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

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Message-ID: <CAHk-=wgru1UAm3kAKSOdnbewPXQMOxYkq9PnAsRadAC6pXCCMQ@mail.gmail.com>
Date:   Thu, 12 Aug 2021 08:10:28 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Al Viro <viro@...iv.linux.org.uk>,
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        Steven Rostedt <rostedt@...dmis.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
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        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
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        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

On Thu, Aug 12, 2021 at 7:48 AM Eric W. Biederman <ebiederm@...ssion.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.

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

The kernel ETXTBUSY thing is purely a courtesy feature, and as people
have noticed it only really works for the main executable because of
various reasons. It's not something user space should even rely on,
it's more of a "ok, you're doing something incredibly stupid, and
we'll help you avoid shooting yourself in the foot when we notice".

Any distro should make sure their upgrade tools don't just
truncate/write to random libraries executables.

And if they do, it's really not a kernel issue.

This patch series basically takes this very historical error return,
and simplifies and clarifies the implementation, and in the process
might change some very subtle corner case (unmapping the original
executable entirely?). I hope (and think) it wouldn't matter exactly
because this is a "courtesy error" rather than anything that a sane
setup would _depend_ on, but hey, insane setups clearly exist.

               Linus