[PATCH v2] nbd/client: Use smarter assert

[Eric Blake]

Assigning strlen() to a uint32_t and then asserting that it isn't too
large doesn't catch the case of an input string 4G in length.
Thankfully, the incoming strings can never be that large: if the
export name or query is reflecting a string the client got from the
server, we already guarantee that we dropped the NBD connection if the
server sent more than 32M in a single reply to our NBD_OPT_* request;
if the export name is coming from qemu, nbd_receive_negotiate()
asserted that strlen(info->name) <= NBD_MAX_STRING_SIZE; and
similarly, a query string via x->dirty_bitmap coming from the user was
bounds-checked in either qemu-nbd or by the limitations of QMP.
Still, it doesn't hurt to be more explicit in how we write our
assertions to not have to analyze whether inadvertent wraparound is
possible.

Fixes: 93676c88 ("nbd: Don't send oversize strings", v4.2.0)
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
---

v2: update subject line and commit message to reflect file being
touched; adjust a second nearby assertion with the same issue

 nbd/client.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/nbd/client.c b/nbd/client.c
index 30d5383cb1..90a6b7b38b 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -658,11 +658,11 @@ static int nbd_send_meta_query(QIOChannel *ioc, uint32_t 
opt,
     char *p;

     data_len = sizeof(export_len) + export_len + sizeof(queries);
-    assert(export_len <= NBD_MAX_STRING_SIZE);
+    assert(strlen(export) <= NBD_MAX_STRING_SIZE);
     if (query) {
         query_len = strlen(query);
         data_len += sizeof(query_len) + query_len;
-        assert(query_len <= NBD_MAX_STRING_SIZE);
+        assert(strlen(query) <= NBD_MAX_STRING_SIZE);
     } else {
         assert(opt == NBD_OPT_LIST_META_CONTEXT);
     }
-- 
2.37.3