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

[Theodore Ts'o]

On Wed, Apr 13, 2005 at 08:40:05AM -0400, Richard B. Johnson wrote:
> The kernel does NOT have to copy data from user-space before
> using it. 

Incorrect.  It must, or the kernel code in question is by definition
buggy.

> In fact, user-mode pointers are valid in kernel-space
> when the kernel is performing a function on behalf of the user-
> mode code. 

On some architectures, this is true.  But not all architectures, and
not in all circumstances.  For example, even on the x86 architecture,
in the 4G/4G mode, a user-mode pointer is *not* valid when kernel code
is running.  You must use copy_to_user()/copy_from_user().  Simply
dereferencing a user-mode pointer is a BUG.  It might work sometimes,
on some architectures, but not everywhere.  Therefore, for correctly
written kernel code, you must not do it.

						- Ted
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