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

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 findthat the system call need to call copy_from_user() to copy the data fromuser space to kernel space before using it . Why not use it directly asthe system call has got the address ? Furthermore , how to distinguishbetween user space and kernel space ?


Thx a lot,

TOM
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