linux-kernel - Re: [PATCH 5/5] vfs: don't allow writes to swap files

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Date:   Wed, 26 Jun 2019 04:51:51 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     matthew.garrett@...ula.com, yuchao0@...wei.com, tytso@....edu,
        ard.biesheuvel@...aro.org, josef@...icpanda.com, hch@...radead.org,
        clm@...com, adilger.kernel@...ger.ca, jack@...e.com,
        dsterba@...e.com, jaegeuk@...nel.org, jk@...abs.org,
        reiserfs-devel@...r.kernel.org, linux-efi@...r.kernel.org,
        devel@...ts.orangefs.org, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-mm@...ck.org, linux-nilfs@...r.kernel.org,
        linux-mtd@...ts.infradead.org, ocfs2-devel@....oracle.com,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 5/5] vfs: don't allow writes to swap files

On Tue, Jun 25, 2019 at 07:33:31PM -0700, Darrick J. Wong wrote:
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -236,6 +236,9 @@ int notify_change(struct dentry * dentry, struct iattr * attr, struct inode **de
>  	if (IS_IMMUTABLE(inode))
>  		return -EPERM;
>  
> +	if (IS_SWAPFILE(inode))
> +		return -ETXTBSY;
> +
>  	if ((ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID | ATTR_TIMES_SET)) &&
>  	    IS_APPEND(inode))
>  		return -EPERM;

Er...  So why exactly is e.g. chmod(2) forbidden for swapfiles?  Or touch(1),
for that matter...

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 596ac98051c5..1ca4ee8c2d60 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3165,6 +3165,19 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  	if (error)
>  		goto bad_swap;
>  
> +	/*
> +	 * Flush any pending IO and dirty mappings before we start using this
> +	 * swap file.
> +	 */
> +	if (S_ISREG(inode->i_mode)) {
> +		inode->i_flags |= S_SWAPFILE;
> +		error = inode_drain_writes(inode);
> +		if (error) {
> +			inode->i_flags &= ~S_SWAPFILE;
> +			goto bad_swap;
> +		}
> +	}

Why are swap partitions any less worthy of protection?