linux-kernel - Re: [patch v2 2/4] x86, ptrace: regset extensions to support xstate

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Date:	Wed, 10 Feb 2010 16:34:43 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Roland McGrath <roland@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, hjl.tools@...il.com
Subject: Re: [patch v2 2/4] x86, ptrace: regset extensions to support xstate

On 02/10, Oleg Nesterov wrote:
>
> static inline int user_regset_copyout(unsigned int *pos, unsigned int *count,
> 				      void **kbuf,
> 				      void __user **ubuf, const void *data,
> 				      const int start_pos, const int end_pos)
> {
> 	if (*count == 0)
> 		return 0;
> 	BUG_ON(*pos < start_pos);
> 	if (end_pos < 0 || *pos < end_pos) {
> 		unsigned int copy = (end_pos < 0 ? *count
> 				     : min(*count, end_pos - *pos));
> 		data += *pos - start_pos;
>                             ^^^
> Is it correct? Shouldn't it be
>
> 		data += *pos + start_pos;
>
> ?

Ah, I seem to understand. start_pos is not the offset inside *data as
I thought. It is needed to compensate the "*pos += copy" addition which
was done by the previous user_regset_copyout().

This means that xstateregs_get() is right, it copies xstate_fx_sw_bytes
but uses sizeof(i387_fxsave_struct) as start_pos.

So tricky... I must admit, I don't understand the point.

Oleg.

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