[PATCH v3 01/14] nbd/client: 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 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 <dave@treblig.org>
Signed-off-by: Eric Blake <eblake@redhat.com>

---

Looking through older branches, I came across this one that was never
applied at the time, but which also had a useful review comment from
Vladimir that invalidates the R-b it had back then.

v2 was here: https://lists.gnu.org/archive/html/qemu-devel/2022-10/msg02733.html
since then - update David's email, use strnlen before strlen
---
 nbd/client.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/nbd/client.c b/nbd/client.c
index 30d5383cb19..ff75722e487 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -650,19 +650,20 @@ static int nbd_send_meta_query(QIOChannel *ioc, uint32_t opt,
                                Error **errp)
 {
     int ret;
-    uint32_t export_len = strlen(export);
+    uint32_t export_len;
     uint32_t queries = !!query;
     uint32_t query_len = 0;
     uint32_t data_len;
     char *data;
     char *p;

+    assert(strnlen(export, NBD_MAX_STRING_SIZE + 1) <= NBD_MAX_STRING_SIZE);
+    export_len = strlen(export);
     data_len = sizeof(export_len) + export_len + sizeof(queries);
-    assert(export_len <= NBD_MAX_STRING_SIZE);
     if (query) {
+        assert(strnlen(query, NBD_MAX_STRING_SIZE + 1) <= NBD_MAX_STRING_SIZE);
         query_len = strlen(query);
         data_len += sizeof(query_len) + query_len;
-        assert(query_len <= NBD_MAX_STRING_SIZE);
     } else {
         assert(opt == NBD_OPT_LIST_META_CONTEXT);
     }
-- 
2.40.1