Message-ID: <425CF7DB.4000407@haha.com>
Date: Wed, 13 Apr 2005 18:43:39 +0800
From: Tomko <tomko@haha.com>
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To: Jan-Benedict Glaw <jbglaw@lug-owl.de>, linux-kernel@vger.kernel.org
Subject: Re: Why system call need to copy the date from the userspace before
 using it
References: <425C9E55.6010607@haha.com> <20050413102916.GS4965@lug-owl.de>
In-Reply-To: <20050413102916.GS4965@lug-owl.de>
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hi ,
Thank you for your reply, can i ask some more question?

Inside the system call , the kernel often copy the data by calling 
copy_from_user() rather than just using strcpy(), is it because the 
memory mapping in kenel space is different from user space?  for example 
, now user program want to pass a pointer *a to kernel space , is it 
true that *a seems means address 0xb000 to user space but actually it is 
at 0xc000 at kernel space?

Thx a lot,
TOM

Jan-Benedict Glaw wrote:

>On Wed, 2005-04-13 12:21:41 +0800, Tomko <tomko@haha.com>
>wrote in message <425C9E55.6010607@haha.com>:
>  
>
>>While i am reading the source code of the linux system call , i find 
>>that the system call need to call copy_from_user() to copy the data from 
>>user space to kernel space before using it . Why not use it directly as 
>>the system call has got the address ?  Furthermore , how to distinguish 
>>between user space and kernel space ?
>>    
>>
>
>Think about the memory access. The page that contains the data could be
>swapped out, so the kernel isn't allowed to just access it, because it's
>not there.
>
>MfG, JBG
>
>  
>

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