linux-kernel - Re: [PATCH v1 0/7] Remove in-tree usage of MAP

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Date:   Thu, 12 Aug 2021 11:01:36 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "David Hildenbrand" <david@...hat.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
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        "H. Peter Anvin" <hpa@...or.com>,
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Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE



On Thu, Aug 12, 2021, at 10:48 AM, Eric W. Biederman wrote:
> "Andy Lutomirski" <luto@...nel.org> writes:

> I had a blind spot, and Florian Weimer made a very reasonable request.
> Apparently userspace for shared libraires uses MAP_PRIVATE.
> 
> So we almost don't care if the library is overwritten.  We loose some
> efficiency and apparently there are some corner cases like the library
> being extended past the end of the exiting file that are problematic.
> 
> 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) that just makes certain that
> everything mapped from the executable is guaranteed to be visible from
> the time of the mmap, and any changes from the filesystem side after
> that are guaranteed to cause a copy on write.
> 
> Once we get that figured out we could consider getting rid of deny-write
> entirely.

Are all of the CoW bits in good enough shape for this to work without just immediately CoWing the whole file?  In principle, write(2) to a file should be able to notice that it needs to CoW some pages, but I doubt that this actually works.

--Andy