linux-ext4 - Re: [heads-up][RFC] ext4_file

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Date:	Sat, 5 Apr 2014 07:53:28 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, Eric Sandeen <sandeen@...hat.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [heads-up][RFC] ext4_file_write() breakage

On Fri, Apr 04, 2014 at 11:15:07PM -0400, Theodore Ts'o wrote:

> Right, but Eric's scneario was talking about unaligned *blocks* not
> *pages*.
> 
> So his scenario was one where the block size was 4k, and the write was
> unaligned with respect to the 4k block size.  For example, if with a
> 4k block size, we had one write starting at offset 0 with a size 512,
> and at the same time another write starting at offset 2048 with a size
> 1024 bytes.  The problem is that we were doing two writes inside the
> same *block*, and so if dio_zero_block() tried to operate on the same
> block at the same time, bad things would happen.
> 
> Does that make sense?

Make the first write take the following iovec array:
	{{good_pointer, 512}, {unmapped_pointer, 4096 - 512}}
and you'll get exact same scenario.   writev() on that iovec is the
same as write(fd, good_pointer, 512).  It certainly should not overwrite
the data at offsets greater than 512.

That's the whole point - it's possible to sneak an equivalent of what
ext4 considers an unaligned write (unaligned wrt fs blocks) past the
check in ext4_unaligned_aio().  You can pad a 512-byte write with
additional iovec segment that will *not* be written (->iov_base points
to something we'd just munmapped), so that the total iovec length looks
good, but passing that to writev()/pwritev()/AIO_PWRITEV will end up
with a short write.
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