linux-kernel - Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of blocks

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Date:	Fri, 11 Mar 2016 13:59:52 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>, Gregory Farnum <greg@...gs42.com>,
	Dave Chinner <david@...morbit.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	shane.seymour@....com, Bruce Fields <bfields@...ldses.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	Eric Sandeen <esandeen@...hat.com>
Subject: Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range
 of blocks

> > We can do the security check at the filesystem level, because we have
> > sb->s_bdev->bd_inode, and if you have read and write permissions to
> > that inode, you might as well have permission to create a unsafe hole.

Not if you don't have access to a block device node to open it, or there
are SELinux rules that control the access. There are cases it isn't
entirely the same thing as far as I can see. Consider within a container
for example.

The paranoid approach would IMHO to have a mount option so you can
explicitly declare a file system mount should trust its owner/group and
then that can also be used to wire up any other "unsafe" activities in a
general "mounted for a special use" option.

Alan