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Re: GNUnet News: vfork and the signal race

From: Thomas Bushnell, BSG
Subject: Re: GNUnet News: vfork and the signal race
Date: Fri, 25 Nov 2011 17:43:11 -0800

Oh, and in the case you describe there:

The hypervisor at start creates a global variable hypervisor_pid, initialized from getpid().

The signal handler in the hypervisor then does this:

if getpid() == hypervisor_pid

In this way, if the child is between fork and exec when the parent gets its kill, and then it tries to kill the child, and the kill happens before the child execs (the problematic case you describe), then the child simply enters the hypervisor's signal handler, notices that it's not the hypervisor, and exits.


On Fri, Nov 25, 2011 at 4:09 PM, Christian Grothoff <christian@grothoff.org> wrote:
On 11/25/2011 07:50 PM, Thomas Bushnell, BSG wrote:
Programs which depend on the special suspend-the-parent behavior of
vfork were always regarded as buggy...

So relying on the well-documented behavior of a system call is a bug? Did you even read about the scenario I described at
https://gnunet.org/vfork ? Before writing this, I looked around for existing information on vfork, but I didn't find anyone making a good argument (or even the claim) that relying on 'suspend-the-parent' was per-se buggy.  (Naturally, the result would be non-portable for systems where fork==vfork, but then maybe implementing vfork as fork is the bug? ;-))

Happy hacking


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