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Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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Date:   Thu, 12 Aug 2021 21:24:00 +0200
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On 12.08.21 20:10, Linus Torvalds wrote:
> On Thu, Aug 12, 2021 at 7:48 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>> Given that MAP_PRIVATE for shared libraries is our strategy for handling
>> writes to shared libraries perhaps we just need to use MAP_POPULATE or a
>> new related flag (perhaps MAP_PRIVATE_NOW)
> 
> No. That would be horrible for the usual bloated GUI libraries. It
> might help some (dynamic page faults are not cheap either), but it
> would hurt a lot.

Right, we most certainly don't want to waste system ram / swap space, 
memory for page tables, and degrade performance just because some 
corner-case nasty user space could harm itself.

> 
> This is definitely a "if you overwrite a system library while it's
> being used, you get to keep both pieces" situation.

Right, play stupid games, win stupid prices. I agree that if there would 
be an efficient way to detect+handle such overwrites gracefully, it 
would be great to have the kernel support that. ETXTBUSY as implemented 
with this series (but also before this series) is really only a 
minimalistic approach to help detect some issues regarding the main 
executable.

-- 
Thanks,

David / dhildenb