From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Daniel Micay <danielmicay@gmail.com>
Date: Wed, 2 Dec 2015 23:37:28 -0500
Subject: [PATCH] switch pthread_atfork handler allocation to mmap

---
 libc/bionic/pthread_atfork.cpp | 35 ++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/libc/bionic/pthread_atfork.cpp b/libc/bionic/pthread_atfork.cpp
index 84e511c2e..651c026df 100644
--- a/libc/bionic/pthread_atfork.cpp
+++ b/libc/bionic/pthread_atfork.cpp
@@ -29,8 +29,11 @@
 #include <errno.h>
 #include <pthread.h>
 #include <stdlib.h>
+#include <sys/mman.h>
+#include <unistd.h>
 
 #include "private/bionic_macros.h"
+#include "private/bionic_prctl.h"
 
 struct atfork_t {
   atfork_t* next;
@@ -43,6 +46,8 @@ struct atfork_t {
   void* dso_handle;
 };
 
+static atfork_t* pool;
+
 class atfork_list_t {
  public:
   constexpr atfork_list_t() : first_(nullptr), last_(nullptr) {}
@@ -101,7 +106,8 @@ class atfork_list_t {
       last_ = entry->prev;
     }
 
-    free(entry);
+    entry->next = pool;
+    pool = entry;
   }
 
   atfork_t* first_;
@@ -154,18 +160,35 @@ void __bionic_atfork_run_parent() {
 // __register_atfork is the name used by glibc
 extern "C" int __register_atfork(void (*prepare)(void), void (*parent)(void),
                                  void(*child)(void), void* dso) {
-  atfork_t* entry = reinterpret_cast<atfork_t*>(malloc(sizeof(atfork_t)));
-  if (entry == nullptr) {
-    return ENOMEM;
+  pthread_mutex_lock(&g_atfork_list_mutex);
+
+  if (!pool) {
+    size_t page_size = getpagesize();
+    char* page = static_cast<char*>(mmap(NULL, page_size, PROT_READ|PROT_WRITE,
+                                         MAP_ANONYMOUS|MAP_PRIVATE, -1, 0));
+    if (page == MAP_FAILED) {
+      pthread_mutex_unlock(&g_atfork_list_mutex);
+      return ENOMEM;
+    }
+
+    prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, page, page_size,
+      "atfork handlers");
+
+    for (char* it = page; it < page + page_size - sizeof(atfork_t); it += sizeof(atfork_t)) {
+      atfork_t* node = reinterpret_cast<atfork_t*>(it);
+      node->next = pool;
+      pool = node;
+    }
   }
 
+  atfork_t* entry = pool;
+  pool = entry->next;
+
   entry->prepare = prepare;
   entry->parent = parent;
   entry->child = child;
   entry->dso_handle = dso;
 
-  pthread_mutex_lock(&g_atfork_list_mutex);
-
   g_atfork_list.push_back(entry);
 
   pthread_mutex_unlock(&g_atfork_list_mutex);
