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(a)redhat.com&gt; --- 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