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diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -159,6 +159,9 @@
}
caddr_t __read_mostly unmapped_buf;
+#ifdef INVARIANTS
+caddr_t poisoned_buf = (void *)-1;
+#endif
/* Used below and for softdep flushing threads in ufs/ffs/ffs_softdep.c */
struct proc *bufdaemonproc;
@@ -1211,6 +1214,9 @@
mtx_init(&bdirtylock, "dirty buf lock", NULL, MTX_DEF);
unmapped_buf = (caddr_t)kva_alloc(maxphys);
+#ifdef INVARIANTS
+ poisoned_buf = unmapped_buf;
+#endif
/* finally, initialize each buffer header and stick on empty q */
for (i = 0; i < nbuf; i++) {
diff --git a/sys/sys/kassert.h b/sys/sys/kassert.h
--- a/sys/sys/kassert.h
+++ b/sys/sys/kassert.h
@@ -38,6 +38,37 @@
extern bool panicked;
#define KERNEL_PANICKED() __predict_false(panicked)
+/*
+ * Trap accesses going through a pointer. Moreover if kasan is available trap
+ * reading the pointer itself.
+ *
+ * Sample usage: you have a struct with numerous fields and by API contract
+ * only some of them get populated, even if the implementation temporary writes
+ * to them. You can DEBUG_POISON_POINTER so the consumer which should no be
+ * looking at the field gets caught.
+ *
+ * DEBUG_POISON_POINTER(obj->ptr);
+ * ....
+ * if (obj->ptr != NULL) // traps with kasan, does not trap otherwise
+ * ....
+ * obj->ptr->field // traps with and without kasan
+ */
+#ifdef INVARIANTS
+
+#include <sys/asan.h>
+
+extern caddr_t poisoned_buf;
+#define DEBUG_POISON_POINTER_VALUE poisoned_buf
+
+#define DEBUG_POISON_POINTER(x) ({ \
+ x = (void *)(DEBUG_POISON_POINTER_VALUE); \
+ kasan_mark(&x, 0, sizeof(x), KASAN_GENERIC_REDZONE); \
+})
+
+#else
+#define DEBUG_POISON_POINTER(x)
+#endif
+
#ifdef INVARIANTS /* The option is always available */
#define VNASSERT(exp, vp, msg) do { \
if (__predict_false(!(exp))) { \

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