Re: [PATCH] Introduce O_CLOEXEC (take >2)

[Davide Libenzi <davidel@xmailserver.org>]

On Thu, 31 May 2007, Ulrich Drepper wrote:

> I've brought this topic up before but didn't provide a patch.  Well, here
> we go again, this time with a patch.  I even throw in a test program.
> 
> The problem is as follows: in multi-threaded code (or more correctly: all
> code using clone() with CLONE_FILES) we have a race when exec'ing.
> 
>    thread #1                       thread #2
> 
>    fd=open()
> 
>                                    fork + exec
> 
>   fcntl(fd,F_SETFD,FD_CLOEXEC)
> 
> In some applications this can happen frequently.  Take a web browser.  One
> thread opens a file and another thread starts, say, an external PDF viewer.
> The result can even be a security issue if that open file descriptor refers
> to a sensitive file and the external program can somehow be tricked into
> using that descriptor.
> 
> Just adding O_CLOEXEC support to open() doesn't solve the whole set of
> problems.  There are other ways to create file descriptors (socket,
> epoll_create, Unix domain socket transfer, etc).  These can and should
> be addressed separately though.  open() is such an easy case that it makes
> not much sense putting the fix off.

Isn't this better be a global process flag? Default should be, for legacy
reasons, !FD_CLOEXEC. But then you can call a sys_task_set_fflags(FD_CLOEXEC)
and all newly created files get that behavior by default. Then, in case 
you want some of them to cross the exec boundary, you explicitly
fcntl(fd, F_SETFD, !FD_CLOEXEC).
Most the MT+exec apps I write, would like FD_CLOEXEC for everything anyway.



- Davide