Re: [PATCH] remember to check return value from __copy_to_user() in cdrom_read_cdda_old()

[viro@parcelfarce.linux.theplanet.co.uk]

On Tue, Sep 07, 2004 at 12:30:31PM +0200, Jens Axboe wrote:
> it boils down to access_ok() not being sufficient on its own, and in
> which case yes we should just use copy_to_user() and kill the check
> completely as per the patch sent out.

access_ok() is just "we can trust MMU to do the right thing when dealing
with access to process address space at that address".  On platforms with
secondary address spaces (e.g. sparc) it's always true.  On something like
i386 we *could* use segments for the same purposes.  In fact, we used to
do just that - access to userland memory went with %fs as segment (thus
the names like extinct memcpy_fromfs() and surviving set_fs()).  However,
it's cheaper to do that check explicitly instead of relying on MMU.  And
that's what access_ok() does.