Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

From: ebiederm@xmission.com (Eric W. Biederman)
To: Alexey Gladkov <gladkov.alexey@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>, Kees Cook <keescook@chromium.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>,
	Oleg Nesterov <oleg@redhat.com>, Linux-MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	io-uring <io-uring@vger.kernel.org>
Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting
Date: Tue, 19 Jan 2021 19:57:36 -0600	[thread overview]
Message-ID: <87eeig74kv.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <20210118205629.zro2qkd3ut42bpyq@example.org> (Alexey Gladkov's message of "Mon, 18 Jan 2021 21:56:29 +0100")

Alexey Gladkov <gladkov.alexey@gmail.com> writes:

> On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote:
>> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov
>> <gladkov.alexey@gmail.com> wrote:
>> >
>> > Sorry about that. I thought that this code is not needed when switching
>> > from int to refcount_t. I was wrong.
>> 
>> Well, you _may_ be right. I personally didn't check how the return
>> value is used.
>> 
>> I only reacted to "it certainly _may_ be used, and there is absolutely
>> no comment anywhere about why it wouldn't matter".
>
> I have not found examples where checked the overflow after calling
> refcount_inc/refcount_add.
>
> For example in kernel/fork.c:2298 :
>
>    current->signal->nr_threads++;                           
>    atomic_inc(&current->signal->live);                      
>    refcount_inc(&current->signal->sigcnt);  
>
> $ semind search signal_struct.sigcnt
> def include/linux/sched/signal.h:83  		refcount_t		sigcnt;
> m-- kernel/fork.c:723 put_signal_struct 		if (refcount_dec_and_test(&sig->sigcnt))
> m-- kernel/fork.c:1571 copy_signal 		refcount_set(&sig->sigcnt, 1);
> m-- kernel/fork.c:2298 copy_process 				refcount_inc(&current->signal->sigcnt);
>
> It seems to me that the only way is to use __refcount_inc and then compare
> the old value with REFCOUNT_MAX
>
> Since I have not seen examples of such checks, I thought that this is
> acceptable. Sorry once again. I have not tried to hide these changes.

The current ucount code does check for overflow and fails the increment
in every case.

So arguably it will be a regression and inferior error handling behavior
if the code switches to the ``better'' refcount_t data structure.

I originally didn't use refcount_t because silently saturating and not
bothering to handle the error makes me uncomfortable.

Not having to acquire the ucounts_lock every time seems nice.  Perhaps
the path forward would be to start with stupid/correct code that always
takes the ucounts_lock for every increment of ucounts->count, that is
later replaced with something more optimal.

Not impacting performance in the non-namespace cases and having good
performance in the other cases is a fundamental requirement of merging
code like this.

Eric

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