close-on-exec alternatives (was: Why kernel kills processes that run out of memory instead of just failing memory allocation system calls?)

[Bernd Walter]

On Fri, May 29, 2009 at 12:31:37PM -0700, Alfred Perlstein wrote:
> * Dag-Erling Sm??rgrav <des at des.no> [090529 02:49] wrote:
> > Alfred Perlstein <alfred at freebsd.org> writes:
> > > Dag-Erling Sm??rgrav <des at des.no> writes:
> > > > Usually, what you see is closer to this:
> > > > 
> > > > if ((pid = fork()) == 0) {
> > > >         for (int fd = 3; fd < getdtablesize(); ++fd)
> > > >                 (void)close(fd);
> > > >         execve(path, argv, envp);
> > > >         _exit(1);
> > > > }
> > >
> > > I'm probably missing something, but couldn't you iterate 
> > > in the parent setting the close-on-exec flag then vfork?
> > 
> > This is an example, Alfred.  Like most examples, it is greatly
> > simplified.  I invite you to peruse the source to find real-world
> > instances of non-trivial fork() / execve() usage.
> 
> It wasn't meant to critisize, just ask a question for the specific
> instance because it made me curious.  I know how bad it can be with
> vfork as I observed a few fixes involving mistaken use of vfork at
> another job.

It is still very interesting, because I currently have a similar problem
and wasn't aware of getdtablesize();
A threaded application which needs to call an external script of unknown
runtime.
I don't have all descriptors under my knowledge, because external libs
might open them.
I also believe there could be a race between retrieving the descriptor
and setting close-on-exec.
The only solution which I found so far is using rfork with RFCFDG.
If I undestand RFCFDG correctly the child process has no descriptors at
all.
This is Ok for me, since I don't need to inherit some.

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.