From mboxrd@z Thu Jan  1 00:00:00 1970
From: Florian Weimer <fweimer@redhat.com>
Subject: Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking ABIs at the same time.
Date: Wed, 06 Nov 2019 15:04:17 +0100
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        (Zack Weinberg's message of "Wed, 6 Nov 2019 09:00:51 -0500")
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To: Zack Weinberg <zackw@panix.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Carlos O'Donell <carlos@redhat.com>, Shawn Landden <shawn@git.icu>, GNU C Library <libc-alpha@sourceware.org>, linux-api@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, Deepa Dinamani <deepa.kernel@gmail.com>, Oleg Nesterov <oleg@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Catalin Marinas <catalin.marinas@arm.com>, Keith Packard <keithp@keithp.com>, Peter Zijlstra <peterz@infradead.org>
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* Zack Weinberg:

> On Tue, Nov 5, 2019 at 9:28 AM Thomas Gleixner <tglx@linutronix.de> wrote=
:
>>
>> The real issue is that the robust list could be circular by incident or
>> malice and there is no way for the kernel to figure that out. That would
>> prevent the task from exiting and make it iterate over the list until
>> doomsday, i.e. a nice unpriviledged DoS.
>
> Why can't the kernel use the standard tortoise-and-hare algorithm for
> detecting circular linked lists here?

It's not guaranteed to terminate if the list is in shared memory.

Thanks,
Florian