linux-kernel - Re: [PATCH v7 09/12] sysfs: fix deadlock race with module removal

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Date:   Mon, 20 Sep 2021 07:53:16 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
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Subject: Re: [PATCH v7 09/12] sysfs: fix deadlock race with module removal

Hello,

On Fri, Sep 17, 2021 at 10:04:27PM -0700, Luis Chamberlain wrote:
> If try_module_get() fails we fail the operation on the kernfs node.
> 
> We use a try method as a full lock means we'd then make our sysfs
> attributes busy us out from possible module removal, and so userspace
> could force denying module removal, a silly form of "DOS" against module
> removal. A try lock on the module removal ensures we give priority to
> module removal and interacting with sysfs attributes only comes second.
> Using a full lock could mean for instance that if you don't stop poking
> at sysfs files you cannot remove a module.

I find this explanation odd because there's no real equivalent to locking
the module (as opposed to try locking) because you can't wait for the
removal to finish and then grant the lock, so any operation which increases
the reference *has* to be a try method unless the caller already holds a
reference to the same module and thus knows that the module is already
pinned. The code isn't wrong, so maybe just drop the related paragraphs in
the commit message?

>  static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
>  					     struct kernfs_node *parent,
>  					     const char *name, umode_t mode,
> +					     struct module *owner,
>  					     kuid_t uid, kgid_t gid,
>  					     unsigned flags)

Is there a particular reason why @owner is added between @mode and @uid?
Sitting between two fs attributes seems a bit awkward. Maybe it can just be
the last one?

Thanks.

-- 
tejun