Re: Why system call need to copy the date from the userspace before using it

[Tomko <tomko@haha.com>]

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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