selftests/mm: fix -Warray-bounds warnings in pkey_sighandler_tests

GCC doesn't like dereferencing a pointer set to 0x1 (when building
at -O2):

pkey_sighandler_tests.c:166:9: warning: array subscript 0 is outside array bounds of 'int[0]' [-Warray-bounds=]
  166 |         *(int *) (0x1) = 1;
      |         ^~~~~~~~~~~~~~
cc1: note: source object is likely at address zero

Using NULL instead seems to make it happy.  This should make no difference
in practice (SIGSEGV with SEGV_MAPERR will be the outcome regardless), we
just need to update the expected si_addr.

[[email protected]: fix clang dereferencing-null issue]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kevin Brodsky <[email protected]>
Cc: Aruna Ramakrishna <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Joey Gouly <[email protected]>
Cc: Keith Lucas <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: kernel test robot <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: kevin-brodsky-arm

tools/testing/selftests/mm/pkey_sighandler_tests.c

@@ -163,7 +163,7 @@ static void *thread_segv_with_pkey0_disabled(void *ptr)
 	__write_pkey_reg(pkey_reg_restrictive_default());
 
 	/* Segfault (with SEGV_MAPERR) */
-	*(int *) (0x1) = 1;
+	*(volatile int *)NULL = 1;
 	return NULL;
 }
 
@@ -179,7 +179,6 @@ static void *thread_segv_pkuerr_stack(void *ptr)
 static void *thread_segv_maperr_ptr(void *ptr)
 {
 	stack_t *stack = ptr;
-	int *bad = (int *)1;
 	u64 pkey_reg;
 
 	/*
@@ -195,7 +194,7 @@ static void *thread_segv_maperr_ptr(void *ptr)
 	__write_pkey_reg(pkey_reg);
 
 	/* Segfault */
-	*bad = 1;
+	*(volatile int *)NULL = 1;
 	syscall_raw(SYS_exit, 0, 0, 0, 0, 0, 0);
 	return NULL;
 }
@@ -234,7 +233,7 @@ static void test_sigsegv_handler_with_pkey0_disabled(void)
 
 	ksft_test_result(siginfo.si_signo == SIGSEGV &&
 			 siginfo.si_code == SEGV_MAPERR &&
-			 siginfo.si_addr == (void *)1,
+			 siginfo.si_addr == NULL,
 			 "%s\n", __func__);
 }
 
@@ -349,7 +348,7 @@ static void test_sigsegv_handler_with_different_pkey_for_stack(void)
 
 	ksft_test_result(siginfo.si_signo == SIGSEGV &&
 			 siginfo.si_code == SEGV_MAPERR &&
-			 siginfo.si_addr == (void *)1,
+			 siginfo.si_addr == NULL,
 			 "%s\n", __func__);
 }