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

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Message-ID: <87eeig74kv.fsf@x220.int.ebiederm.org>
Date:   Tue, 19 Jan 2021 19:57:36 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Alexey Gladkov <gladkov.alexey@...il.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

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.

Eric