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

On 26.08.21 19:48, Andy Lutomirski wrote:

On Fri, Aug 13, 2021, at 5:54 PM, Linus Torvalds wrote:

quoted

On Fri, Aug 13, 2021 at 2:49 PM Andy Lutomirski [off-list ref] wrote:

quoted

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