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Re: [Qemu-devel] [PATCH for-2.11] nbd: Don't crash when server reports N

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH for-2.11] nbd: Don't crash when server reports NBD_CMD_READ failure
Date: Mon, 13 Nov 2017 14:37:13 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

12.11.2017 04:39, Eric Blake wrote:
If a server fails a read, for example with EIO, but the connection
is still live, then we would crash trying to print a non-existent
error message.  Bug introduced in commit f140e300.

Signed-off-by: Eric Blake <address@hidden>
  block/nbd-client.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/nbd-client.c b/block/nbd-client.c
index b44d4d4a01..5f3375a970 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -78,7 +78,7 @@ static coroutine_fn void nbd_read_reply_entry(void *opaque)
      while (!s->quit) {
          assert(s->reply.handle == 0);
          ret = nbd_receive_reply(s->ioc, &s->reply, &local_err);


/* nbd_receive_reply
 * Returns 1 on success
 *         0 on eof, when no data was read (errp is not set)
 *         negative errno on failure (errp is set)
int nbd_receive_reply(QIOChannel *ioc, NBDReply *reply, Error **errp)

- looks like errp should be set if ret < 0. may be the function should be fixed? - don't you think that this function returns transferred server error? it doesn't.

-        if (ret < 0) {
+        if (local_err) {
          if (ret <= 0) {
@@ -681,7 +681,7 @@ int nbd_client_co_preadv(BlockDriverState *bs, uint64_t 

      ret = nbd_co_receive_cmdread_reply(client, request.handle, offset, qiov,
-    if (ret < 0) {
+    if (local_err) {
      return ret;

but here, it looks like you are right. My first attempt was to store server error to ret and server error msg to errp (bad idea, as I can see now), but you proposed to do not print every server error msg and deleted this functionality. And now we have a set of functions which can return ret < 0 and not set errp.. It looks very confusing, I think. Better solution is to refactor this somehow,
may be not mixing server-error-reply with local normal errors..

For now, for second chunk:

Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

Best regards,

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