On Mon, Feb 12, 2007 at 10:35:52PM -0800, Andrew Morton wrote:
> On Sun, 11 Feb 2007 23:23:30 +0300 Alexey Dobriyan <[email protected]> wrote:
>
> > [PATCH v4] Fix rmmod/read/write races in /proc entries
>
> This:
>
> 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;
> ssize_t rv = -EIO;
>
> dp = PDE(inode);
>
> if (!dp->write_proc)
> goto out;
>
> spin_lock(&dp->pde_unload_lock);
> if (!dp->proc_fops)
> /*
> * remove_proc_entry() marked PDE as "going away".
> * No new writers allowed.
> */
> goto out_unlock;
>
> versus
>
> spin_lock(&de->pde_unload_lock);
> /*
> * Stop accepting new readers/writers. If you're dynamically
> * allocating ->proc_fops, save a pointer somewhere.
> */
> de->proc_fops = NULL;
> /* Wait until all existing readers/writers are done. */
> if (de->pde_users > 0) {
> struct completion c;
>
> init_completion(&c);
> if (!de->pde_unload_completion)
> de->pde_unload_completion = &c;
>
> spin_unlock(&de->pde_unload_lock);
> spin_unlock(&proc_subdir_lock);
>
> wait_for_completion(de->pde_unload_completion);
>
> spin_lock(&proc_subdir_lock);
> goto continue_removing;
> }
> spin_unlock(&de->pde_unload_lock);
> <here>
> ...
> <free de>
>
> What prevents proc_file_write() from looking up and playing with this de in
> <here>?
If I understood your two-column diagram correctly, scenario below can't
happen because of PDE's own refcount (->count) and existence of
->deleted (0/1)
remove_proc_entry() sees positive ->count and doesn't immediately free
PDE. remove_proc_entry() will at most a) lock b) access to check
->proc_fops which is NULL now, and c) unlock which is fine because
memory is in place.
->count is bumped in proc_get_inode after checking PDEs lists, but our
PDE was already removed from it.
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