linux-kernel - Re: [PATCH v3 3/3] generic sys

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Date:	Fri, 8 Jan 2010 16:34:43 +0100
From:	Christoph Hellwig <hch@....de>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Christoph Hellwig <hch@....de>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux@....linux.org.uk, starvik@...s.com, jesper.nilsson@...s.com,
	dhowells@...hat.com, ysato@...rs.sourceforge.jp,
	takata@...ux-m32r.org, geert@...ux-m68k.org, zippel@...ux-m68k.org,
	gerg@...inux.org, ralf@...ux-mips.org, benh@...nel.crashing.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	lethal@...ux-sh.org, davem@...emloft.net, jdike@...toit.com,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	viro@...iv.linux.org.uk
Subject: Re: [PATCH v3 3/3] generic sys_ipc wrapper

On Fri, Jan 08, 2010 at 11:57:17AM +0100, Arnd Bergmann wrote:
> > Add a generic implementation of the ipc demultiplexer syscall.  Except for
> > s390 and sparc64 all implementations of the sys_ipc are nearly identical.
> 
> I think the s390 version is trivial to add as well, like
> 
> SYSCALL_DEFINE5(s390_ipc, uint, call, int, first, unsigned long, second,
>  		unsigned long, third, void __user *, ptr)
> {
> 	if (call == SEMTIMEDOP)
> 		return sys_semtimedop(first, (struct sembuf __user *)ptr,
> 				      (unsigned) second,
> 				      (const struct timespec __user *)third);
> 
> 	return sys_ipc(call & 0xffff, first, second, third, ptr, 0);
> }

Possiblly.  Not sure if it's worth it, though.  If the s390 maintainers
want it I'd say do it as a separate patch.

> But while going over the code again, I noticed that you broke sign extension
> at least on powerpc and s390, possibly on all 64 bit machines:

> > +			return -EINVAL;
> > +		if (get_user(fourth.__pad, (void __user * __user *) ptr))
> > +			return -EFAULT;
> > +		return sys_semctl(first, second, third, fourth);
> 
> 		return sys_semctl(first, (int)second, third, fourth);

What does the explicit case buy us in terms of sign-extension over
the implicit one given that the second argument to sys_semctl already is
types as int?

> This is needed to make sure the upper half of the register is filled with
> zero-extended or sign-extended correctly and does not contain random garbage
> in the native 64 bit case. IIRC, x86_64 does not have this problem and mips64
> may have the wrong code already. Alpha, parisc and ia64 don't have a native
> sys_ipc and the rest are 32 bit, so they don't care.

I can fix it up, but I don't quite understand the need.

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