Re: [PATCH v3 3/3] exec: do unshare_files after de_thread

[Oleg Nesterov <oleg@redhat.com>]

On 09/14, Jeff Layton wrote:
>
> 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.
>
> Fix this by doing unshare_files later during exec,

See my reply to 1/3... if we can forget about the races with get_files_struct()
we can probably make a much simpler patch, plus we do not need 2/2, afaics.

What I really can't understand is why we need to _change_ current->files
early in do_execve().

IOW. Lets ignore do_close_on_exec(), lets ignore the fact that unshare_fd()
can fail and thus it makes sense to call it before point-of-no-return.

Any other reason why we can't simply call unshare_files() at the end of
__do_execve_file() on success?

Oleg.