Message-Id: <b629cda1-becd-4725-b16c-13208ff478d3@www.fastmail.com>
Date:   Fri, 13 Aug 2021 17:49:19 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "David Laight" <David.Laight@...lab.com>,
        "David Hildenbrand" <david@...hat.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
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        Linux-MM <linux-mm@...ck.org>,
        "Florian Weimer" <fweimer@...hat.com>,
        "Michael Kerrisk" <mtk.manpages@...il.com>
Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE



On Fri, Aug 13, 2021, at 5:31 PM, Linus Torvalds wrote:
> On Fri, Aug 13, 2021 at 10:18 AM Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
> >
> > Florian Weimer, would it be possible to get glibc's ld.so implementation to use
> > MAP_SHARED?  Just so people reading the code know what to expect of the
> > kernel?  As far as I can tell there is not a practical difference
> > between a read-only MAP_PRIVATE and a read-only MAP_SHARED.
> 
> There's a huge difference.
> 
> For one, you actually don't necessarily want read-only. Doing COW on
> library images is quite common for things like relocation etc (you'd
> _hope_ everything is PC-relative, but no)
> 
> So no. Never EVER use MAP_SHARED unless you literally expect to have
> two different mappings that need to be kept in sync and one writes the
> other.
> 
> I'll just repeat: stop arguing about this case. If somebody writes to
> a busy library, THAT IS A FUNDAMENTAL BUG, and nobody sane should care
> at all about it apart from the "you get what you deserve".
> 
> What's next? Do you think glibc should also map every byte in the user
> address space so that user programs don't get SIGSEGV when they have
> wild pointers?
> 
> Again - that's a user BUG and trying to "work around" a wild pointer
> is a worse fix than the problem it tries to fix.
> 
> The exact same thing is true for shared library (or executable)
> mappings. Trying to work around people writing to them is *worse* than
> the bug of doing so.
> 
> Stop this completely inane discussion already.
> 

I’ll bite.  How about we attack this in the opposite direction: remove the deny write mechanism entirely.

In my life, I’ve encountered -ETXTBUSY intermittently, and it invariably means that I somehow failed to finish killing a program fast enough for whatever random rebuild I’m doing to succeed. It’s at best erratic — it only applies for static binaries, and it has never once saved me from a problem I care about. If the program I’m recompiling crashes, I don’t care — it’s probably already part way through dying from an unrelated fatal signal.  What actually happens is that I see -ETXTBUSY, think “wait, this isn’t Windows, why are there file sharing rules,” then think “wait, Linux has *one* half baked file sharing rule,” and go on with my life. [0]

Seriously, can we deprecate and remove the whole thing?

[0] we have mandatory locks, too. Sigh.

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