[Qemu-block] [PATCH v3 7/6] nbd/client: Ignore inaccessible tail of inconsistent server

[Eric Blake]

The NBD spec suggests that a server should never advertise a size
inconsistent with its minimum block alignment, as that tail is
effectively inaccessible to a compliant client obeying those block
constraints. Although the block layer likes to round up, here, we'd
prefer to truncate down to obey the spec, and note that it is the
server's fault for advertising bogus size.

Does not impact either qemu (which always sends properly aligned
sizes) or nbdkit (which does not send minimum block requirements yet);
so this is mostly theoretical, to avoid potential asserts elsewhere in
the code that assume the size is aligned.

Signed-off-by: Eric Blake <address@hidden>
---
 nbd/client.c     | 8 +++++++-
 nbd/trace-events | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/nbd/client.c b/nbd/client.c
index de7da48246b..be437051429 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2018 Red Hat, Inc.
+ *  Copyright (C) 2016-2019 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <address@hidden>
  *
  *  Network Block Device Client Side
@@ -426,6 +426,12 @@ static int nbd_opt_info_or_go(QIOChannel *ioc, uint32_t 
opt,
                 nbd_send_opt_abort(ioc);
                 return -1;
             }
+            if (info->min_block &&
+                !QEMU_IS_ALIGNED(info->size, info->min_block)) {
+                trace_nbd_opt_info_go_unaligned_size(info->size,
+                                                     info->min_block);
+                info->size = QEMU_ALIGN_DOWN(info->size, info->min_block);
+            }
             trace_nbd_receive_negotiate_size_flags(info->size, info->flags);
             break;

diff --git a/nbd/trace-events b/nbd/trace-events
index a6cca8fdf83..49dd48c9620 100644
--- a/nbd/trace-events
+++ b/nbd/trace-events
@@ -8,6 +8,7 @@ nbd_reply_err_unsup(uint32_t option, const char *name) "server 
doesn't understan
 nbd_receive_list(const char *name, const char *desc) "export list includes 
'%s', description '%s'"
 nbd_opt_info_go_start(const char *opt, const char *name) "Attempting %s for 
export '%s'"
 nbd_opt_info_go_success(const char *opt) "Export is ready after %s request"
+nbd_opt_info_go_unaligned_size(uint64_t size, uint32_t align) "Server reported 
unaligned size 0x%" PRIx64 " for alignment 0x%x; truncating"
 nbd_opt_info_unknown(int info, const char *name) "Ignoring unknown info %d 
(%s)"
 nbd_opt_info_block_size(uint32_t minimum, uint32_t preferred, uint32_t 
maximum) "Block sizes are 0x%" PRIx32 ", 0x%" PRIx32 ", 0x%" PRIx32
 nbd_receive_query_exports_start(const char *wantname) "Querying export list 
for '%s'"
-- 
2.20.1