linux-kernel - Re: [RFC] ptrace: add generic SET

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Date:	Fri, 7 Nov 2014 12:11:19 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"roland@...k.frob.com" <roland@...k.frob.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>
Subject: Re: [RFC] ptrace: add generic SET_SYSCALL request

On Fri, Nov 07, 2014 at 01:03:00PM +0100, Arnd Bergmann wrote:
> On Friday 07 November 2014 11:55:51 Will Deacon wrote:
> > We need this for arm64 and, since all architectures seem to have a mechanism
> > for setting a system call via ptrace, moving it to generic code should make
> > sense for new architectures too, no?
> 
> It makes a little more sense now, but I still don't understand why you
> need to set the system call number via ptrace. What is this used for,
> and why doesn't any other architecture have this?

All other architectures have a way.  x86, for example, you set orig_eax
(or orig_rax) to change the syscall number.  On ARM, that doesn't work
because we don't always pass the syscall number in a register.

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