Date: Tue, 16 Sep 2008 20:05:37 +0400 From: Andrey Chernov <ache@nagual.pp.ru> To: Daniel Eischen <deischen@freebsd.org> Cc: Attilio Rao <attilio@freebsd.org>, current@freebsd.org Subject: Re: Is fork() hook ever possible? Message-ID: <20080916160535.GA40676@nagual.pp.ru> In-Reply-To: <Pine.GSO.4.64.0809161125120.8677@sea.ntplx.net> References: <20080916140319.GA34447@nagual.pp.ru> <BBB443F5-042C-444E-A2F4-592B66FF2003@gid.co.uk> <20080916144502.GA39765@nagual.pp.ru> <3bbf2fe10809160753o7e5e8a78q7c6bd44c02bfd5c2@mail.gmail.com> <20080916150120.GA40087@nagual.pp.ru> <Pine.GSO.4.64.0809161125120.8677@sea.ntplx.net>
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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@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?
--
http://ache.pp.ru/
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