Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking ABIs at the same time.

[Florian Weimer <fweimer@redhat.com>]

* Thomas Gleixner:

> On Tue, 5 Nov 2019, Carlos O'Donell wrote:
>> On 11/5/19 6:56 AM, Thomas Gleixner wrote:
>> The other issue is this:
>> 
>> "Robust mutexes do not take ROBUST_LIST_LIMIT into account"
>> https://sourceware.org/bugzilla/show_bug.cgi?id=19089
>
>   "The kernel limits the length of the robust mutex list to 2048 entries.
>    This constant does not seem to be exported to user space."
>
> FWIW, the constant is defined in the UAPI futex header.
>
> The main concern here is not the actual number of futexes held by a task.
>
> 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.
>
> So I fear the kernel cannot really help with this one.

I'm actually fine with treating ROBUST_LIST_LIMIT as an ABI constant.
It's just not clear to me if the constant has this status today.  I
suspect it was just split from the implementation headers at one point.

Thanks,
Florian