kcov: Integrate with KMSAN

- kern_kcov.c needs to be compiled with -fsanitize=kernel-memory when
  KMSAN is configured since it calls into various other subsystems.
- Disable address and memory sanitizers in kcov(4)'s coverage sanitizer
  callbacks, as they do not provide useful checking.  Moreover, with
  KMSAN we may otherwise get false positives since the caller (coverage
  sanitizer runtime) is not instrumented.
- Disable KASAN and KMSAN interceptors in subr_coverage.c, as they do
  not provide any benefit but do introduce overhead when fuzzing.

Sponsored by:	The FreeBSD Foundation

sys/conf/files

@@ -3826,7 +3826,7 @@ kern/kern_idle.c		standard
 kern/kern_intr.c		standard
 kern/kern_jail.c		standard
 kern/kern_kcov.c		optional kcov			\
-	compile-with "${NORMAL_C:N-fsanitize*}"
+	compile-with "${NORMAL_C:N-fsanitize*} ${NORMAL_C:M-fsanitize=kernel-memory}"
 kern/kern_khelp.c		standard
 kern/kern_kthread.c		standard
 kern/kern_ktr.c			optional ktr

sys/kern/kern_kcov.c

@@ -35,7 +35,8 @@
  * $FreeBSD$
  */
 
-#ifdef KCSAN
+/* Interceptors are required for KMSAN. */
+#if defined(KASAN) || defined(KCSAN)
 #define	SAN_RUNTIME
 #endif
 
@@ -191,7 +192,7 @@ get_kinfo(struct thread *td)
 	return (info);
 }
 
-static void
+static void __nosanitizeaddress __nosanitizememory
 trace_pc(uintptr_t ret)
 {
 	struct thread *td;
@@ -223,7 +224,7 @@ trace_pc(uintptr_t ret)
 	buf[0] = index + 1;
 }
 
-static bool
+static bool __nosanitizeaddress __nosanitizememory
 trace_cmp(uint64_t type, uint64_t arg1, uint64_t arg2, uint64_t ret)
 {
 	struct thread *td;

sys/kern/subr_coverage.c

@@ -35,7 +35,7 @@
  * $FreeBSD$
  */
 
-#ifdef KCSAN
+#ifdef SAN_NEEDS_INTERCEPTORS
 #define	SAN_RUNTIME
 #endif