linux-kernel - Re: [PATCH] mm: optionally disable brk()

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Date:   Mon, 5 Oct 2020 12:47:03 +0300
From:   Topi Miettinen <toiwoton@...il.com>
To:     David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: optionally disable brk()

On 5.10.2020 12.13, David Hildenbrand wrote:
> On 05.10.20 08:12, Michal Hocko wrote:
>> On Sat 03-10-20 00:44:09, Topi Miettinen wrote:
>>> On 2.10.2020 20.52, David Hildenbrand wrote:
>>>> On 02.10.20 19:19, Topi Miettinen wrote:
>>>>> The brk() system call allows to change data segment size (heap). This
>>>>> is mainly used by glibc for memory allocation, but it can use mmap()
>>>>> and that results in more randomized memory mappings since the heap is
>>>>> always located at fixed offset to program while mmap()ed memory is
>>>>> randomized.
>>>>
>>>> Want to take more Unix out of Linux?
>>>>
>>>> Honestly, why care about disabling? User space can happily use mmap() if
>>>> it prefers.
>>>
>>> brk() interface doesn't seem to be used much and glibc is happy to switch to
>>> mmap() if brk() fails, so why not allow disabling it optionally? If you
>>> don't care to disable, don't do it and this is even the default.
>>
>> I do not think we want to have config per syscall, do we?
> 
> I do wonder if grouping would be a better option then (finding a proper
> level of abstraction ...).

If hardening and compatibility are seen as tradeoffs, perhaps there 
could be a top level config choice (CONFIG_HARDENING_TRADEOFF) for this. 
It would have options
- "compatibility" (default) to gear questions for maximum compatibility, 
deselecting any hardening options which reduce compatibility
- "hardening" to gear questions for maximum hardening, deselecting any 
compatibility options which reduce hardening
- "none/manual": ask all questions like before

-Topi