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

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Message-Id: <b60e9bd1-7232-472d-9c9c-1d6593e9e85e@www.fastmail.com>
Date:   Thu, 26 Aug 2021 10:48:55 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "David Laight" <David.Laight@...lab.com>,
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Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

On Fri, Aug 13, 2021, at 5:54 PM, Linus Torvalds wrote:
> On Fri, Aug 13, 2021 at 2:49 PM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > I’ll bite.  How about we attack this in the opposite direction: remove the deny write mechanism entirely.
> 
> I think that would be ok, except I can see somebody relying on it.
> 
> It's broken, it's stupid, but we've done that ETXTBUSY for a _loong_ time.

Someone off-list just pointed something out to me, and I think we should push harder to remove ETXTBSY.  Specifically, we've all been focused on open() failing with ETXTBSY, and it's easy to make fun of anyone opening a running program for write when they should be unlinking and replacing it.

Alas, Linux's implementation of deny_write_access() is correct^Wabsurd, and deny_write_access() *also* returns ETXTBSY if the file is open for write.  So, in a multithreaded program, one thread does:

fd = open("some exefile", O_RDWR | O_CREAT | O_CLOEXEC);
write(fd, some stuff);

<--- problem is here

close(fd);
execve("some exefile");

Another thread does:

fork();
execve("something else");

In between fork and execve, there's another copy of the open file description, and i_writecount is held, and the execve() fails.  Whoops.  See, for example:

https://github.com/golang/go/issues/22315

I propose we get rid of deny_write_access() completely to solve this.

Getting rid of i_writecount itself seems a bit harder, since a handful of filesystems use it for clever reasons.

(OFD locks seem like they might have the same problem.  Maybe we should have a clone() flag to unshare the file table and close close-on-exec things?)

> 
> But you are right that we have removed parts of it over time (no more
> MAP_DENYWRITE, no more uselib()) so that what we have today is a
> fairly weak form of what we used to do.
> 
> And nobody really complained when we weakened it, so maybe removing it
> entirely might be acceptable.
> 
>               Linus
>