Implement guard pages.

Adding guarded extents, which are regular extents surrounded by guard pages
(mprotected).  To reduce syscalls, small guarded extents are cached as a
separate eset in ecache, and decay through the dirty / muzzy / retained pipeline
as usual.

src/guard.c

@@ -0,0 +1,63 @@
+#include "jemalloc/internal/jemalloc_preamble.h"
+#include "jemalloc/internal/jemalloc_internal_includes.h"
+
+#include "jemalloc/internal/assert.h"
+#include "jemalloc/internal/ehooks.h"
+#include "jemalloc/internal/guard.h"
+#include "jemalloc/internal/tsd.h"
+
+/* The sanitizer options. */
+size_t opt_san_guard_large = SAN_GUARD_LARGE_EVERY_N_EXTENTS_DEFAULT;
+size_t opt_san_guard_small = SAN_GUARD_SMALL_EVERY_N_EXTENTS_DEFAULT;
+
+void
+guard_pages(tsdn_t *tsdn, ehooks_t *ehooks, edata_t *edata, emap_t *emap) {
+	emap_deregister_boundary(tsdn, emap, edata);
+
+	size_t size_with_guards = edata_size_get(edata);
+	size_t usize = size_with_guards - PAGE_GUARDS_SIZE;
+
+	uintptr_t guard1 = (uintptr_t)edata_base_get(edata);
+	uintptr_t addr = guard1 + PAGE;
+	uintptr_t guard2 = addr + usize;
+
+	assert(edata_state_get(edata) == extent_state_active);
+	ehooks_guard(tsdn, ehooks, (void *)guard1, (void *)guard2);
+
+	/* Update the guarded addr and usable size of the edata. */
+	edata_size_set(edata, usize);
+	edata_addr_set(edata, (void *)addr);
+	edata_guarded_set(edata, true);
+
+	/* The new boundary will be registered on the pa_alloc path. */
+}
+
+void
+unguard_pages(tsdn_t *tsdn, ehooks_t *ehooks, edata_t *edata, emap_t *emap) {
+	/* Remove the inner boundary which no longer exists. */
+	emap_deregister_boundary(tsdn, emap, edata);
+
+	size_t size = edata_size_get(edata);
+	size_t size_with_guards = size + PAGE_GUARDS_SIZE;
+
+	uintptr_t addr =  (uintptr_t)edata_base_get(edata);
+	uintptr_t guard1 = addr - PAGE;
+	uintptr_t guard2 = addr + size;
+
+	assert(edata_state_get(edata) == extent_state_active);
+	ehooks_unguard(tsdn, ehooks, (void *)guard1, (void *)guard2);
+
+	/* Update the true addr and usable size of the edata. */
+	edata_size_set(edata, size_with_guards);
+	edata_addr_set(edata, (void *)guard1);
+	edata_guarded_set(edata, false);
+
+	/* Then re-register the outer boundary including the guards. */
+	emap_register_boundary(tsdn, emap, edata, SC_NSIZES, /* slab */ false);
+}
+
+void
+tsd_san_init(tsd_t *tsd) {
+	*tsd_san_extents_until_guard_smallp_get(tsd) = opt_san_guard_small;
+	*tsd_san_extents_until_guard_largep_get(tsd) = opt_san_guard_large;
+}