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Date: Wed, 26 Jun 2019 05:11:33 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH 2/5] vfs: create a generic checking function for
 FS_IOC_FSSETXATTR
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To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: matthew.garrett@nebula.com, yuchao0@huawei.com, tytso@mit.edu, shaggy@kernel.org, ard.biesheuvel@linaro.org, josef@toxicpanda.com, hch@infradead.org, clm@fb.com, adilger.kernel@dilger.ca, jk@ozlabs.org, jack@suse.com, dsterba@suse.com, jaegeuk@kernel.org, cluster-devel@redhat.com, jfs-discussion@lists.sourceforge.net, linux-efi@vger.kernel.org, Jan Kara <jack@suse.cz>, reiserfs-devel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-xfs@vger.kernel.org, linux-nilfs@vger.kernel.org, linux-mtd@lists.infradead.org, ocfs2-devel@oss.oracle.com, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org

On Tue, Jun 25, 2019 at 07:32:18PM -0700, Darrick J. Wong wrote:
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -373,10 +373,9 @@ static int check_xflags(unsigned int flags)
>  static int btrfs_ioctl_fsgetxattr(struct file *file, void __user *arg)
>  {
>  	struct btrfs_inode *binode = BTRFS_I(file_inode(file));
> -	struct fsxattr fa;
> -
> -	memset(&fa, 0, sizeof(fa));
> -	fa.fsx_xflags = btrfs_inode_flags_to_xflags(binode->flags);
> +	struct fsxattr fa = {
> +		.fsx_xflags = btrfs_inode_flags_to_xflags(binode->flags),
> +	};

Umm...  Sure, there's no padding, but still - you are going to copy that thing
to userland...  How about

static inline void simple_fill_fsxattr(struct fsxattr *fa, unsigned xflags)
{
	memset(fa, 0, sizeof(*fa));
	fa->fsx_xflags = xflags;
}

and let the compiler optimize the crap out?