Re: Provision for filesystem specific open flags

On 11/13/2017 09:19 PM, Fu, Rodney wrote:

Yes, an ioctl open is possible but not ideal.  The interface would require an
additional open to perform the ioctl against.  The open system call is really a
great place to pass control information to the filesystem and any other solution
seems less elegant.

But with the FS-specific open flag, you would have to do an open call 
with O_PATH, check that the file system is what you expect, and then 
openat the O_PATH descriptor to get a full descriptor.  If you don't 
follow this protocol, you might end up using a custom open flag with a 
different file system which has completely different semantics for the flag.

So ioctl actually is much simpler here and needs fewer system calls.

(Due to per-file bind mounts, there is no way to figure out the file 
system on which a file is located without actually opening the file.)

Thanks,
Florian