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      make 'user_access_begin()' do 'access_ok()' · 594cc251
      Linus Torvalds 提交于
      Originally, the rule used to be that you'd have to do access_ok()
      separately, and then user_access_begin() before actually doing the
      direct (optimized) user access.
      
      But experience has shown that people then decide not to do access_ok()
      at all, and instead rely on it being implied by other operations or
      similar.  Which makes it very hard to verify that the access has
      actually been range-checked.
      
      If you use the unsafe direct user accesses, hardware features (either
      SMAP - Supervisor Mode Access Protection - on x86, or PAN - Privileged
      Access Never - on ARM) do force you to use user_access_begin().  But
      nothing really forces the range check.
      
      By putting the range check into user_access_begin(), we actually force
      people to do the right thing (tm), and the range check vill be visible
      near the actual accesses.  We have way too long a history of people
      trying to avoid them.
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      594cc251