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>
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(¤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.
Eric
next prev parent reply other threads:[~2021-01-20 1:59 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 [this message]
2021-01-20 1:58 ` Eric W. Biederman
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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