Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

[David Hildenbrand]

On 26.08.21 19:48, Andy Lutomirski wrote:
> On Fri, Aug 13, 2021, at 5:54 PM, Linus Torvalds wrote:
> > On Fri, Aug 13, 2021 at 2:49 PM Andy Lutomirski <luto@xxxxxxxxxx> 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?)

It's not like this issue is new (^2017) or relevant in practice. So no 
need to hurry IMHO. One step at a time: it might make perfect sense to 
remove ETXTBSY, but we have to be careful to not break other user space 
that actually cares about the current behavior in practice.

--
Thanks,

David / dhildenb