) {

include/block/nbd.h

@@ -331,8 +331,8 @@ typedef struct NBDExport NBDExport;
 typedef struct NBDClient NBDClient;
 
 BlockExport *nbd_export_create(BlockExportOptions *exp_args, Error **errp);
-NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
+NBDExport *nbd_export_new(BlockDriverState *bs,
-                          uint64_t size, const char *name, const char *desc,
+                          const char *name, const char *desc,
                           const char *bitmap, bool readonly, bool shared,
                           void (*close)(NBDExport *), bool writethrough,
                           BlockBackend *on_eject_blk, Error **errp);

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,
+NBDExport *nbd_export_new(BlockDriverState *bs,
-                          uint64_t size, const char *name, const char *desc,
+                          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,
+            ret = blk_pread(exp->blk, offset + progress, data + progress, pnum);
-                            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,
+    ret = blk_pread(exp->blk, request->from, data, request->len);
-                    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,
+        ret = blk_pwrite(exp->blk, request->from, data, request->len, flags);
-                         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,
+            ret = blk_pwrite_zeroes(exp->blk, request->from, len, flags);
-                                    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,
+            ret = blk_co_pdiscard(exp->blk, request->from, len);
-                                  len);
             request->len -= len;
             request->from += len;
         }

qemu-nbd.c

@@ -524,7 +524,6 @@ int main(int argc, char **argv)
     const char *port = NULL;
     char *sockpath = NULL;
     char *device = NULL;
-    int64_t fd_size;
     QemuOpts *sn_opts = NULL;
     const char *sn_id_or_name = NULL;
     const char *sopt = "hVb:o:p:rsnc:dvk:e:f:tl:x:T:D:B:L";
@@ -1037,6 +1036,17 @@ int main(int argc, char **argv)
     }
     bs = blk_bs(blk);
 
+    if (dev_offset) {
+        QDict *raw_opts = qdict_new();
+        qdict_put_str(raw_opts, "driver", "raw");
+        qdict_put_str(raw_opts, "file", bs->node_name);
+        qdict_put_int(raw_opts, "offset", dev_offset);
+        bs = bdrv_open(NULL, NULL, raw_opts, flags, &error_fatal);
+        blk_remove_bs(blk);
+        blk_insert_bs(blk, bs, &error_fatal);
+        bdrv_unref(bs);
+    }
+
     blk_set_enable_write_cache(blk, !writethrough);
 
     if (sn_opts) {
@@ -1054,21 +1064,8 @@ int main(int argc, char **argv)
     }
 
     bs->detect_zeroes = detect_zeroes;
-    fd_size = blk_getlength(blk);
-    if (fd_size < 0) {
-        error_report("Failed to determine the image length: %s",
-                     strerror(-fd_size));
-        exit(EXIT_FAILURE);
-    }
 
-    if (dev_offset >= fd_size) {
+    export = nbd_export_new(bs, export_name,
-        error_report("Offset (%" PRIu64 ") has to be smaller than the image "
-                     "size (%" PRId64 ")", dev_offset, fd_size);
-        exit(EXIT_FAILURE);
-    }
-    fd_size -= dev_offset;
-
-    export = nbd_export_new(bs, dev_offset, fd_size, export_name,
                             export_description, bitmap, readonly, shared > 1,
                             nbd_export_closed, writethrough, NULL,
                             &error_fatal);