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nbd-client: Short-circuit 0-length operations

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The NBD spec was recently clarified to state that clients should
not send 0-length requests to the server, as the server behavior
is undefined [1].  We know that qemu-nbd's behavior is a successful
no-op (once it has filtered for read-only exports), but other NBD
implementations might return an error.  To avoid any questionable
server implementations, it is better to just short-circuit such
requests on the client side (we are relying on the block layer to
already filter out requests such as invalid offset, write to a
read-only volume, and so forth); do the short-circuit as late as
possible to still benefit from protections from assertions that
the block layer is not violating our assumptions.

[1] https://github.com/NetworkBlockDevice/nbd/commit/ee926037

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171108215703.9295-6-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
This commit is contained in:
Eric Blake 2017-11-08 15:57:01 -06:00
parent efdc0c103d
commit 9d8f818cde
1 changed files with 10 additions and 1 deletions
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11
block/nbd-client.c
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@ -674,6 +674,9 @@ int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
assert(bytes <= NBD_MAX_BUFFER_SIZE);
assert(!flags);
if (!bytes) {
return 0;
}
ret = nbd_co_send_request(bs, &request, NULL);
if (ret < 0) {
return ret;
@ -705,6 +708,9 @@ int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t offset,
assert(bytes <= NBD_MAX_BUFFER_SIZE);
if (!bytes) {
return 0;
}
return nbd_co_request(bs, &request, qiov);
}
@ -731,6 +737,9 @@ int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
request.flags |= NBD_CMD_FLAG_NO_HOLE;
}
if (!bytes) {
return 0;
}
return nbd_co_request(bs, &request, NULL);
}
@ -759,7 +768,7 @@ int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t offset, int bytes)
};
assert(!(client->info.flags & NBD_FLAG_READ_ONLY));
if (!(client->info.flags & NBD_FLAG_SEND_TRIM)) {
if (!(client->info.flags & NBD_FLAG_SEND_TRIM) || !bytes) {
return 0;
}