Re: [PATCH] devtmpfs: move NULLing the thread pointer before unregistering fs

Greg Kroah-Hartman
2022-12-02T15:56:16Z

On Fri, Dec 02, 2022 at 02:45:01PM +0200, Alexander Atanasov wrote:
> In commit
> 31c779f293b3 ("devtmpfs: fix the dangling pointer of global devtmpfsd thread")
> a dangling pointer on an error condition was fixed. But the fix
> left the dangling pointer during unregister_filesystem and printk calls.

And how could it be used there?

> Improve the fix to clear the pointer before unregistration to close
> the window where the dangling pointer can be potentially used.

Again, how can that happen? And you have an extra ' ' in that line :(

> Make it clear the pointer at only one place in the function.
>
> Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
> ---
> drivers/base/devtmpfs.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
> index e4bffeabf344..773e66ef5642 100644
> --- a/drivers/base/devtmpfs.c
> +++ b/drivers/base/devtmpfs.c
> @@ -472,17 +472,15 @@ int __init devtmpfs_init(void)
> }
>
> thread = kthread_run(devtmpfsd, &err, "kdevtmpfs");
> - if (!IS_ERR(thread)) {
> + if (!IS_ERR(thread))
> wait_for_completion(&setup_done);
> - } else {
> + else
> err = PTR_ERR(thread);
> - thread = NULL;
> - }
>
> if (err) {
> + thread = NULL;
> printk(KERN_ERR "devtmpfs: unable to create devtmpfs %i\n", err);
> unregister_filesystem(&dev_fs_type);
> - thread = NULL;
> return err;
> }

This all feels wrong and way too complex to have to clean up from a call
to kthread_run(). Are you sure this is the correct way to do this?

And how was this "issue" found? How does the call to kthread_run() ever
fail for you?

thanks,

greg k-h