powerpc: make stack walking KASAN-safe

Make our stack-walking code KASAN-safe by using __no_sanitize_address.
Generic code, arm64, s390 and x86 all make accesses unchecked for similar
sorts of reasons: when unwinding a stack, we might touch memory that KASAN
has marked as being out-of-bounds. In ppc64 KASAN development, I hit this
sometimes when checking for an exception frame - because we're checking
an arbitrary offset into the stack frame.

See commit 20955746 ("s390/kasan: avoid false positives during stack
unwind"), commit bcaf669b ("arm64: disable kasan when accessing
frame->fp in unwind_frame"), commit 91e08ab0 ("x86/dumpstack:
Prevent KASAN false positive warnings") and commit 6e22c836
("tracing, kasan: Silence Kasan warning in check_stack of stack_tracer").
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210614120907.1952321-1-dja@axtens.net

arch/powerpc/kernel/process.c

@@ -2133,8 +2133,9 @@ unsigned long get_wchan(struct task_struct *p)
 
 static int kstack_depth_to_print = CONFIG_PRINT_STACK_DEPTH;
 
-void show_stack(struct task_struct *tsk, unsigned long *stack,
-		const char *loglvl)
+void __no_sanitize_address show_stack(struct task_struct *tsk,
+				      unsigned long *stack,
+				      const char *loglvl)
 {
 	unsigned long sp, ip, lr, newsp;
 	int count = 0;

arch/powerpc/kernel/stacktrace.c

@@ -23,8 +23,8 @@
 
 #include <asm/paca.h>
 
-void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
-		     struct task_struct *task, struct pt_regs *regs)
+void __no_sanitize_address arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
+					   struct task_struct *task, struct pt_regs *regs)
 {
 	unsigned long sp;
 
@@ -61,8 +61,8 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
  *
  * If the task is not 'current', the caller *must* ensure the task is inactive.
  */
-int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
-			     void *cookie, struct task_struct *task)
+int __no_sanitize_address arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
+						   void *cookie, struct task_struct *task)
 {
 	unsigned long sp;
 	unsigned long newsp;

arch/powerpc/perf/callchain.c

@@ -40,7 +40,7 @@ static int valid_next_sp(unsigned long sp, unsigned long prev_sp)
 	return 0;
 }
 
-void
+void __no_sanitize_address
 perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs)
 {
 	unsigned long sp, next_sp;