[PATCH] Improve LinuxThreads compatibility in rfork()

[Kostik Belousov]

On Mon, Jul 11, 2011 at 12:54:06PM +0200, Robert Millan wrote:
> This patch made by Petr Salinger improves compatibility with
> LinuxThreads in rfork() syscall.  The Linux clone() implementation
> allows specifying the signal sent to parent when child terminates
> (instead of SIGCHLD).
> 
> As the threading implementation in Debian GNU/kFreeBSD is
> LinuxThreads-based, we had to diverge from upstream kFreeBSD ABI and
> implement this extension.
> 
> I hope it is acceptable for you to use the same encoding, this way we
> would archieve ABI compatibility to run Debian GNU/kFreeBSD inside a
> chroot/jail on top of a FreeBSD system.
> 
> Thanks for considering
> 
> -- 
> Robert Millan

> --- a/sys/kern/kern_fork.c
> +++ b/sys/kern/kern_fork.c
> @@ -477,7 +477,13 @@
>  		p2->p_sigacts = newsigacts;
>  	}
>  	if (flags & RFLINUXTHPN) 
> -	        p2->p_sigparent = SIGUSR1;
> +	{
> +		int sig;
> +		sig = RFTHPNSIGNUM(flags);
> +	        if (sig == 0) sig = SIGUSR1;
> +	        if (sig == SIGCHLD) sig = 0;
> +	        p2->p_sigparent = sig;
> +	}
>  	else
>  	        p2->p_sigparent = SIGCHLD;
>  
> --- a/sys/sys/unistd.h
> +++ b/sys/sys/unistd.h
> @@ -182,6 +182,10 @@
>  #define	RFHIGHPID	(1<<18)	/* use a pid higher than 10 (idleproc) */
>  #define	RFPPWAIT	(1<<31)	/* parent sleeps until child exits (vfork) */
>  #define	RFKERNELONLY	(RFSTOPPED | RFHIGHPID | RFPPWAIT)
> +#define RFTHPNSHIFT	24	/* reserve bits 24-30 */
> +#define RFTHPNMASK	0x7F    /* for compatibility with linuxthreads/clone()   */
> +				/* allow to specify  "clone exit parent notification" signal */
> +#define RFTHPNSIGNUM(flags)	(((flags) >> RFTHPNSHIFT) & RFTHPNMASK)
>  
>  #endif /* __BSD_VISIBLE */
>  
I looked at this patch some time ago already.

Can you, please, describe the reasoning behind the
> +	        if (sig == SIGCHLD) sig = 0;
line ?
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