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

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
> 
>

On Fri, Feb 03, 2023 at 08:00:16AM -0500, Brian Foster wrote:
> 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

Hm, so switch the order of i_size_write() and truncate_pagecache()?
There could still be a store between old-EOF and new-EOF from another
thread, which would then be visible, but I don't think you could prove
that store should have been zeroed.  Not from the thread doing the
ftruncate() anyway -- I think the thread doing the store could prove
it, but that thread is relying on undefined behaviour anyway.