selftests: sigaltstack: fix -Wuninitialized

Building sigaltstack with clang via:
$ ARCH=x86 make LLVM=1 -C tools/testing/selftests/sigaltstack/

produces the following warning:
warning: variable 'sp' is uninitialized when used here [-Wuninitialized]
if (sp < (unsigned long)sstack ||
^~

Clang expects these to be declared at global scope; we've fixed this in
the kernel proper by using the macro `current_stack_pointer`. This is
defined in different headers for different target architectures, so just
create a new header that defines the arch-specific register names for
the stack pointer register, and define it for more targets (at least the
ones that support current_stack_pointer/ARCH_HAS_CURRENT_STACK_POINTER).

Reported-by: Linux Kernel Functional Testing <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Nick Desaulniers <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Tested-by: Linux Kernel Functional Testing <[e-mail]>
Tested-by: Anders Roxell <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/sigaltstack/current_stack_pointer.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#if __alpha__
+register unsigned long sp asm("$30");
+#elif __arm__ || __aarch64__ || __csky__ || __m68k__ || __mips__ || __riscv
+register unsigned long sp asm("sp");
+#elif __i386__
+register unsigned long sp asm("esp");
+#elif __loongarch64
+register unsigned long sp asm("$sp");
+#elif __ppc__
+register unsigned long sp asm("r1");
+#elif __s390x__
+register unsigned long sp asm("%15");
+#elif __sh__
+register unsigned long sp asm("r15");
+#elif __x86_64__
+register unsigned long sp asm("rsp");
+#elif __XTENSA__
+register unsigned long sp asm("a1");
+#else
+#error "implement current_stack_pointer equivalent"
+#endif

tools/testing/selftests/sigaltstack/sas.c

···
 #include <sys/auxv.h>
 
 #include "../kselftest.h"
+#include "current_stack_pointer.h"
 
 #ifndef SS_AUTODISARM
 #define SS_AUTODISARM  (1U << 31)
···
 	int err;
 	stack_t stk;
 	struct stk_data *p;
-
-#if __s390x__
-	register unsigned long sp asm("%15");
-#else
-	register unsigned long sp asm("sp");
-#endif
 
 	if (sp < (unsigned long)sstack ||
 			sp >= (unsigned long)sstack + stack_size) {