[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [PATCH v2 12/22] nbd/client: Improve error handling in

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH v2 12/22] nbd/client: Improve error handling in nbd_negotiate_simple_meta_context()
Date: Thu, 20 Dec 2018 14:13:54 +0000

17.12.2018 18:30, Eric Blake wrote:
> On 12/15/18 9:19 AM, Richard W.M. Jones wrote:
>> On Sat, Dec 15, 2018 at 07:53:14AM -0600, Eric Blake wrote:
>>> Always allocate space for the reply returned by the server and
>>> hoist the trace earlier, as it is more interesting to trace the
>>> server's reply (even if it is unexpected) than parroting our
>>> request only on success.  After all, skipping the allocation
>>> for a wrong size was merely a micro-optimization that only
>>> benefitted a broken server, rather than the common case of a
>>> compliant server that meets our expectations.
>>> Then turn the reply handling into a loop (even though we still
>>> never iterate more than once), to make this code easier to use
>>> when later patches do support multiple server replies.  This
>>> changes the error message for a server with two replies (a
>>> corner case we are unlikely to hit in practice) from:
>>> Unexpected reply type 4 (meta context), expected 0 (ack)
>>> to:
>>> Server replied with more than one context
>>> Signed-off-by: Eric Blake <address@hidden>
>>> ---
>>> v2: split patch into easier-to-review pieces [Rich, Vladimir]
>>> ---
>>>   nbd/client.c | 16 ++++++++++++----
>>>   1 file changed, 12 insertions(+), 4 deletions(-)
>>> diff --git a/nbd/client.c b/nbd/client.c
>>> index bcccd5f555e..b6a85fc3ef8 100644
>>> --- a/nbd/client.c
>>> +++ b/nbd/client.c
>>> @@ -684,10 +684,11 @@ static int 
>>> nbd_negotiate_simple_meta_context(QIOChannel *ioc,
>>>           return ret;
>>>       }
>>> -    if (reply.type == NBD_REP_META_CONTEXT) {
>>> +    while (reply.type == NBD_REP_META_CONTEXT) {
>> I'm not sure I understand why this change is safe.
>> As far as I can see reply.type is only updated in the loop by
>> nbd_receive_option_reply, and that reads from the server, and so the
>> server might keep sending NBD_REP_META_CONTEXT packets (instead of the
>> expected NBD_REP_ACK), so it could now loop forever against a
>> malicious server?  (This is not taking into account any later patches)
> Hmm - now that I've already responded to why the conversion to a loop does 
> not change this code, I'm now wondering if I even need this patch. In v1 of 
> the series, both SET and LIST shared a common function, and since LIST needs 
> the loop, converting SET to use a loop that exits early if it executes more 
> than once was needed to make the two actions share a common entry point.  But 
> since v2 uses different entry points (because it separated the common code 
> into separate helper functions, leaving the SET entry point unchanged and 
> adding a new LIST entry point), where only the LIST entry point actually has 
> to loop, I might be able to just drop this patch entirely and still achieve 
> the same effect.

Are you going to resend series without this patch? In this case I'd prefer to 
continue review on already rebased patches. Or it doesn't make much difference?

Best regards,

reply via email to

[Prev in Thread] Current Thread [Next in Thread]