Re: [PATCH] locks: change POSIX lock ownership on execve when files_struct is displaced

[Al Viro]

On Sat, Mar 17, 2018 at 10:25:20AM -0400, Jeff Layton wrote:
> From: Jeff Layton <jlayton@xxxxxxxxxx>
> 
> POSIX mandates that open fds and their associated file locks should be
> preserved across an execve. This works, unless the process is
> multithreaded at the time that execve is called.
> 
> In that case, we'll end up unsharing the files_struct but the locks will
> still have their fl_owner set to the address of the old one. Eventually,
> when the other threads die and the last reference to the old
> files_struct is put, any POSIX locks get torn down since it looks like
> a close occurred on them.
> 
> The result is that all of your open files will be intact with none of
> the locks you held before execve. The simple answer to this is "use OFD
> locks", but this is a nasty surprise and it violates the spec.
> 
> On a successful execve, change ownership of any POSIX file_locks
> associated with the old files_struct to the new one, if we ended up
> swapping it out.

TBH, I don't like the way you implement that.  Why not simply use
iterate_fd()?