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From: Alexey Dobriyan <adobriyan@openvz.org>
To: Andrew Morton <akpm@osdl.org>
Cc: Al Viro <viro@ftp.linux.org.uk>,
	linux-kernel@vger.kernel.org, devel@openvz.org
Subject: Re: [PATCH] Fix rmmod/read/write races in /proc entries
Date: Wed, 24 Jan 2007 18:22:51 +0300	[thread overview]
Message-ID: <20070124152251.GA11116@localhost.sw.ru> (raw)
In-Reply-To: <20070123125801.7982c23f.akpm@osdl.org>

On Tue, Jan 23, 2007 at 12:58:01PM -0800, Andrew Morton wrote:
> <head spins>
>
> Looks a bit hacky.  Can this race not be fixed by addition of suitable
> locking, or possibly refcounting-under-locking?

I'll think about it.

> > @@ -76,6 +77,12 @@ proc_file_read(struct file *file, char _
> >  	if (!(page = (char*) __get_free_page(GFP_KERNEL)))
> >  		return -ENOMEM;
> >
> > +	if (!dp->proc_fops)
> > +		goto out_free;
> > +	atomic_inc(&dp->pde_users);
> > +	if (!dp->proc_fops)
> > +		goto out_dec;
> > +
>
> You'll be shocked to know that I'd prefer more comments in there.  Enough
> for a later maintainer to be able to understand what's going on.

Here is replacement patch with rewritten changelog and comments in
place. HTH.



[PATCH] Fix rmmod/read/write races in /proc entries

Current /proc creation interfaces suffer from at least two types of races:
--------------------------------------------------------
1. Write via ->write_proc sleeps in copy_from_user(). Module disappears
   meanwhile.

   pde = create_proc_entry()
   if (!pde)
	return -ENOMEM;
   pde->write_proc = ...
				open
				write
				copy_from_user
   pde = create_proc_entry();
   if (!pde) {
	remove_proc_entry();
	return -ENOMEM;
	/* module unloaded */
   }
				*boom*
--------------------------------------------------------
2. Read/write happens when PDE only partially initialized. ->data is NULL
   when create_proc_entry() returns. Almost all ->read_proc and
   ->write_proc handlers assume that ->data is valid.

   pde = create_proc_entry();
   if (pde) {
	/* which dereferences ->data */
	pde->write_proc = ...
				open
				write
	pde->data = ...
   }
--------------------------------------------------------

The following plan is going to be executed (as per Al Viro's explanations):

PDE gets atomic counter counting reads and writes in progress done
via ->read_proc, ->write_proc, ->get_info . Generic proc code will bump
PDE's counter before calling into module-specific method and decrement
it after it returns.

remove_proc_entry() will wait until all readers and writers are done.
To do this reliably it will set ->proc_fops to NULL and generic proc
code won't call into module it it sees NULL ->proc_fops.

This patch implements part above. So far, no changes in proc users
required. Patch fixes races of type 1.



Unfortunately, fixing races of type #2 will require changing in some modules.

We need an indicator of PDE readinness of accepting reads and writes.
->proc_fops nicely fits. It is going to get new semantics:

* if ->proc_fops is valid, PDE will accept reads and writes via ->read_proc,
  ->write_proc, ->get_info.
* if ->proc_fops is NULL, PDE won't call into module's code.

remove_proc_entry() and only remove_proc_entry() will clear ->proc_fops.
create_proc_entry() will not set ->proc_fops. Helpers will be required.

Helpers will set ->proc_fops last (after ->data, particularly).

	set_proc_entry_data_fops(pde, data, fops);
	set_proc_entry_data_read_write(pde, data, read_proc, write_proc);

When all necessary helpers will be plugged, create_proc_entry will stop
setting default proc_fops and helpers will start setting it. Races of
type #2 will be fixed.

If module sets ->proc_fops only, or uses create_proc_read_entry(), or uses
create_proc_info_entry(), there won't be any changes for module.
---

 fs/proc/generic.c       |   61 +++++++++++++++++++++++++++++++++++++++++++++---
 include/linux/proc_fs.h |   15 +++++++++++
 2 files changed, 73 insertions(+), 3 deletions(-)

--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -19,6 +19,7 @@ #include <linux/init.h>
 #include <linux/idr.h>
 #include <linux/namei.h>
 #include <linux/bitops.h>
+#include <linux/delay.h>
 #include <linux/spinlock.h>
 #include <asm/uaccess.h>
 
@@ -76,6 +77,25 @@ proc_file_read(struct file *file, char _
 	if (!(page = (char*) __get_free_page(GFP_KERNEL)))
 		return -ENOMEM;
 
+	if (!dp->proc_fops)
+		/*
+		 * remove_proc_entry() marked PDE as "going away".
+		 * No new readers allowed.
+		 */
+		goto out_free;
+	/*
+	 * We are going to call into module's code via ->get_info or
+	 * ->read_proc. Bump refcount so that remove_proc_entry() will
+	 * wait for read to complete.
+	 */
+	atomic_inc(&dp->pde_users);
+	if (!dp->proc_fops)
+		/*
+		 * While we're busy bumping refcount, remove_proc_entry()
+		 * marked PDE as "going away". Obey.
+		 */
+		goto out_dec;
+
 	while ((nbytes > 0) && !eof) {
 		count = min_t(size_t, PROC_BLOCK_SIZE, nbytes);
 
@@ -195,6 +215,9 @@ proc_file_read(struct file *file, char _
 		buf += n;
 		retval += n;
 	}
+out_dec:
+	atomic_dec(&dp->pde_users);
+out_free:
 	free_page((unsigned long) page);
 	return retval;
 }
@@ -205,14 +228,33 @@ proc_file_write(struct file *file, const
 {
 	struct inode *inode = file->f_path.dentry->d_inode;
 	struct proc_dir_entry * dp;
+	ssize_t rv;
 	
 	dp = PDE(inode);
 
 	if (!dp->write_proc)
 		return -EIO;
+	/*
+	 * remove_proc_entry() marked PDE as "going away".
+	 * No new writers allowed.
+	 */
+	if (!dp->proc_fops)
+		return -EIO;
 
-	/* FIXME: does this routine need ppos?  probably... */
-	return dp->write_proc(file, buffer, count, dp->data);
+	rv = -EIO;
+	/*
+	 * We are going to call into module's code via ->write_proc .
+	 * Bump refcount so that module won't dissapear while ->write_proc
+	 * sleeps in copy_from_user(). remove_proc_entry() will wait for
+	 * write to complete.
+	 */
+	atomic_inc(&dp->pde_users);
+	if (dp->proc_fops)
+		/* PDE is ready, refcount bumped, call into module. */
+		/* FIXME: does this routine need ppos?  probably... */
+		rv = dp->write_proc(file, buffer, count, dp->data);
+	atomic_dec(&dp->pde_users);
+	return rv;
 }
 
 
@@ -717,12 +759,25 @@ void remove_proc_entry(const char *name,
 	if (!parent && xlate_proc_name(name, &parent, &fn) != 0)
 		goto out;
 	len = strlen(fn);
-
+again:
 	spin_lock(&proc_subdir_lock);
 	for (p = &parent->subdir; *p; p=&(*p)->next ) {
 		if (!proc_match(len, fn, *p))
 			continue;
 		de = *p;
+
+		/*
+		 * Stop accepting new readers/writers. If you're dynamically
+		 * allocating ->proc_fops, save a pointer somewhere.
+		 */
+		de->proc_fops = NULL;
+		/* Wait until all readers/writers are done. */
+		if (atomic_read(&de->pde_users) > 0) {
+			spin_unlock(&proc_subdir_lock);
+			msleep(1);
+			goto again;
+		}
+
 		*p = de->next;
 		de->next = NULL;
 		if (S_ISDIR(de->mode))
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -56,6 +56,19 @@ struct proc_dir_entry {
 	gid_t gid;
 	loff_t size;
 	struct inode_operations * proc_iops;
+	/*
+	 * NULL ->proc_fops means "PDE is going away RSN" or
+	 * "PDE is just created". In either case ->get_info, ->read_proc,
+	 * ->write_proc won't be called because it's too late or too early,
+	 * respectively.
+	 *
+	 * Valid ->proc_fops means "use this file_operations" or
+	 * "->data is setup, it's safe to call ->read_proc, ->write_proc which
+	 * dereference it".
+	 *
+	 * If you're allocating ->proc_fops dynamically, save a pointer
+	 * somewhere.
+	 */
 	const struct file_operations * proc_fops;
 	get_info_t *get_info;
 	struct module *owner;
@@ -66,6 +79,8 @@ struct proc_dir_entry {
 	atomic_t count;		/* use count */
 	int deleted;		/* delete flag */
 	void *set;
+	atomic_t pde_users;	/* number of readers + number of writers via
+				 * ->read_proc, ->write_proc, ->get_info */
 };
 
 struct kcore_list {


      reply	other threads:[~2007-01-24 15:16 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2006-12-27 13:42 Racy /proc creations interfaces Alexey Dobriyan
2006-12-27 13:56 ` Al Viro
2006-12-28  8:21   ` Alexey Dobriyan
2007-01-15 15:39     ` [PATCH] Fix rmmod/read/write races in /proc entries Alexey Dobriyan
2007-01-23 20:58       ` Andrew Morton
2007-01-24 15:22         ` Alexey Dobriyan [this message]

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