linux-kernel - Re: [PATCH] fs: allow do_renameat2() over bind mounts of the same filesystem.

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Message-ID: <87wo1hf8o9.fsf@platform.sh>
Date:   Sat, 29 Aug 2020 23:23:34 +0200
From:   Florian Margaine <florian@...tform.sh>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: allow do_renameat2() over bind mounts of the same filesystem.

Al Viro <viro@...iv.linux.org.uk> writes:

> On Fri, Aug 28, 2020 at 10:40:35PM +0200, Florian Margaine wrote:
>> There's currently this seemingly unnecessary limitation that rename()
>> cannot work over bind mounts of the same filesystem,
>
> ... is absolutely deliberate - that's how you set a boundary in the
> tree, preventing both links and renames across it.

Sorry, I'm not not sure I understand what you're saying.

As I understand it, the tree is the superblock there, not the mount. As
in, the dentries are relative to the superblock, and the mountpoint is
no more than a pointer to a superblock's dentry.

In addition, I noticed this snippet in fs/read_write.c:

    /*
     * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
     * the same mount. Practically, they only need to be on the same file
     * system.
     */
    if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
        return -EXDEV;

Which seems to confirm my understanding.

What am I getting wrong there?

>
> Incidentally, doing that would have fun effects for anyone with current
> directory inside the subtree you'd moved - try and see.

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