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

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Message-ID: <5b0d7c1e73ca43ef9ce6665fec6c4d7e@AcuMS.aculab.com>
Date:   Fri, 13 Aug 2021 09:05:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Eric W. Biederman'" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Andy Lutomirski <luto@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
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Subject: RE: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

From: Eric W. Biederman
> Sent: 12 August 2021 19:47
...
> So today the best advice I can give to userspace is to mark their
> executables and shared libraries as read-only and immutable.  Otherwise
> a change to the executable file can change what is mapped into memory.
> MAP_PRIVATE does not help.

While 'immutable' might be ok for files installed by distributions
it would be a PITA in development.

ETXTBUSY is a useful reminder that the file you are copying from
machine A to machine B (etc) is still running and probably ought
to be killed/stopped before you get confused.

I've never really understood why it doesn't stop shared libraries
being overwritten - but they do tend to be updated less often.

Overwriting an in-use shared library could be really confusing.
It is likely that all the code is actually in memory.
So everything carries on running as normal.
Until the kernel gets under memory pressure and discards a page.
Then a page from the new version is faulted in and random
programs start getting SEGVs.
This could be days after the borked update.

	David

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