linux-kernel - Re: [RFC PATCH v3 1/8] Use refcount

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Message-ID: <20210121120427.iiggfmw3tpsmyzeb@example.org>
Date:   Thu, 21 Jan 2021 13:04:27 +0100
From:   Alexey Gladkov <gladkov.alexey@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

On Tue, Jan 19, 2021 at 07:57:36PM -0600, Eric W. Biederman wrote:
> Alexey Gladkov <gladkov.alexey@...il.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@...il.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.

Did I understand your suggestion correctly that you suggest to use
spin_lock for atomic_read and atomic_inc ?

If so, then we are already incrementing the counter under ucounts_lock.

	...
	if (atomic_read(&ucounts->count) == INT_MAX)
		ucounts = NULL;
	else
		atomic_inc(&ucounts->count);
	spin_unlock_irq(&ucounts_lock);
	return ucounts;

something like this ?

-- 
Rgrds, legion