linux-kernel - Re: [PATCH 2/4] x86&x86-64 support for sys

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Message-ID: <alpine.LFD.0.9999.0711150852470.2786@woody.linux-foundation.org>
Date:	Thu, 15 Nov 2007 09:02:02 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ulrich Drepper <drepper@...hat.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/4] x86&x86-64 support for sys_indirect

[ Ingo, Thomas - see the whole series on linux-kernel ]

On Thu, 15 Nov 2007, Ulrich Drepper wrote:
>
> This part adds support for sys_indirect on x86 and x86-64.

The thing is, not all system calls can do this.

Some system calls are magic, and don't just take the arguments in 
registers: they also care about the actual stack pointer and the whole 
pt_regs struct when returning to user mode. 

So this does need more infrastructure: some way of marking which system 
calls cannot be executed indirectly.

The magic system calls are things like:

 - sys_iopl() - this one changes the eflags value restored on iret
 - execve/clone/vfork() - need direct access to pt_regs
 - vm86() - does magic with the stack, cares about pt_regs
 - sigreturn - magic pt_regs accesses again

and there may be others I have forgotten about.

Calling these system calls from C code will just corrupt the kernel stack, 
and is a big big no-no.

		Linus
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