Re: [Qemu-devel] [PATCH 1/4] monitor: Less restrictive fd matching for fd sets

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

On 10/05/2012 02:35 PM, Eric Blake wrote:
> On 10/05/2012 12:07 PM, Corey Bryant wrote:
>> Currently, in order to use a file descriptor that resides in an
>> fd set, the access mode flag of the qemu_open() call has to be
>> an exact match of the access mode flag of an fd in the requested
>> fd set.
>>
>> This patch lightens up that restriction.  For example, if QEMU
>> attempts to qemu_open() "/dev/fdset/2" with O_RDONLY or O_WRONLY,
>> and an fd in fd set 2 has O_RDWR, the call will now be successful.
>
> I was right for wondering if we'd need this, back when fdsets were first
> implemented. :)

Yeah yeah yeah :)

>
> However...
>
>>
>> Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
>> ---
>>   monitor.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/monitor.c b/monitor.c
>> index a0e3ffb..34a968c 100644
>> --- a/monitor.c
>> +++ b/monitor.c
>> @@ -2304,7 +2304,8 @@ int monitor_fdset_get_fd(int64_t fdset_id, int flags)
>>                   return -1;
>>               }
>>
>> -            if ((flags & O_ACCMODE) == (mon_fd_flags & O_ACCMODE)) {
>> +            if ((mon_fd_flags & O_ACCMODE) == O_RDWR ||
>> +                (mon_fd_flags & O_ACCMODE) == (flags & O_ACCMODE)) {
>>                   return mon_fdset_fd->fd;
>>               }
>
> Is this always the right fd to return?  Suppose I create a set that
> contains two fds, one O_RDONLY and another O_RDWR; and that the current
> qemu_open() is requesting O_RDONLY.  It feels like we would want to get
> the O_RDONLY fd returned, even if it comes later in the set, since it is
> nominally safer.
>
> That is, I wonder if this should really be implemented as an iteration
> over the set that selects the best possible match, where more than one
> fd can match, but at the end of the iteration, we have the closest match
> possible, rather than the first (possibly inexact) match.
>

Actually I think we can go back to requiring an exact match if we use 
your new proposal of adding a new command that allows multiple fd's to 
be added to an fd set over the command line.

-- 
Regards,
Corey Bryant