diff --git a/Makefile b/Makefile
index 49fb35c..15a29a8 100644
--- a/Makefile
+++ b/Makefile
@@ -9,9 +9,6 @@ CFLAGS=-Wall -std=c89 -Werror -pedantic -fPIC -g -Iinclude -I. -Wno-unused-funct
 
 tests: string_tests htable_tests
 
-with_valgrind:
-	make EXTRACFLAGS=-DUNS_VALGRIND
-
 COMMON_OBJECTS=uns.o
 
 uns.o: uns.c include/uns.h
diff --git a/cheney_c89.c b/cheney_c89.c
index b94fd55..a8fb3eb 100644
--- a/cheney_c89.c
+++ b/cheney_c89.c
@@ -16,14 +16,21 @@
 */
 #include <stdio.h>
 
+/* XXX: Currently valgrind marks the header region as valid for all
+ * functions, when it should only be valid inside allocator calls.
+ */
 #ifdef UNS_VALGRIND
 # include <valgrind/valgrind.h>
+# include <valgrind/memcheck.h>
 # define REDZONE 16
 #else
 # define REDZONE 0
 # define VALGRIND_CREATE_MEMPOOL(pool, rzB, is_zeroed) (void)0
 # define VALGRIND_DESTROY_MEMPOOL(pool) (void)0
 # define VALGRIND_MEMPOOL_ALLOC(pool, ptr, siz) (void)0
+# define VALGRIND_CHECK_MEM_IS_ADDRESSABLE(p, len) 0
+# define VALGRIND_MAKE_MEM_DEFINED(p, len) (void)0
+# define VALGRIND_MAKE_MEM_NOACCESS(p, len) (void)0
 #endif
 
 #include <stdlib.h>
@@ -134,7 +141,13 @@ static void hdr_read(struct hdr *hdr, unsigned char *p)
 	assert(hdr);
 	assert(p);
 
+	if (VALGRIND_CHECK_MEM_IS_ADDRESSABLE(p + HDR_LEN, 1) != 0)
+		abort();
+
+	VALGRIND_MAKE_MEM_DEFINED(p, HDR_LEN);
 	memcpy(&hdr->len, p, HDR_LEN);
+	VALGRIND_MAKE_MEM_NOACCESS(p, HDR_LEN);
+
 	if (hdr->len < 0) {
 		hdr->typ = RECORD;
 		hdr->len = -hdr->len;
@@ -144,6 +157,9 @@ static void hdr_read(struct hdr *hdr, unsigned char *p)
 	} else {
 		hdr->typ = REGION;
 	}
+
+	if (VALGRIND_CHECK_MEM_IS_ADDRESSABLE(p + HDR_LEN, hdr->len) != 0)
+		abort();
 }
 
 /** Destructure header from a pointer to a region.
@@ -178,7 +194,12 @@ static void hdr_write_direct(struct hdr *hdr, unsigned char *p)
 	case RECORD: s = -hdr->len; break;
 	}
 
+	if (VALGRIND_CHECK_MEM_IS_ADDRESSABLE(p + HDR_LEN, hdr->len) != 0)
+		abort();
+
+	VALGRIND_MAKE_MEM_DEFINED(p, HDR_LEN);
 	memcpy(p, &s, HDR_LEN);
+	VALGRIND_MAKE_MEM_NOACCESS(p, HDR_LEN);
 }
 
 /** Write to the header of a region.
@@ -225,7 +246,7 @@ static void *raw_alloc(struct ctx *ctx, uns_sword len, int is_record)
 	assert(enough_space(ctx, len));
 
 	p = ctx->tospace_alloc;
-	VALGRIND_MEMPOOL_ALLOC(ctx->tospace, p, len);
+	VALGRIND_MEMPOOL_ALLOC(ctx->tospace, p + HDR_LEN, len - HDR_LEN);
 
 	hdr_write_direct(&hdr, p);
 	ctx->tospace_alloc += len + REDZONE;