Re: Is fork() hook ever possible?

On Tue, Sep 16, 2008 at 11:36:03AM -0400, Daniel Eischen wrote:
> On Tue, 16 Sep 2008, Andrey Chernov wrote:
> 
> > On Tue, Sep 16, 2008 at 04:53:54PM +0200, Attilio Rao wrote:
> >> 2008/9/16, Andrey Chernov <ache_at_nagual.pp.ru>:
> >>> On Tue, Sep 16, 2008 at 03:38:16PM +0100, Bob Bishop wrote:
> >>> > Hi,
> >>>
> >>>>
> >>> > On 16 Sep 2008, at 15:03, Andrey Chernov wrote:
> >>> >
> >>> >> I need some sort of fork() hook to detect that pid is changed to re-
> >>> >> stir
> >>> >> ar4random() after that (in the child), simple flag variable with
> >>> >> child's pid is needed.
> >>> >>
> >>> >> Currently OpenBSD does almost that checking getpid() every time
> >>> >> arc4random() called, but it is very slow way to use getpid() syscall
> >>> >> repeatedly, about 12-15 times slower than just arc4random() without
> >>> >> getpid().
> >>> >>
> >>> >> Any ideas?
> >>> >
> >>>
> >>>> How about something hacky using mmap()/minherit()?
> >>>
> >>>  Could you please provide working low cost example to detect that we are in
> >>>  the child (pid changed or something else)? Calling getpid() as OpenBSD
> >>>  does definitely is very high cost. :(
> >>
> >> An idea would be to implement a shared page between process and system
> >> which exports such informations.
> >> I'm sure we have a SoC project (2007) implementing this and perforce
> >> branches for it, I'm just not sure how far it did end.
> >
> > Please keep in mind that the hook itself must be invisible to user
> > application, we have standard API only - fork() and arc4random() family,
> > no additional setup or functions are possible outside of existen API. I.e.
> > the low cost hack must be completely inside ether the fork() wrapper or
> > arc4random().
> 
> Well, you could speed up getpid() by having libc wrap all fork()
> variants.  The idea is that getpid() would only call __sys_getpid()
> the first time it was called and then only after a fork().  It
> would return the saved process id for all other cases.
> 
> This wouldn't work if the application made its own syscall
> without going through libc.
> 
> The shared page between process and system has been tossed around
> before and would probably be more benficial.  Having access to
> time without making a syscall would be nice.

Yes, speeding up getpid() by caching its pid is nice idea.
But I am completely unaware how to create syscall wrappers inside libc. :(
I think about something like that:

__weak_reference(_fork, fork);

pid_t _fork(void);

pid_t
_fork(void)
{
	pid_t ret;

	if ((ret = __sys_fork()) == 0)
		_curr_pid = -1;

	return (ret);
}

But how it will coexists with the same __weak in thread/thr_fork.c ?
Are some threading locks required in this code?

-- 
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