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 19:58:44 -0600 [thread overview]
Message-ID: <878s8o74iz.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <87eeig74kv.fsf@x220.int.ebiederm.org> (Eric W. Biederman's message of "Tue, 19 Jan 2021 19:57:36 -0600")
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(¤t->signal->live);
>> refcount_inc(¤t->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(¤t->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
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next prev parent reply other threads:[~2021-01-20 2:00 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-15 14:57 [RFC PATCH v3 0/8] Count rlimits in each user namespace Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting Alexey Gladkov
2021-01-18 6:06 ` c25050162e: WARNING:at_lib/refcount.c:#refcount_warn_saturate kernel test robot
2021-01-18 19:14 ` [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting Linus Torvalds
2021-01-18 19:45 ` Alexey Gladkov
2021-01-18 20:34 ` Linus Torvalds
2021-01-18 20:56 ` Alexey Gladkov
2021-01-19 4:35 ` Kaiwan N Billimoria
2021-01-20 1:57 ` Eric W. Biederman
2021-01-20 1:58 ` Eric W. Biederman [this message]
2021-01-21 12:04 ` Alexey Gladkov
2021-01-21 15:50 ` Eric W. Biederman
2021-01-21 16:07 ` Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 2/8] Add a reference to ucounts for each cred Alexey Gladkov
2021-01-18 6:47 ` 14c3c8a27f: kernel_BUG_at_kernel/cred.c kernel test robot
2021-01-18 8:31 ` [PATCH v4 2/8] Add a reference to ucounts for each cred Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 3/8] Move RLIMIT_NPROC counter to ucounts Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 4/8] Move RLIMIT_MSGQUEUE " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 5/8] Move RLIMIT_SIGPENDING " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 6/8] Move RLIMIT_MEMLOCK " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 7/8] Move RLIMIT_NPROC check to the place where we increment the counter Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 8/8] kselftests: Add test to check for rlimit changes in different user namespaces Alexey Gladkov
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