PR target/96260 - KASAN should work even back-end not porting anything.

- Most KASAN function don't need any porting anything in back-end
   except asan stack protection.

 - However kernel will given shadow offset when enable asan stack
   protection, so eveything in KASAN can work if shadow offset is given.

 - Verified with x86 and risc-v.

 - Verified with RISC-V linux kernel.

gcc/ChangeLog:

	PR target/96260
	* asan.c (asan_shadow_offset_set_p): New.
	* asan.h (asan_shadow_offset_set_p): Ditto.
	* toplev.c (process_options): Allow -fsanitize=kernel-address
	even TARGET_ASAN_SHADOW_OFFSET not implemented, only check when
	asan stack protection is enabled.

gcc/testsuite/ChangeLog:

	PR target/96260
	* gcc.target/riscv/pr91441.c: Update warning message.
	* gcc.target/riscv/pr96260.c: New.

gcc/asan.c

@@ -344,6 +344,13 @@ asan_shadow_offset ()
   return asan_shadow_offset_value;
 }
 
+/* Returns Asan shadow offset has been set.  */
+bool
+asan_shadow_offset_set_p ()
+{
+  return asan_shadow_offset_computed;
+}
+
 alias_set_type asan_shadow_set = -1;
 
 /* Pointer types to 1, 2 or 4 byte integers in shadow memory.  A separate

gcc/asan.h

@@ -129,6 +129,8 @@ asan_var_and_redzone_size (unsigned HOST_WIDE_INT size)
 
 extern bool set_asan_shadow_offset (const char *);
 
+extern bool asan_shadow_offset_set_p ();
+
 extern void set_sanitized_sections (const char *);
 
 extern bool asan_sanitize_stack_p (void);

gcc/testsuite/gcc.target/riscv/pr91441.c

@@ -7,4 +7,4 @@ int *f( int a)
 {
   return bar(&a);
 }
-/* { dg-warning ".'-fsanitize=address' and '-fsanitize=kernel-address' are not supported for this target" "" { target *-*-* } 0 } */
+/* { dg-warning ".'-fsanitize=kernel-address' with stack protection is not supported without '-fasan-shadow-offset=' for this target" "" { target *-*-* } 0 } */

gcc/testsuite/gcc.target/riscv/pr96260.c

@@ -0,0 +1,9 @@
+/* PR target/96260 */
+/* { dg-do compile } */
+/* { dg-options "--param asan-stack=1 -fsanitize=kernel-address -fasan-shadow-offset=0x100000" } */
+
+int *bar(int *);
+int *f( int a)
+{
+  return bar(&a);
+}

gcc/toplev.c

@@ -1835,7 +1835,7 @@ process_options (void)
   /* Address Sanitizer needs porting to each target architecture.  */
 
   if ((flag_sanitize & SANITIZE_ADDRESS)
-      && (!FRAME_GROWS_DOWNWARD || targetm.asan_shadow_offset == NULL))
+      && !FRAME_GROWS_DOWNWARD)
     {
       warning_at (UNKNOWN_LOCATION, 0,
 		  "%<-fsanitize=address%> and %<-fsanitize=kernel-address%> "
@@ -1843,6 +1843,26 @@ process_options (void)
       flag_sanitize &= ~SANITIZE_ADDRESS;
     }
 
+  if ((flag_sanitize & SANITIZE_USER_ADDRESS)
+      && targetm.asan_shadow_offset == NULL)
+    {
+      warning_at (UNKNOWN_LOCATION, 0,
+		  "%<-fsanitize=address%> not supported for this target");
+      flag_sanitize &= ~SANITIZE_ADDRESS;
+    }
+
+  if ((flag_sanitize & SANITIZE_KERNEL_ADDRESS)
+      && (targetm.asan_shadow_offset == NULL
+	  && param_asan_stack
+	  && !asan_shadow_offset_set_p ()))
+    {
+      warning_at (UNKNOWN_LOCATION, 0,
+		  "%<-fsanitize=kernel-address%> with stack protection "
+		  "is not supported without %<-fasan-shadow-offset=%> "
+		  "for this target");
+      flag_sanitize &= ~SANITIZE_ADDRESS;
+    }
+
  /* Do not use IPA optimizations for register allocation if profiler is active
     or patchable function entries are inserted for run-time instrumentation
     or port does not emit prologue and epilogue as RTL.  */