linux-kernel - Re: [PATCH] ftrace: unify arch_syscall

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Date:	Fri, 22 Jan 2010 17:37:00 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mike Frysinger <vapier@...too.org>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH] ftrace: unify arch_syscall_addr() implementations

On Fri, Jan 22, 2010 at 09:36:17AM -0500, Steven Rostedt wrote:
> [ Added Heiko Carstens and Paul Mundt to Cc ]
> 
> On Fri, 2010-01-22 at 08:43 -0500, Mike Frysinger wrote:
> > Every arch_syscall_addr() implementation thus far is the same, so unify
> > them as a default weak in common code so more arches don't have to waste
> > time copying & pasting this simple function.  The Blackfin version is
> > going to be exactly the same.
> > 
> > Signed-off-by: Mike Frysinger <vapier@...too.org>
> > ---
> > note: only thing that needs double checking is s390 and sparc where they
> > declared the sys_call_table as an array of ints.  considering this table
> > is supposed to be an array of function pointers, this seems like more of
> > a typo to me ...
> 
> I would not be too sure. s390 is very strange, and I would definitely
> want to get an Ack from the arch maintainers first.

It's not a typo. The syscall table on s390 contains always 32 bit pointers
since we know that the address of the function to be called is (way) below
4GB. So this saves us a few bytes.
In addition this makes syscall patching done by some security modules a
bit more difficult, since they would need to store a 64 bit pointer.
That's because we make sure that module addresses are always above 4GB.
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