linux-kernel - Re: [PATCH 1/3] arch/x86: Use offsetof

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Date:	Thu, 27 Dec 2007 02:44:33 +0100 (CET)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Pavel Machek <pavel@....cz>, Julia Lawall <julia@...u.dk>,
	tglx@...utronix.de, viro@....linux.org.uk,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 1/3] arch/x86: Use offsetof


On Dec 26 2007 17:01, H. Peter Anvin wrote:
>> > @@ -215,7 +215,9 @@ asmlinkage int sys_vm86old(struct pt_reg
>> >   ret = -EFAULT;
>> >   if (tmp)
>> > 		goto out;
>> > -	memset(&info.vm86plus, 0, (int)&info.regs32 - (int)&info.vm86plus);
>> > +	memset(&info.vm86plus, 0,
>> > +	       offsetof(struct kernel_vm86_struct, regs32) -
>> > +	       offsetof(struct kernel_vm86_struct, vm86plus));
>> 
>> I do not think this makes it more readable... (int) -> (char *) would
>> make it portable and readable, AFAICT.
>>         Pavel
>
> The right way to do it is:
>
> 	memset(&info.vm86plus, 0, sizeof info.vm86plus);

Either way, downcasting a pointer to (int) is dangerous,
even if this one occurrence happens to be in 32-bit-only code.
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