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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Mar 17, 2018 at 11:43:28AM -0400, Jeff Layton wrote:
> On Sat, 2018-03-17 at 15:05 +0000, Al Viro wrote:
> > On Sat, Mar 17, 2018 at 10:25:20AM -0400, Jeff Layton wrote:
> > > From: Jeff Layton <jlayton@redhat.com>
> > > 
> > > 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()?
> 
> Ahh, I wasn't aware of it. I copied the loop in change_lock_owners from
> close_files. I'll have a look at iterate_fd().

BTW, if iterate_fd() turns out to be slower, it might make sense to have it
look at the bitmap to skip unpopulated parts of descriptor table faster -
other users might also benefit from that.