Re: [PATCH 2/2] monitor: Report EBADFD if fdset contains invalid FD

["Michal Prívozník" <mprivozn@redhat.com>]

On 8/17/21 11:59 AM, Marc-André Lureau wrote:
> Hi
> 
> On Tue, Aug 17, 2021 at 12:56 PM Michal Privoznik <mprivozn@redhat.com>
> wrote:
> 
>> When opening a path that starts with "/dev/fdset/" the control
>> jumps into qemu_parse_fdset() and then into
>> monitor_fdset_dup_fd_add(). In here, corresponding fdset is found
>> and then all FDs from the set are iterated over trying to find an
>> FD that matches expected access mode. For instance, if caller
>> wants O_WRONLY then the FD set has to contain an O_WRONLY FD.
>>
>> If no such FD is found then errno is set to EACCES which results
>> in very misleading error messages, for instance:
>>
>>   Could not dup FD for /dev/fdset/3 flags 441: Permission denied
>>
>> There is no permission issue, the problem is that there was no FD
>> within given fdset that was in expected access mode. Therefore,
>> let's set errno to EBADFD, which gives us somewhat better
>> error messages:
>>
>>   Could not dup FD for /dev/fdset/3 flags 441: File descriptor in bad state
>>
>> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
>>
> 
> I am not sure this is any better. If you try to open a read-only file, the
> system also reports EACCES (Permission denied). This is what the current
> code models, I believe.

Fair enough. Another idea I had was that if an FD that's O_RDWR was
passed but only read or only write access was requested then such FD
could be accepted because it is capable of reading/writing.

But since patch 1/2 was accepted then I guess 2/2 is not that much needed.

Michal