Re: [PATCH 1/5] truncate: Zero bytes after 'oldsize' if we're expanding the file

[Brian Foster <bfoster@redhat.com>]

On Thu, Feb 02, 2023 at 08:44:23PM +0000, Matthew Wilcox (Oracle) wrote:
> POSIX requires that "If the file size is increased, the extended area
> shall appear as if it were zero-filled".  It is possible to use mmap to
> write past EOF and that data will become visible instead of zeroes.
> This fixes the problem for the filesystems which simply call
> truncate_setsize().  More complex filesystems will need their own
> patches.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/truncate.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/truncate.c b/mm/truncate.c
> index 7b4ea4c4a46b..cebfc5415e9a 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -763,9 +763,12 @@ void truncate_setsize(struct inode *inode, loff_t newsize)
>  	loff_t oldsize = inode->i_size;
>  
>  	i_size_write(inode, newsize);
> -	if (newsize > oldsize)
> +	if (newsize > oldsize) {
>  		pagecache_isize_extended(inode, oldsize, newsize);
> -	truncate_pagecache(inode, newsize);
> +		truncate_pagecache(inode, oldsize);
> +	} else {
> +		truncate_pagecache(inode, newsize);
> +	}

I don't think this alone quite addresses the problem. Looking at ext4
for example, if the eof page is dirty and writeback occurs between the
i_size update (because writeback also zeroes the post-eof portion of the
page) and the truncate_setsize() call, we end up with pagecache
inconsistency because pagecache truncate doesn't dirty the page it
zeroes.

So for example, with this series plus a nefariously placed
filemap_flush() in ext4_setattr():

# xfs_io -fc "truncate 1" -c "mmap 0 1k" -c "mwrite 0 10" -c "truncate 5" -c "mread -v 0 5" /mnt/file
00000000:  58 00 00 00 00  X....
# umount /mnt/; mount <dev> /mnt/
# xfs_io -c "mmap 0 1k" -c "mread -v 0 5" /mnt/file 
00000000:  58 58 58 58 58  XXXXX

Brian

>  }
>  EXPORT_SYMBOL(truncate_setsize);
>  
> -- 
> 2.35.1
> 
>