x86: Unify copy_from_user() size checking

x86: Unify copy_from_user() size checking

Commits 4a3127693001c61a21d1ce680db6340623f52e93 ("x86: Turn the
copy_from_user check into an (optional) compile time warning")
and 63312b6a6faae3f2e5577f2b001e3b504f10a2aa ("x86: Add a
Kconfig option to turn the copy_from_user warnings into errors")
touched only the 32-bit variant of copy_from_user(), whereas the
original commit 9f0cf4adb6aa0bfccf675c938124e68f7f06349d ("x86:
Use __builtin_object_size() to validate the buffer size for
copy_from_user()") also added the same code to the 64-bit one.

Further the earlier conversion from an inline WARN() to the call
to copy_from_user_overflow() went a little too far: When the
number of bytes to be copied is not a constant (e.g. [looking at
3.11] in drivers/net/tun.c:__tun_chr_ioctl() or
drivers/pci/pcie/aer/aer_inject.c:aer_inject_write()), the
compiler will always have to keep the funtion call, and hence
there will always be a warning. By using __builtin_constant_p()
we can avoid this.

And then this slightly extends the effect of
CONFIG_DEBUG_STRICT_USER_COPY_CHECKS in that apart from
converting warnings to errors in the constant size case, it
retains the (possibly wrong) warnings in the non-constant size
case, such that if someone is prepared to get a few false
positives, (s)he'll be able to recover the current behavior
(except that these diagnostics now will never be converted to
errors).

Since the 32-bit variant (intentionally) didn't call
might_fault(), the unification results in this being called
twice now. Adding a suitable #ifdef would be the alternative if
that's a problem.

I'd like to point out though that with
__compiletime_object_size() being restricted to gcc before 4.6,
the whole construct is going to become more and more pointless
going forward. I would question however that commit
2fb0815c9ee6b9ac50e15dd8360ec76d9fa46a2 ("gcc4: disable
__compiletime_object_size for GCC 4.6+") was really necessary,
and instead this should have been dealt with as is done here
from the beginning.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/5265056D02000078000FC4F3@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>