[PATCH] nbd/server: Use smarter assert

[Eric Blake <eblake@redhat.com>]

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 string can never be that large: if the export
name is reflecting what the client asked about, we already guarantee
that we drop the NBD connection if the client tries to send more than
32M in a single NBD_OPT_* request; and if the export name is coming
from qemu, nbd_receive_negotiate() asserted that strlen(info->name) <=
NBD_MAX_STRING_SIZE.  Still, it doesn't hurt to be more explicit in
how we write our assertion that we are aware that no 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>
---
 nbd/client.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/nbd/client.c b/nbd/client.c
index 60c9f4941a..b601ee97e5 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -658,7 +658,7 @@ 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;
-- 
2.37.3