Re: [PATCH] fs: fix guard_bio_eod to check for real EOD errors

[Dave Chinner <david@fromorbit.com>]

On Sat, Feb 23, 2019 at 07:33:21AM +0800, Ming Lei wrote:
> Hi Carlos,
> 
> Cc block list given it is related with interface between fs and block layer.
> 
> On Fri, Feb 22, 2019 at 10:14 PM Carlos Maiolino <cmaiolino@redhat.com> wrote:
> >
> > guard_bio_eod() can truncate a segment in bio to allow it to do IO on
> > odd last sectors of a device.
> >
> > It already checks if the IO starts past EOD, but it does not consider
> > the possibility of an IO request starting within device boundaries can
> > contain more than one segment past EOD.
> >
> > In such cases, truncated_bytes can be bigger than PAGE_SIZE, and will
> > underflow bvec->bv_len.
> 
> It can cause memory corruption even for < PAGE_SIZE, also it can be correct
> to see > PAGE_SIZE truncated_bytes:
> 
> - xfs is going to support big block size which may be 64k

FYI, this isn't an XFS problem and never will be - XFS doesn't use
bufferheads and mpage_readpages() anymore, it goes down the
iomap_readpages() path which does not need this whacky
guard_bio_eod() thingy.

> - suppose fs block size is 4k, bio sector is 1022 and size is 4k, and
> disk size is
> 1024,

XFS won't do that, either - it checks at mount time if it can read
the very last sector of the filesystem via uncached IO (see
xfs_check_sizes() and xfs_rtmount_init()). If any of the EOD reads
fail, it won't mount.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com