um: Allow builds with Clang

Add SUBARCH target for Clang+um (which must go last, not alphabetically,
so the other SUBARCHes are assigned). Remove open-coded "DEFINE"
macro, instead using linux/kbuild.h's version which was updated to use
Clang-friendly assembly in commit cf0c3e68aa ("kbuild: fix asm-offset
generation to work with clang"). Redefine "DEFINE_LONGS" in terms of
"COMMENT" and "DEFINE" so that the intended coment actually has useful
content. Add a missed "break" to avoid implicit fall-through warnings.

This lets me run KUnit tests with Clang:

$ ./tools/testing/kunit/kunit.py run --make_options LLVM=1
...

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: David Gow <davidgow@google.com>
Cc: linux-um@lists.infradead.org
Cc: linux-kbuild@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Cc: llvm@lists.linux.dev
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/lkml/Yg2YubZxvYvx7%2Fnm@dev-arch.archlinux-ax161/
Tested-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/lkml/CABVgOSk=oFxsbSbQE-v65VwR2+mXeGXDDjzq8t7FShwjJ3+kUg@mail.gmail.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v1: https://lore.kernel.org/lkml/20220217002843.2312603-1-keescook@chromium.org
v2: https://lore.kernel.org/lkml/20220224055831.1854786-1-keescook@chromium.org
v3:
 - use kbuild.h to avoid duplication (Masahiro)
 - fix intended comments (Masahiro)
 - use SUBARCH (Nathan)

arch/um/os-Linux/execvp.c

@@ -93,6 +93,7 @@ int execvp_noalloc(char *buf, const char *file, char *const argv[])
 					   up finding no executable we can use, we want to diagnose
 					   that we did find one but were denied access.  */
 					got_eacces = 1;
+					break;
 				case ENOENT:
 				case ESTALE:
 				case ENOTDIR:

arch/x86/um/user-offsets.c

@@ -8,12 +8,11 @@
 #define __FRAME_OFFSETS
 #include <linux/ptrace.h>
 #include <asm/types.h>
+#include <linux/kbuild.h>
 
-#define DEFINE(sym, val) \
-	asm volatile("\n->" #sym " %0 " #val : : "i" (val))
-
-#define DEFINE_LONGS(sym, val) \
-	asm volatile("\n->" #sym " %0 " #val : : "i" (val/sizeof(unsigned long)))
+#define DEFINE_LONGS(sym, val)	\
+	COMMENT(#val " / sizeof(unsigned long)");	\
+	DEFINE(sym, val / sizeof(unsigned long))
 
 void foo(void)
 {

scripts/Makefile.clang

@@ -10,6 +10,7 @@ CLANG_TARGET_FLAGS_powerpc	:= powerpc64le-linux-gnu
 CLANG_TARGET_FLAGS_riscv	:= riscv64-linux-gnu
 CLANG_TARGET_FLAGS_s390		:= s390x-linux-gnu
 CLANG_TARGET_FLAGS_x86		:= x86_64-linux-gnu
+CLANG_TARGET_FLAGS_um		:= $(CLANG_TARGET_FLAGS_$(SUBARCH))
 CLANG_TARGET_FLAGS		:= $(CLANG_TARGET_FLAGS_$(SRCARCH))
 
 ifeq ($(CROSS_COMPILE),)