linux-kernel - Re: [PATCH 1/2] proc: do PDE usecounting even for ->read_proc, ->write

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Message-ID: <20090128132415.6ff52584@tleilax.poochiereds.net>
Date:	Wed, 28 Jan 2009 13:24:15 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] proc: do PDE usecounting even for ->read_proc,
 ->write_proc

On Tue, 27 Jan 2009 02:31:51 +0300
Alexey Dobriyan <adobriyan@...il.com> wrote:

> Not exactly tested but one should get the idea.
> 
> 
> 
> -----------------------------------------------
> struct proc_dir_entry::owner is going to be removed. Now it's only necessary
> to protect PDEs which are using ->read_proc, ->write_proc hooks.
> 
> However, ->owner assignments are racy and make it very easy for someone to switch
> ->owner on live PDE (as some subsystems do) without fixing refcounts and so on.
> 
> http://bugzilla.kernel.org/show_bug.cgi?id=12454
> 
> So, ->owner is on death row.
> 
> Proxy file operations exist already (proc_file_operations), just bump usecount
> when necessary.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
>  fs/proc/generic.c  |   48 ++++++++++++++++++++++++++++++++++++++----------
>  fs/proc/inode.c    |    2 +-
>  fs/proc/internal.h |    1 +
>  3 files changed, 40 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/proc/generic.c b/fs/proc/generic.c
> index 5ee636a..65cc1ee 100644
> --- a/fs/proc/generic.c
> +++ b/fs/proc/generic.c
> @@ -37,7 +37,7 @@ static int proc_match(int len, const char *name, struct proc_dir_entry *de)
>  #define PROC_BLOCK_SIZE	(PAGE_SIZE - 1024)
>  
>  static ssize_t
> -proc_file_read(struct file *file, char __user *buf, size_t nbytes,
> +__proc_file_read(struct file *file, char __user *buf, size_t nbytes,
>  	       loff_t *ppos)
>  {
>  	struct inode * inode = file->f_path.dentry->d_inode;
> @@ -183,19 +183,47 @@ proc_file_read(struct file *file, char __user *buf, size_t nbytes,
>  }
>  
>  static ssize_t
> +proc_file_read(struct file *file, char __user *buf, size_t nbytes,
> +	       loff_t *ppos)
> +{
> +	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
> +	ssize_t rv = -EIO;
> +
> +	spin_lock(&pde->pde_unload_lock);
> +	if (!pde->proc_fops) {
> +		spin_unlock(&pde->pde_unload_lock);
> +		return rv;
> +	}
> +	pde->pde_users++;
> +	spin_unlock(&pde->pde_unload_lock);
> +
> +	rv = __proc_file_read(file, buf, nbytes, ppos);
> +
> +	pde_users_dec(pde);
> +	return rv;
> +}
> +
> +static ssize_t
>  proc_file_write(struct file *file, const char __user *buffer,
>  		size_t count, loff_t *ppos)
>  {
> -	struct inode *inode = file->f_path.dentry->d_inode;
> -	struct proc_dir_entry * dp;
> -	
> -	dp = PDE(inode);
> -
> -	if (!dp->write_proc)
> -		return -EIO;
> +	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
> +	ssize_t rv = -EIO;
> +
> +	if (pde->write_proc) {
> +		spin_lock(&pde->pde_unload_lock);
> +		if (!pde->proc_fops) {
> +			spin_unlock(&pde->pde_unload_lock);
> +			return rv;
> +		}
> +		pde->pde_users++;
> +		spin_unlock(&pde->pde_unload_lock);
>  
> -	/* FIXME: does this routine need ppos?  probably... */
> -	return dp->write_proc(file, buffer, count, dp->data);
> +		/* FIXME: does this routine need ppos?  probably... */
> +		rv = pde->write_proc(file, buffer, count, pde->data);
> +		pde_users_dec(pde);
> +	}
> +	return rv;
>  }
>  
>  
> diff --git a/fs/proc/inode.c b/fs/proc/inode.c
> index 0bbb937..7a55346 100644
> --- a/fs/proc/inode.c
> +++ b/fs/proc/inode.c
> @@ -127,7 +127,7 @@ static void __pde_users_dec(struct proc_dir_entry *pde)
>  		complete(pde->pde_unload_completion);
>  }
>  
> -static void pde_users_dec(struct proc_dir_entry *pde)
> +void pde_users_dec(struct proc_dir_entry *pde)
>  {
>  	spin_lock(&pde->pde_unload_lock);
>  	__pde_users_dec(pde);
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index ab62ede..5b890ba 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -102,5 +102,6 @@ struct pde_opener {
>  	int (*release)(struct inode *, struct file *);
>  	struct list_head lh;
>  };
> +void pde_users_dec(struct proc_dir_entry *pde);
>  
>  extern struct list_head proc_automounts;

I did some testing with this today and it seems to all work as expected. Nice work, Alexey.

Tested-by: Jeff Layton <jlayton@...hat.com>
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