kmsan: introduce memset_no_sanitize_memory()

Add a wrapper for memset() that prevents unpoisoning.  This is useful for
filling memory allocator redzones.

Link: https://lkml.kernel.org/r/20240621113706.315500-13-iii@linux.ibm.comSigned-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: <kasan-dev@googlegroups.com>
Cc: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/kmsan.h

@@ -255,6 +255,19 @@ void kmsan_enable_current(void);
  */
 void kmsan_disable_current(void);
 
+/**
+ * memset_no_sanitize_memory(): Fill memory without KMSAN instrumentation.
+ * @s: address of kernel memory to fill.
+ * @c: constant byte to fill the memory with.
+ * @n: number of bytes to fill.
+ *
+ * This is like memset(), but without KMSAN instrumentation.
+ */
+static inline void *memset_no_sanitize_memory(void *s, int c, size_t n)
+{
+	return __memset(s, c, n);
+}
+
 #else
 
 static inline void kmsan_init_shadow(void)
@@ -362,6 +375,11 @@ static inline void kmsan_disable_current(void)
 {
 }
 
+static inline void *memset_no_sanitize_memory(void *s, int c, size_t n)
+{
+	return memset(s, c, n);
+}
+
 #endif
 
 #endif /* _LINUX_KMSAN_H */