uaccess: reimplement probe_kernel_address() using probe_kernel_read()

probe_kernel_address() is basically the same as the (later added)
probe_kernel_read().

The return value on EFAULT is a bit different: probe_kernel_address()
returns number-of-bytes-not-copied whereas probe_kernel_read() returns
-EFAULT.  All callers have been checked, none cared.

probe_kernel_read() can be overridden by the architecture whereas
probe_kernel_address() cannot.  parisc, blackfin and um do this, to insert
additional checking.  Hence this patch possibly fixes obscure bugs,
although there are only two probe_kernel_address() callsites outside
arch/.

My first attempt involved removing probe_kernel_address() entirely and
converting all callsites to use probe_kernel_read() directly, but that got
tiresome.

This patch shrinks mm/slab_common.o by 218 bytes.  For a single
probe_kernel_address() callsite.

Cc: Steven Miao <realmz6@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Helge Deller <deller@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/maccess.c

@@ -13,6 +13,11 @@
  *
  * Safely read from address @src to the buffer at @dst.  If a kernel fault
  * happens, handle that and return -EFAULT.
+ *
+ * We ensure that the copy_from_user is executed in atomic context so that
+ * do_page_fault() doesn't attempt to take mmap_sem.  This makes
+ * probe_kernel_read() suitable for use within regions where the caller
+ * already holds mmap_sem, or other locks which nest inside mmap_sem.
  */
 
 long __weak probe_kernel_read(void *dst, const void *src, size_t size)