Re: [RFC PATCH 0/2] dm crypt: Allow unaligned buffer lengths for skcipher devices

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 24 Sep 2020, Sudhakar Panneerselvam wrote:

> Hi Mikulas,
> 
> > > Windows Guest <--> Vhost-Scsi <--> LIO(scsi/target/blockio) <-->  dm-crypt
> > <--> iSCSI block device
> > >
> > > One real example out of my debugging: Windows sends a I/O request with
> > > 6656 bytes to vhost-scsi interface. Vhost-scsi uses translate_desc() in
> > > drivers/vhost/vhost.c to convert windows user space memory buffers to
> > > kernel iovecs. Vhost-scsi then converts the iovecs to sg entries in
> > > vhost_scsi_mapal() which is then handed over to "target" subsystem and
> > > eventually submitted to dm-crypt. This 6656 bytes IO has got 3 segments,
> > > first segment had 1584, second 4096 and the last had 976 bytes. Dm-crypt
> > > rejects the I/O after seeing the first segment length 1584 which is not
> > > a 512 byte multiple.
> > >
> > > Let me know if there are further questions.
> > >
> > > Thanks
> > > Sudhakar
> > 
> > Hi
> > 
> > I think it should be fixed in vhost-scsi.
> 
> In the above example of 6656 bytes I/O, windows allocates 6656 bytes 
> virtually contiguous I/O. This IO, when it lands in the kernel, 
> translates to 3 physically discontiguous pages, that's why 
> translate_desc() had to create 3 iovecs to handle this I/O. I don't 
> understand how vhost-scsi could have solved this issue.

By copying it to a temporary aligned buffer and issuing I/O on this 
buffer.

> Only other 
> possibility I see is to have windows fix it by always sending 512 byte 
> aligned buffer lengths, but going with my earlier point that every other 
> component in the Linux IO path handles this case well except for 
> dm-crypt, so it make more sense to fix it in dm-crypt.
> 
> Thanks
> Sudhakar

Are you sure that the problem is only with dm-crypt? You haven't tried all 
the existing block device drivers, have you?

Mikulas