linux-kernel - Re: [PATCH 2.6.21.1] i386: save registers before intra-privilege syscall

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Date:	Fri, 18 May 2007 01:27:06 +0200
From:	Philipp Kohlbecher <pk031698@...-greifswald.de>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Dave Jones <davej@...emonkey.org.uk>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	linux-assembly@...r.kernel.org
Subject: Re: [PATCH 2.6.21.1] i386: save registers before intra-privilege
 syscall

H. Peter Anvin wrote:
> Philipp Kohlbecher wrote:
>> From: Philipp Kohlbecher <pk031698@...-greifswald.de>
>>
>> The kernel_execve function issues a software interrupt (int 0x80) to make
>> a system call to sys_execve. This function expects to find the stack segment
>> and stack pointer of the function that issued the system call in the pt_regs
>> struct. The syscall entry code that sets up this struct expects the stack
>> segment and the stack pointer of the issuing function already on the stack.
>> But the Intel processor saves these registers only if a stack-switch occurs,
>> i.e. for inter-privilege interrupts and exceptions (cf. Intel Software
>> Developer’s Manual, Vol. 3A, p. 5-17,
>> http://www.intel.com/design/processor/manuals/253668.pdf).
>> For an intra-privilege interrupt like the one issued in kernel_execve, these
>> registers must be saved manually.
>>
> 
> Could you describe the failure scenario this causes?

I don't know of any problems this causes. The kernel needs to be aware
of the fact that the xss and esp fields of the pt_regs struct may
contain wrong values anyway, as hardware interrupts arriving while the
CPU is in kernel mode would also lead to this condition.
The file include/asm-i386/processor.h contains a comment to that effect
(lines 483-492).
With kernel_execve we can predict this, however, and account for it.
(This may be superfluous, but I don't think it hurts and it might
prevent future errors.)

- Phil Kohlbecher
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