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

[Carlos Maiolino <cmaiolino@redhat.com>]

> MNT=/mnt
> IMG=./DISK.img
> DEV=/dev/loop0
> 

My apologies. I forgot to mention I had ./DISK.img already created in my working
tree, it's a simple

`dd if=/dev/zero of=./DISK.img bs=1m count=20`.

The image should be slighter larger than 16247280, so the FS attempts to step
out of the device.


> mkfs.vfat $IMG
> mount $IMG $MNT
> cp -R /etc $MNT &> /dev/null
> umount $MNT
> 
> losetup -D
> 
> losetup --find --show --sizelimit 16247280 $IMG
> mount $DEV $MNT
> 
> find $MNT -type f -exec cat {} + >/dev/null
> 
> Kudos to Eric Sandeen for coming up with the reproducer above
> 
> Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> ---
> 
> P.S. I'm not 100% proficient in bio internals, so I'm not sure if this is the
> right fix, so comments are much appreciated.
> Thanks
> 
>  fs/buffer.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 784de3dbcf28..32dc5cd2f6ba 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -3063,6 +3063,13 @@ void guard_bio_eod(int op, struct bio *bio)
>  	/* Uhhuh. We've got a bio that straddles the device size! */
>  	truncated_bytes = bio->bi_iter.bi_size - (maxsector << 9);
>  
> +	/*
> +	 * The bio contains more than one segment which spans EOD, just return
> +	 * and let IO layer turn it into an EIO
> +	 */
> +	if (truncated_bytes > PAGE_SIZE)
> +		return;
> +
>  	/* Truncate the bio.. */
>  	bio->bi_iter.bi_size -= truncated_bytes;
>  	bvec->bv_len -= truncated_bytes;
> -- 
> 2.17.2
> 

-- 
Carlos