[Libguestfs] [nbdkit PATCH] truncate: Detect large image overflow with round-up

[Eric Blake]

It is possible for the round-up parameter to cause our desired size to
exceed 2**63-1.  But this error message is cryptic:

$ ./nbdkit -f --filter=truncate pattern $(((1<<63)-1)) round-up=1m &
$ qemu-nbd --list
nbdkit: pattern[1]: error: .get_size function returned invalid value (-9223372036854775808)
qemu-nbd: Failed to read initial magic: Unexpected end-of-file before all bytes were read

Better is the result of this patch:
nbdkit: pattern[1]: error: cannot round size 9223372036854775807 up to next boundary of 1048576

Alas, we are still limited in the fact that we can only detect the
problem on a per-connection basis (at .prepare, rather than at .load)
(because we support plugins that have a different size per
connection); and that nbdkit in general is still rather rude to
clients when .prepare prevents the use of a connection (perhaps nbdkit
should instead be taught to proceed with the socekt, but respond to
NBD_OPT_LIST with 0 items and to NBD_OPT_GO with NBD_REP_ERR_UNKNOWN
or NBD_REP_ERR_SHUTDOWN to give the client a hint that this server has
nothing further to offer).

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

I pushed this one, but am posting to the list because of the
conversation points it brings up about what to do when .prepare fails
(there's a difference between refusing to start nbdkit because .config
failed, vs. starting but being useless to a client because
.open/.prepare failed).

 filters/truncate/truncate.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/filters/truncate/truncate.c b/filters/truncate/truncate.c
index 6408c35..bed5a03 100644
--- a/filters/truncate/truncate.c
+++ b/filters/truncate/truncate.c
@@ -38,6 +38,7 @@
 #include <string.h>
 #include <limits.h>
 #include <errno.h>
+#include <inttypes.h>

 #include <nbdkit-filter.h>

@@ -172,8 +173,14 @@ truncate_prepare (struct nbdkit_next_ops *next_ops, void *nxdata,
    */
   if (truncate_size >= 0)
     h->size = truncate_size;
-  if (round_up > 0)
+  if (round_up > 0) {
+    if (ROUND_UP (h->size, round_up) > INT64_MAX) {
+      nbdkit_error ("cannot round size %" PRId64 " up to next boundary of %u",
+                    h->size, round_up);
+      return -1;
+    }
     h->size = ROUND_UP (h->size, round_up);
+  }
   if (round_down > 0)
     h->size = ROUND_DOWN (h->size, round_down);

-- 
2.20.1