Re: [PATCH] btrfs: fix fallocate to use file_modified to update permissions consistently

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Mar 11, 2022 at 12:28:58PM +0000, Filipe Manana wrote:
> On Thu, Mar 10, 2022 at 11:22:45AM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > Since the initial introduction of (posix) fallocate back at the turn of
> > the century, it has been possible to use this syscall to change the
> > user-visible contents of files.  This can happen by extending the file
> > size during a preallocation, or through any of the newer modes (punch,
> > zero range).  Because the call can be used to change file contents, we
> > should treat it like we do any other modification to a file -- update
> > the mtime, and drop set[ug]id privileges/capabilities.
> > 
> > The VFS function file_modified() does all this for us if pass it a
> > locked inode, so let's make fallocate drop permissions correctly.
> > 
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> > Note: I plan to add fstests to test this behavior, but after the
> > generic/673 mess, I'm holding back on them until I can fix the three
> > major filesystems and clean up the xfs setattr_copy code.
> > 
> > https://lore.kernel.org/linux-ext4/20220310174410.GB8172@magnolia/T/#u
> > ---
> >  fs/btrfs/file.c |   23 +++++++++++++++++++----
> >  1 file changed, 19 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> > index a0179cc62913..79e61c88b9e7 100644
> > --- a/fs/btrfs/file.c
> > +++ b/fs/btrfs/file.c
> > @@ -2918,8 +2918,9 @@ int btrfs_replace_file_extents(struct btrfs_inode *inode,
> >  	return ret;
> >  }
> >  
> > -static int btrfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> > +static int btrfs_punch_hole(struct file *file, loff_t offset, loff_t len)
> >  {
> > +	struct inode *inode = file_inode(file);
> >  	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> >  	struct btrfs_root *root = BTRFS_I(inode)->root;
> >  	struct extent_state *cached_state = NULL;
> > @@ -2951,6 +2952,10 @@ static int btrfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> >  		goto out_only_mutex;
> >  	}
> >  
> > +	ret = file_modified(file);
> > +	if (ret)
> > +		goto out_only_mutex;
> > +
> >  	lockstart = round_up(offset, btrfs_inode_sectorsize(BTRFS_I(inode)));
> >  	lockend = round_down(offset + len,
> >  			     btrfs_inode_sectorsize(BTRFS_I(inode))) - 1;
> > @@ -3177,11 +3182,12 @@ static int btrfs_zero_range_check_range_boundary(struct btrfs_inode *inode,
> >  	return ret;
> >  }
> >  
> > -static int btrfs_zero_range(struct inode *inode,
> > +static int btrfs_zero_range(struct file *file,
> >  			    loff_t offset,
> >  			    loff_t len,
> >  			    const int mode)
> >  {
> > +	struct inode *inode = file_inode(file);
> >  	struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
> >  	struct extent_map *em;
> >  	struct extent_changeset *data_reserved = NULL;
> > @@ -3202,6 +3208,12 @@ static int btrfs_zero_range(struct inode *inode,
> >  		goto out;
> >  	}
> >  
> > +	ret = file_modified(file);
> > +	if (ret) {
> > +		free_extent_map(em);
> > +		goto out;
> > +	}
> > +
> 
> Could be done before getting the extent map, to make the code a bit shorter, or
> see the comment below.

The trouble is, if getting the extent map fails, we didn't change the
file, so there's no reason to bump the timestamps and whatnot...

> 
> >  	/*
> >  	 * Avoid hole punching and extent allocation for some cases. More cases
> >  	 * could be considered, but these are unlikely common and we keep things
> > @@ -3391,7 +3403,7 @@ static long btrfs_fallocate(struct file *file, int mode,
> >  		return -EOPNOTSUPP;
> >  
> >  	if (mode & FALLOC_FL_PUNCH_HOLE)
> > -		return btrfs_punch_hole(inode, offset, len);
> > +		return btrfs_punch_hole(file, offset, len);
> >  
> >  	/*
> >  	 * Only trigger disk allocation, don't trigger qgroup reserve
> > @@ -3446,7 +3458,7 @@ static long btrfs_fallocate(struct file *file, int mode,
> >  		goto out;
> >  
> >  	if (mode & FALLOC_FL_ZERO_RANGE) {
> > -		ret = btrfs_zero_range(inode, offset, len, mode);
> > +		ret = btrfs_zero_range(file, offset, len, mode);
> >  		btrfs_inode_unlock(inode, BTRFS_ILOCK_MMAP);
> >  		return ret;
> >  	}
> > @@ -3528,6 +3540,9 @@ static long btrfs_fallocate(struct file *file, int mode,
> >  		cur_offset = last_byte;
> >  	}
> >  
> > +	if (!ret)
> > +		ret = file_modified(file);
> 
> If call file_modified() before the if that checks for the zero range case,
> then we avoid having to call file_modified() at btrfs_zero_range() too,
> and get the behaviour for both plain fallocate and zero range.

...and the reason I put it here is to make sure the ordered IO finishes
ok and that we pass the quota limit checks before we start modifying
things.

That said -- you all know btrfs far better than I do, so if you'd rather
I put these calls further up (probably right after the inode_newsize_ok
check?) then I'm happy to do that. :)

--D

> 
> Otherwise, it looks good.
> 
> Thanks for doing this, I had it on my todo list since I noticed the generic/673
> failure with reflinks and the suid/sgid bits.
> 
> > +
> >  	/*
> >  	 * If ret is still 0, means we're OK to fallocate.
> >  	 * Or just cleanup the list and exit.