Re: [PATCH 3/3] vfs: change sb->s_maxbytes to a loff_t

[Mandeep Singh Baines <msb@google.com>]

Jeff Layton (jlayton@redhat.com) wrote:
> sb->s_maxbytes is supposed to indicate the maximum size of a file that
> can exist on the filesystem. It's declared as an unsigned long long.
> 
> Even if a filesystem has no inherent limit that prevents it from using
> every bit in that unsigned long long, it's still problematic to set it
> to anything larger than MAX_LFS_FILESIZE. There are places in the kernel
> that cast s_maxbytes to a signed value. If it's set too large then this
> cast makes it a negative number and generally breaks the comparison.
> 
> Change s_maxbytes to be loff_t instead. That should help eliminate the
> temptation to set it too large by making it a signed value.
> 
> Also, add a warning for couple of releases to help catch filesystems
> that set s_maxbytes too large. Eventually we can either convert this to
> a BUG() or just remove it and in the hope that no one will get it wrong
> now that it's a signed value.
> 
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  fs/super.c         |   10 ++++++++++
>  include/linux/fs.h |    2 +-
>  2 files changed, 11 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 2761d3e..660d437 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -889,6 +889,16 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
>   	if (error)
>   		goto out_sb;
>  
> +	/*
> +	 * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
> +	 * but s_maxbytes was an unsigned long long for many releases. Throw
> +	 * this warning for a little while to try and catch filesystems that
> +	 * violate this rule. This warning should be either removed or
> +	 * converted to a BUG() in 2.6.34. 
> +	 */
> +	WARN((mnt->mnt_sb->s_maxbytes < 0), "WARNING: %s set sb->s_maxbytes to "
> +		"negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);
> +

Minor nit. "WARNING:" is redundant. I believe WARN() will already print
"WARNING: at <file>:<line>".

>  	mnt->mnt_mountpoint = mnt->mnt_root;
>  	mnt->mnt_parent = mnt;
>  	up_write(&mnt->mnt_sb->s_umount);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 67888a9..bfe3c08 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1316,7 +1316,7 @@ struct super_block {
>  	unsigned long		s_blocksize;
>  	unsigned char		s_blocksize_bits;
>  	unsigned char		s_dirt;
> -	unsigned long long	s_maxbytes;	/* Max file size */
> +	loff_t			s_maxbytes;	/* Max file size */

Looks good to me. Thanks!

>  	struct file_system_type	*s_type;
>  	const struct super_operations	*s_op;
>  	struct dquot_operations	*dq_op;
> -- 
> 1.6.0.6
>