syscalls: use uaccess_kernel in addr_limit_user_check

Patch series "clean up address limit helpers", v2.

In preparation for eventually phasing out direct use of set_fs(), this
series removes the segment_eq() arch helper that is only used to implement
or duplicate the uaccess_kernel() API, and then adds descriptive helpers
to force the kernel address limit.

This patch (of 6):

Use the uaccess_kernel helper instead of duplicating it.

[hch@lst.de: arm: don't call addr_limit_user_check for nommu]
  Link: http://lkml.kernel.org/r/20200721045834.GA9613@lst.deSigned-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Tested-by: NGuenter Roeck <linux@roeck-us.net>
Acked-by: NLinus Torvalds <torvalds@linux-foundation.org>
Cc: Nick Hu <nickhu@andestech.com>
Cc: Greentime Hu <green.hu@gmail.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Link: http://lkml.kernel.org/r/20200714105505.935079-1-hch@lst.de
Link: http://lkml.kernel.org/r/20200710135706.537715-1-hch@lst.de
Link: http://lkml.kernel.org/r/20200710135706.537715-2-hch@lst.deSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/arm/kernel/signal.c

@@ -713,7 +713,9 @@ struct page *get_signal_page(void)
 /* Defer to generic check */
 asmlinkage void addr_limit_check_failed(void)
 {
+#ifdef CONFIG_MMU
 	addr_limit_user_check();
+#endif
 }
 
 #ifdef CONFIG_DEBUG_RSEQ

include/linux/syscalls.h

@@ -263,7 +263,7 @@ static inline void addr_limit_user_check(void)
 		return;
 #endif
 
-	if (CHECK_DATA_CORRUPTION(!segment_eq(get_fs(), USER_DS),
+	if (CHECK_DATA_CORRUPTION(uaccess_kernel(),
 				  "Invalid address limit on user-mode return"))
 		force_sig(SIGKILL);