Re: [PATCH for-9.1 v2 02/11] libvhost-user: fail vu_message_write() if sendmsg() is failing

[David Hildenbrand <david@redhat.com>]

On 26.03.24 15:34, Eric Blake wrote:
> On Tue, Mar 26, 2024 at 02:39:27PM +0100, Stefano Garzarella wrote:
>> In vu_message_write() we use sendmsg() to send the message header,
>> then a write() to send the payload.
>>
>> If sendmsg() fails we should avoid sending the payload, since we
>> were unable to send the header.
>>
>> Discovered before fixing the issue with the previous patch, where
>> sendmsg() failed on macOS due to wrong parameters, but the frontend
>> still sent the payload which the backend incorrectly interpreted
>> as a wrong header.
>>
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>>   subprojects/libvhost-user/libvhost-user.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/subprojects/libvhost-user/libvhost-user.c b/subprojects/libvhost-user/libvhost-user.c
>> index 22bea0c775..a11afd1960 100644
>> --- a/subprojects/libvhost-user/libvhost-user.c
>> +++ b/subprojects/libvhost-user/libvhost-user.c
>> @@ -639,6 +639,11 @@ vu_message_write(VuDev *dev, int conn_fd, VhostUserMsg *vmsg)
>>           rc = sendmsg(conn_fd, &msg, 0);
>>       } while (rc < 0 && (errno == EINTR || errno == EAGAIN));
>>   
>> +    if (rc <= 0) {
> 
> Is rejecting a 0 return value correct?  Technically, a 0 return means
> a successful write of 0 bytes - but then again, it is unwise to
> attempt to send an fd or other auxilliary ddata without at least one
> regular byte, so we should not be attempting a write of 0 bytes.  So I
> guess this one is okay, although I might have used < instead of <=.

I was wondering if we could see some partial sendmsg()/write succeeding. 
Meaning, we transferred some bytes but not all, and we'd actually need 
to loop ...

-- 
Cheers,

David / dhildenb