Why system call need to copy the date from the userspace before usingit

[Tomko]

Hi all,

I am new to linux , hope someone can help me.
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 ?

Thx a lot,

TOM
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