Slab: head now uses bitmask for used/free nodes info instead of lists

From now, there are no auxiliary pointers stored in the free slab nodes.
This led to strange debugging problems if use-after-free happened in
slab-allocated structures, especially if the structure's first member is
a next pointer.

This also reduces the memory needed by 1 pointer per allocated object.
OTOH, we now rely on pages being aligned to their size's multiple, which
is quite common anyway.

1 files changed, 80 insertions, 0 deletions

diff --git a/sysdep/unix/alloc.c b/sysdep/unix/alloc.c
new file mode 100644
index 00000000..c525f713
--- /dev/null
+++ b/sysdep/unix/alloc.c
@@ -0,0 +1,80 @@
+/*
+ *	BIRD Internet Routing Daemon -- Raw allocation
+ *
+ *	(c) 2020  Maria Matejka <mq@ucw.cz>
+ *
+ *	Can be freely distributed and used under the terms of the GNU GPL.
+ */
+
+#include "nest/bird.h"
+#include "lib/resource.h"
+
+#include <stdlib.h>
+#include <unistd.h>
+
+#ifdef HAVE_MMAP
+#include <sys/mman.h>
+#endif
+
+#ifdef HAVE_MMAP
+static u64 page_size = 0;
+static _Bool use_fake = 0;
+#else
+static const u64 page_size = 4096; /* Fake page size */
+#endif
+
+u64 get_page_size(void)
+{
+  if (page_size)
+    return page_size;
+
+#ifdef HAVE_MMAP
+  if (page_size = sysconf(_SC_PAGESIZE))
+  {
+    if ((u64_popcount(page_size) > 1) || (page_size > 16384))
+    {
+      /* Too big or strange page, use the aligned allocator instead */
+      page_size = 4096;
+      use_fake = 1;
+    }
+    return page_size;
+  }
+
+  bug("Page size must be non-zero");
+#endif
+}
+
+void *
+alloc_page(void)
+{
+#ifdef HAVE_MMAP
+  if (!use_fake)
+  {
+    void *ret = mmap(NULL, get_page_size(), PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    if (ret == MAP_FAILED)
+      bug("mmap(%lu) failed: %m", page_size);
+    return ret;
+  }
+  else
+#endif
+  {
+    void *ret = aligned_alloc(page_size, page_size);
+    if (!ret)
+      bug("aligned_alloc(%lu) failed", page_size);
+    return ret;
+  }
+}
+
+void
+free_page(void *ptr)
+{
+#ifdef HAVE_MMAP
+  if (!use_fake)
+  {
+    if (munmap(ptr, get_page_size()) < 0)
+      bug("munmap(%p) failed: %m", ptr);
+  }
+  else
+#endif
+    free(ptr);
+}