linux-kernel - Re: [RFC] HACK: overlayfs: Optimize overlay/restore creds

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Date: Mon, 18 Dec 2023 13:57:31 -0800
From: Vinicius Costa Gomes <vinicius.gomes@...el.com>
To: Christian Brauner <brauner@...nel.org>, Amir Goldstein <amir73il@...il.com>
Cc: hu1.chen@...el.com, miklos@...redi.hu, malini.bhandaru@...el.com,
 tim.c.chen@...el.com, mikko.ylinen@...el.com, lizhen.you@...el.com,
 linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, linux-fsdevel
 <linux-fsdevel@...r.kernel.org>, Linus Torvalds
 <torvalds@...ux-foundation.org>, David Howells <dhowells@...hat.com>, Seth
 Forshee <sforshee@...nel.org>
Subject: Re: [RFC] HACK: overlayfs: Optimize overlay/restore creds

Christian Brauner <brauner@...nel.org> writes:

>> > Yes, the important thing is that an object cannot change
>> > its non_refcount property during its lifetime -
>> 
>> ... which means that put_creds_ref() should assert that
>> there is only a single refcount - the one handed out by
>> prepare_creds_ref() before removing non_refcount or
>> directly freeing the cred object.
>> 
>> I must say that the semantics of making a non-refcounted copy
>> to an object whose lifetime is managed by the caller sounds a lot
>> less confusing to me.
>
> So can't we do an override_creds() variant that is effectively just:
>
> /* caller guarantees lifetime of @new */
> const struct cred *foo_override_cred(const struct cred *new)
> {
> 	const struct cred *old = current->cred;
> 	rcu_assign_pointer(current->cred, new);
> 	return old;
> }
>
> /* caller guarantees lifetime of @old */
> void foo_revert_creds(const struct cred *old)
> {
> 	const struct cred *override = current->cred;
> 	rcu_assign_pointer(current->cred, old);
> }
>
> Maybe I really fail to understand this problem or the proposed solution:
> the single reference that overlayfs keeps in ovl->creator_cred is tied
> to the lifetime of the overlayfs superblock, no? And anyone who needs a
> long term cred reference e.g, file->f_cred will take it's own reference
> anyway. So it should be safe to just keep that reference alive until
> overlayfs is unmounted, no? I'm sure it's something quite obvious why
> that doesn't work but I'm just not seeing it currently.

My read of the code says that what you are proposing should work. (what
I am seeing is that in the "optimized" cases, the only practical effect
of override/revert is the rcu_assign_pointer() dance)

I guess that the question becomes: Do we want this property (that the
'cred' associated with a subperblock/similar is long lived and the
"inner" refcount can be omitted) to be encoded in the constructor? Or do
we want it to be "encoded" in a call by call basis?

I can see both working.


Cheers,
-- 
Vinicius