Re: [Qemu-devel] [PATCH v2 5/6] nbd/server: Add helper functions for parsing option payload

[Eric Blake <eblake@redhat.com>]

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On 01/12/2018 04:20 AM, Vladimir Sementsov-Ogievskiy wrote:
> 11.01.2018 02:08, Eric Blake wrote:
>> Rather than making every callsite perform length sanity checks
>> and error reporting, add the helper functions nbd_opt_read()
>> and nbd_opt_drop() that use the length stored in the client
>> struct; also add an assertion that optlen is reduced to zero
>> after each option is handled.
>>
>> Note that the call in nbd_negotiate_handle_export_name() does
>> not use the new helper (in part because the server cannot
>> reply to NBD_OPT_EXPORT_NAME - it either succeeds or the
>> connection drops).
>>

>>
>> +/* Drop remainder of the current option, after sending a reply with
> 
> looks a bit weird: actually you drop the remainder _before_ sending a
> reply)

Good catch. I'll fix it with s/after/and/

> 
>> + * the given error type and message. Return -errno on read or write
> 
> also, unrelated note, -errno is always forced to -EIO, because of
> nbd_read realization.
> and this note applies to many other places here. It is correct (EIO is
> errno, why not?),
> but it may be not bad to note it somewhere..

Someday nbd_read() might fail with something other than EIO (ESHUTDOWN,
perhaps?), in which case leaving this documented as -errno would be
better than hardcoding that EIO is the only failure for now.

>> @@ -812,14 +819,9 @@ static int nbd_negotiate_options(NBDClient
>> *client, uint16_t myflags,
>>                   break;
>>
>>               default:
>> -                if (nbd_drop(client->ioc, length, errp) < 0) {
>> -                    return -EIO;
>> -                }
>> -                ret = nbd_negotiate_send_rep_err(client,
>> -                                                 NBD_REP_ERR_UNSUP,
>> errp,
>> -                                                 "Unsupported option
>> 0x%"
>> -                                                 PRIx32 " (%s)", option,
>> -                                                
>> nbd_opt_lookup(option));
>> +                ret = nbd_opt_drop(client, NBD_REP_ERR_UNSUP, errp,
>> +                                   "Unsupported option 0x%" PRIx32 "
>> (%s)",
>> +                                   option, nbd_opt_lookup(option));
>>                   break;
>>               }
>>           } else {
>> @@ -842,6 +844,7 @@ static int nbd_negotiate_options(NBDClient
>> *client, uint16_t myflags,
>>           if (ret < 0) {
>>               return ret;
>>           }
>> +        assert(!client->optlen);
> 
> isn't it from 2/6?

No, this is a second instance of the assertion, the one that applies
between each option (which I couldn't do in 2/6 because not all options
were manipulating optlen back then).  Maybe I can tweak the commit
messages to make that more obvious.

> 
>>       }
>>   }
>>
> 
> anyway,
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Thanks for the careful attention to detail.


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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