[Qemu-devel] [PATCH v4 05/21] nbd/server: Hoist length check to qmp_nbd_server_add

[Eric Blake <eblake@redhat.com>]

We only had two callers to nbd_export_new; qemu-nbd.c always
passed a valid offset/length pair (because it already checked
the file length, to ensure that offset was in bounds), while
blockdev-nbd.c always passed 0/-1.  Then nbd_export_new reduces
the size to a multiple of BDRV_SECTOR_SIZE (can only happen
when offset is not sector-aligned, since bdrv_getlength()
currently rounds up), which can result in offset being greater
than the enforced length, but that's not fatal (the server
rejects client requests that exceed the advertised length).

However, I'm finding it easier to work with the code if we are
consistent on having both callers pass in a valid length, and
just assert that things are sane in nbd_export_new, meaning
that no negative values were passed, and that offset+size does
not exceed 63 bits (as that really is a fundamental limit to
later operations, whether we use off_t or uint64_t).

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v4: rework asserts [Vladimir], retitle
v3: new patch
---
 blockdev-nbd.c | 10 +++++++++-
 nbd/server.c   | 10 +++-------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index c76d5416b90..d73ac1b026a 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -146,6 +146,7 @@ void qmp_nbd_server_add(const char *device, bool has_name, const char *name,
     BlockDriverState *bs = NULL;
     BlockBackend *on_eject_blk;
     NBDExport *exp;
+    int64_t len;

     if (!nbd_server) {
         error_setg(errp, "NBD server not running");
@@ -168,6 +169,13 @@ void qmp_nbd_server_add(const char *device, bool has_name, const char *name,
         return;
     }

+    len = bdrv_getlength(bs);
+    if (len < 0) {
+        error_setg_errno(errp, -len,
+                         "Failed to determine the NBD export's length");
+        return;
+    }
+
     if (!has_writable) {
         writable = false;
     }
@@ -175,7 +183,7 @@ void qmp_nbd_server_add(const char *device, bool has_name, const char *name,
         writable = false;
     }

-    exp = nbd_export_new(bs, 0, -1, name, NULL, bitmap,
+    exp = nbd_export_new(bs, 0, len, name, NULL, bitmap,
                          writable ? 0 : NBD_FLAG_READ_ONLY,
                          NULL, false, on_eject_blk, errp);
     if (!exp) {
diff --git a/nbd/server.c b/nbd/server.c
index 6b136019f82..51ee8094e02 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1495,17 +1495,13 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset, off_t size,
     exp->refcount = 1;
     QTAILQ_INIT(&exp->clients);
     exp->blk = blk;
+    assert(dev_offset >= 0 && dev_offset <= INT64_MAX);
     exp->dev_offset = dev_offset;
     exp->name = g_strdup(name);
     exp->description = g_strdup(description);
     exp->nbdflags = nbdflags;
-    exp->size = size < 0 ? blk_getlength(blk) : size;
-    if (exp->size < 0) {
-        error_setg_errno(errp, -exp->size,
-                         "Failed to determine the NBD export's length");
-        goto fail;
-    }
-    exp->size -= exp->size % BDRV_SECTOR_SIZE;
+    assert(size >= 0 && size <= INT64_MAX - dev_offset);
+    exp->size = QEMU_ALIGN_DOWN(size, BDRV_SECTOR_SIZE);

     if (bitmap) {
         BdrvDirtyBitmap *bm = NULL;
-- 
2.20.1