linux-kernel - Re: [PATCH] scsi: Silence unnecessary warnings about ioctl to partition

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Date:	Wed, 2 May 2012 16:10:38 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
	LKML <linux-kernel@...r.kernel.org>,
	James Bottomley <JBottomley@...allels.com>,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH] scsi: Silence unnecessary warnings about ioctl to
 partition

> > Also I tend to side with Alan that I don't quite see
> > the point in trying to restrict CAP_SYS_RAWIO threads and thus breaking the
> > compatibility
> 
> For example, we have a customer that wants this:
> 
> * a VM should be able to send vendor-specific commands to a disk via
> SG_IO (vendor-specific commands require CAP_SYS_RAWIO).
> 
> * they want to assign logical volumes or partitions to the same VM
> without letting it read or write outside the logical volume or partition.

And if the process has CAP_SYS_RAWIO it can do it anyway. So not only is
your agenda destructive - it doesn't actually work.

> Of course a better solution for this would be customizable filters for
> SG_IO commands, where a privileged application would open the block
> device with CAP_SYS_RAWIO, set the filter and hand the file descriptor
> to QEMU.  Or alternatively some extension of the device cgroup.  But
> either solution would require a large amount of work.

Or you could just do the special case ioctl magic out of band in the apps.
It's hardly an ultra performance critical path for the SG_IO cases.

Customisable filters are not hard. We've got all the filtering code in
kernel and the ability to verify filters, even the ability to JIT them.
Just support adding/removing/running a BPF filter on the channel in
question.

So it shouldn't be much code to do what you want.

Alan



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