linux-kernel - Re: [PATCH] arch: tile: kernel: kgdb.c: Use memcpy() instead of pointer copy one by one

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Date:	Thu, 13 Nov 2014 08:08:04 +0800
From:	Chen Gang <gang.chen.5i5j@...il.com>
To:	Jeff Epler <jepler@...ythonic.net>
Cc:	cmetcalf@...era.com,
	" linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch: tile: kernel: kgdb.c: Use memcpy() instead of
 pointer copy one by one

Thank you for your detail information, and what you said sounds
reasonable to me.

Send from Lenovo A788t.

Jeff Epler <jepler@...ythonic.net> wrote:

>On Wed, Nov 12, 2014 at 11:43:08PM +0800, Chen Gang wrote:
>> >                                                       (I assume the
>> > compiler could do things like replace an intended load from memory with
>> > a constant load or even no load at all)
>> > 
>> 
>> Excuse me, my English is not quite well, I can not understand what you
>> said above. (If necessary, please help provide more details for it).
>
>I am concerned that writing regs[TREG_TP] is "undefined behavior"
>according to the C standard.
>
>This expression is equivalent to *(regs + TREG_TP).  The expression
>(regs + TREG_TP) does not result in a pointer to any element of regs[],
>so dereferencing it is undefined behavior.  (Source: C99 draft standard
>WG14/N1256, annex J.2, "[The behavior is undefined if t]he operand of
>the unary * operator has an invalid value")
>
>That is why the compiler showed the original diagnostic, but the same
>logic that made the loop's behavior undefined also makes the expression
>regs[TREG_TP] undefined whereever it appears.
>
>None of this is a specific problem with your proposed patch.  Rather, it
>is a suggestion that the whole structure's design needs to be revisited
>in light of compilers beginning to notice that regs[TREG_TP] is
>undefined behavior and change their generated code as a result.
>
>Unfortunately it looks like this header is also a part of the userspace
>API, so it can't simply be changed just in case all in-kernel uses are
>changed.
>
>Jeff