linux-kernel - Re: [PATCH] procfs: make /proc style symlinks behave like "normal" symlinks

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Date:	Thu, 19 Nov 2009 09:07:16 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeff Layton <jlayton@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	pavel@....cz
Subject: Re: [PATCH] procfs: make /proc style symlinks behave like "normal" symlinks


Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

This is broken.  If the referenced file is in a different mount namespace
the path returned could point to a completely different path in your
own mount namespace.  Even in your own mount namespace this makes the
proc symlinks racy and not guaranteed to return the file of interest.

I don't see any hope of this approach ever working.

Eric


Jeff Layton <jlayton@...hat.com> writes:

> -static int proc_pid_readlink(struct dentry * dentry, char __user * buffer, int buflen)
> -{
> -	int error = -EACCES;
> -	struct inode *inode = dentry->d_inode;
> +	struct page *page = NULL;
>  	struct path path;
> +	int error;
>  
>  	/* Are we allowed to snoop on the tasks file descriptors? */
> -	if (!proc_fd_access_allowed(inode))
> +	if (!proc_fd_access_allowed(inode)) {
> +		pathname = ERR_PTR(-EACCES);
>  		goto out;
> +	}
>  
>  	error = PROC_I(inode)->op.proc_get_link(inode, &path);
> -	if (error)
> +	if (error) {
> +		pathname = ERR_PTR(error);
>  		goto out;
> +	}
> +
> +	page = alloc_page(GFP_HIGHUSER);
> +	if (!page) {
> +		pathname = ERR_PTR(-ENOMEM);
> +		goto out_path_put;
> +	}
> +
> +	pathname = kmap(page);
> +	pathname = d_path(&path, pathname, PAGE_SIZE);

This is just nonsense.


> +	if (IS_ERR(pathname)) {
> +		kunmap(page);
> +		__free_page(page);
> +		page = NULL;
> +	}
>  
> -	error = do_proc_readlink(&path, buffer, buflen);
> +out_path_put:
>  	path_put(&path);
>  out:
> -	return error;
> +	nd_set_link(nd, pathname);
> +	return page;
> +}
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