linux-kernel - Re: RFC: permit link(2) to work across --bind mounts ?

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Message-ID: <47685054.6080603@rtr.ca>
Date:	Tue, 18 Dec 2007 17:57:24 -0500
From:	Mark Lord <lkml@....ca>
To:	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: RFC: permit link(2)  to work across --bind mounts ?

Mark Lord wrote:
> Why does link(2) not support hard-linking across bind mount points
> of the same underlying filesystem ?
> 
> Is it as simple as something like this patch below (minus the printk)?
> Not likely, but then I'm not a filesystem guru.
> 
> ???
> 
> --- old/fs/namei.c    2007-12-15 12:33:13.000000000 -0500
> +++ linux/fs/namei.c    2007-12-18 17:37:04.000000000 -0500
> @@ -2398,8 +2398,11 @@
>     if (error)
>         goto out;
>     error = -EXDEV;
> -    if (old_nd.mnt != nd.mnt)
> -        goto out_release;
> +    if (old_nd.mnt != nd.mnt) {
> +        if (old_nd.mnt->mnt_sb != nd.mnt->mnt_sb)
> +            goto out_release;
> +        printk("sys_linkat: old_nd.mnt != nd.mnt, but sb is the same. 
> Continuing..\n");
> +    }
>     new_dentry = lookup_create(&nd, 0);
>     error = PTR_ERR(new_dentry);
>     if (IS_ERR(new_dentry))
..

The patch seems to work for me after some light testing on ext3 here.
But I have no idea about other filesystems, or if there's some kind of
race condition or something.  Or maybe we just never bothered ?

Cheers
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