Re: [PATCH v2] btrfs: fix fallocate to use file_modified to update permissions consistently

[Filipe Manana <fdmanana@kernel.org>]

On Mon, Mar 14, 2022 at 10:55:32AM -0700, 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>
> ---
> v2: move up file_modified to catch a case where we could modify file
> contents but fail on something else before we end up calling
> file_modified


Reviewed-by: Filipe Manana <fdmanana@suse.com>

Thanks, it looks good Darrick.
It also survived a few full fstests runs.

> ---
>  fs/btrfs/file.c |   13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index a0179cc62913..28ddd9cf2069 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;
> @@ -3391,7 +3396,7 @@ 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
> @@ -3413,6 +3418,10 @@ static long btrfs_fallocate(struct file *file, int mode,
>  			goto out;
>  	}
>  
> +	ret = file_modified(file);
> +	if (ret)
> +		goto out;
> +
>  	/*
>  	 * TODO: Move these two operations after we have checked
>  	 * accurate reserved space, or fallocate can still fail but