linux-kernel - Re: [PATCH 3/4] vfs: change sb->s_maxbytes to a loff

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Message-ID: <20090807225146.GC28019@cmpxchg.org>
Date:	Sat, 8 Aug 2009 00:51:46 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, hch@...radead.org, rlove@...gle.com,
	msb@...gle.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH 3/4] vfs: change sb->s_maxbytes to a loff_t

On Fri, Aug 07, 2009 at 02:57:40PM -0400, Jeff Layton 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. A lot of places implicitly
> cast s_maxbytes to a signed value when doing comparisons against it
> (usually using loff_t on the other side of the comparison). 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.
> 
> Signed-off-by: Jeff Layton <jlayton@...hat.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..929d55d 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 can be removed in 2.6.34.
> +	 */
> +	WARN(((unsigned long long) mnt->mnt_sb->s_maxbytes > MAX_LFS_FILESIZE),
> +		"WARNING: %s sets sb->s_maxbytes too large (%llu)", type->name,
> +		(unsigned long long) mnt->mnt_sb->s_maxbytes);

Since it's signed now, you could just check for it being < 0, no?

I don't like the warning much, though.  It seems to be a random check
for a bug that has been fixed now.  We don't check other errors from
->get_sb() either.
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