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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Alexey Gladkov <gladkov.alexey@gmail.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        io-uring <io-uring@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
        Kees Cook <keescook@chromium.org>,
        Oleg Nesterov <oleg@redhat.com>
References: <cover.1610722473.git.gladkov.alexey@gmail.com>
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        <CAHk-=wjsg0Lgf1Mh2UiJE4sqBDDo0VhFVBUbhed47ot2CQQwfQ@mail.gmail.com>
        <20210118194551.h2hrwof7b3q5vgoi@example.org>
        <CAHk-=wiNpc5BS2BfZhdDqofJx1G=uasBa2Q1eY4cr8O59Rev2A@mail.gmail.com>
        <20210118205629.zro2qkd3ut42bpyq@example.org>
        <87eeig74kv.fsf@x220.int.ebiederm.org>
Date:   Tue, 19 Jan 2021 19:58:44 -0600
In-Reply-To: <87eeig74kv.fsf@x220.int.ebiederm.org> (Eric W. Biederman's
        message of "Tue, 19 Jan 2021 19:57:36 -0600")
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Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting
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ebiederm@xmission.com (Eric W. Biederman) writes:

> 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.

So starting with something easy to comprehend and simple, may make it
easier to figure out how to optimize the code.

Eric