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Re: [PATCH] Fix overflow in jobs


From: Chet Ramey
Subject: Re: [PATCH] Fix overflow in jobs
Date: Fri, 9 Feb 2018 15:32:17 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 2/7/18 3:30 PM, Chet Ramey wrote:

>> Unfortunately, the proposed patch does not fix the case for 32 bit
>> architectures.
> 
> That's interesting. It seems to me that the kernel should reject attempts
> to set the maximum number of processes larger than 2**(sizeof (pid_t)).

I looked at this on a 32-bit debian system and discovered that setting the
limit to anything above 2**32 - 1 will basically give you `unlimited'.
Setting it to anything below that but above 2**31 - 1 will give you a value
that `ulimit' will display, but sysconf() will return as negative, which
the code handles fine. It's when you have values between 2**30 and 2**31-1
where there's a problem in the code that calculates the max needed table
size.

However, when you correct that...


>> I think the proper fix may be re-enable the js.c_childmax capping. What
>> do you think?
> 
> That's the wrong place. If your original patch identifies the issue
> correctly, we need to bound the size of bgpids.storage to something a
> pid_t (aliased to ps_index_t) variable can address.
> 
> So instead of using js.c_childmax directly, we compute the maximum
> table size using js.c_childmax unless it exceeds TYPE_MAXIMUM(pid_t).

This is true, but doing that will probably result in the kernel killing
you when the code goes to malloc some 32 gig of memory, especially on a
32-bit system. So it seems appropriate to limit the table size to some
reasonable value, but the question is what constitutes `reasonable'. We
could start with some value like 32768 and see what happens.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://tiswww.cwru.edu/~chet/



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