diff --git a/libc/Android.mk b/libc/Android.mk
index 4f6afe8..2ea7f1f 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -171,7 +171,6 @@
     bionic/recv.cpp \
     bionic/rename.cpp \
     bionic/rmdir.cpp \
-    bionic/sbrk.cpp \
     bionic/scandir.cpp \
     bionic/sched_getaffinity.cpp \
     bionic/sched_getcpu.cpp \
diff --git a/libc/bionic/brk.cpp b/libc/bionic/brk.cpp
index 633b914..4d08a61 100644
--- a/libc/bionic/brk.cpp
+++ b/libc/bionic/brk.cpp
@@ -26,16 +26,49 @@
  * SUCH DAMAGE.
  */
 
+#include <errno.h>
 #include <unistd.h>
 
-/* Shared with sbrk.c. */
-extern "C" void* __bionic_brk; // TODO: should be __LIBC_HIDDEN__ but accidentally exported by NDK :-(
+#if __LP64__
+static void* __bionic_brk;
+#else
+void* __bionic_brk; // Accidentally exported by the NDK.
+#endif
 
 int brk(void* end_data) {
-  void* new_brk = __brk(end_data);
-  if (new_brk != end_data) {
+  __bionic_brk = __brk(end_data);
+  if (__bionic_brk < end_data) {
+    errno = ENOMEM;
     return -1;
   }
-  __bionic_brk = new_brk;
   return 0;
 }
+
+void* sbrk(ptrdiff_t increment) {
+  // Initialize __bionic_brk if necessary.
+  if (__bionic_brk == NULL) {
+    __bionic_brk = __brk(NULL);
+  }
+
+  // Don't ask the kernel if we already know the answer.
+  if (increment == 0) {
+    return __bionic_brk;
+  }
+
+  // Avoid overflow.
+  intptr_t old_brk = reinterpret_cast<intptr_t>(__bionic_brk);
+  if ((increment > 0 && INTPTR_MAX - increment > old_brk) ||
+      (increment < 0 && (increment == PTRDIFF_MIN || old_brk < -increment))) {
+    errno = ENOMEM;
+    return reinterpret_cast<void*>(-1);
+  }
+
+  void* desired_brk = reinterpret_cast<void*>(old_brk + increment);
+  __bionic_brk = __brk(desired_brk);
+
+  if (__bionic_brk < desired_brk) {
+    errno = ENOMEM;
+    return reinterpret_cast<void*>(-1);
+  }
+  return reinterpret_cast<void*>(old_brk);
+}
diff --git a/libc/bionic/sbrk.cpp b/libc/bionic/sbrk.cpp
deleted file mode 100644
index 6c9b534..0000000
--- a/libc/bionic/sbrk.cpp
+++ /dev/null
@@ -1,55 +0,0 @@
-/*
- * Copyright (C) 2008 The Android Open Source Project
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- *  * Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- *  * Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in
- *    the documentation and/or other materials provided with the
- *    distribution.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
- * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
- * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
- * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
- * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
- * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
- * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
- * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
- * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-
-#include <unistd.h>
-#include <errno.h>
-
-/* Shared with brk.c. */
-extern "C" {
-  void* __bionic_brk; // TODO: should be __LIBC_HIDDEN__ but accidentally exported by NDK :-(
-}
-
-void* sbrk(ptrdiff_t increment) {
-  if (__bionic_brk == NULL) {
-    __bionic_brk = __brk(NULL);
-  }
-
-  void* original_brk = __bionic_brk;
-  void* desired_brk = (void*) ((uintptr_t) original_brk + increment);
-
-  void* new_brk = __brk(desired_brk);
-  if (new_brk == (void*) -1) {
-    return new_brk;
-  } else if (new_brk < desired_brk) {
-    errno = ENOMEM;
-    return (void*) -1;
-  }
-
-  __bionic_brk = new_brk;
-  return original_brk;
-}
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index 70b23bd..f56b767 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -18,6 +18,8 @@
 #include "ScopedSignalHandler.h"
 #include "TemporaryFile.h"
 
+#define __STDC_LIMIT_MACROS // For glibc.
+
 #include <errno.h>
 #include <fcntl.h>
 #include <stdint.h>
@@ -29,14 +31,49 @@
   ASSERT_GT(sysconf(_SC_MONOTONIC_CLOCK), 0);
 }
 
-TEST(unistd, sbrk) {
-  void* initial_break = sbrk(0);
+static void* get_brk() {
+  return sbrk(0);
+}
 
-  void* new_break = reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(initial_break) + 2000);
+static void* page_align(uintptr_t addr) {
+  uintptr_t mask = sysconf(_SC_PAGE_SIZE) - 1;
+  return reinterpret_cast<void*>((addr + mask) & ~mask);
+}
+
+TEST(unistd, brk) {
+  void* initial_break = get_brk();
+
+  // The kernel aligns the break to a page.
+  void* new_break = reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(initial_break) + 1);
   ASSERT_EQ(0, brk(new_break));
+  ASSERT_GE(get_brk(), new_break);
 
-  void* final_break = sbrk(0);
-  ASSERT_EQ(final_break, new_break);
+  new_break = page_align(reinterpret_cast<uintptr_t>(initial_break) + sysconf(_SC_PAGE_SIZE));
+  ASSERT_EQ(0, brk(new_break));
+  ASSERT_EQ(get_brk(), new_break);
+}
+
+TEST(unistd, brk_ENOMEM) {
+  ASSERT_EQ(-1, brk(reinterpret_cast<void*>(-1)));
+  ASSERT_EQ(ENOMEM, errno);
+}
+
+TEST(unistd, sbrk_ENOMEM) {
+  intptr_t current_brk = reinterpret_cast<intptr_t>(get_brk());
+
+  // Can't increase by so much that we'd overflow.
+  ASSERT_EQ(reinterpret_cast<void*>(-1), sbrk(PTRDIFF_MAX));
+  ASSERT_EQ(ENOMEM, errno);
+
+  // Can't reduce by more than the current break.
+  ASSERT_EQ(reinterpret_cast<void*>(-1), sbrk(-(current_brk + 1)));
+  ASSERT_EQ(ENOMEM, errno);
+
+#if !defined(__GLIBC__)
+  // The maximum negative value is an interesting special case that glibc gets wrong.
+  ASSERT_EQ(reinterpret_cast<void*>(-1), sbrk(PTRDIFF_MIN));
+  ASSERT_EQ(ENOMEM, errno);
+#endif
 }
 
 TEST(unistd, truncate) {