diff --git a/libc/bionic/brk.cpp b/libc/bionic/brk.cpp
index 4d08a61..4e46193 100644
--- a/libc/bionic/brk.cpp
+++ b/libc/bionic/brk.cpp
@@ -56,19 +56,19 @@
   }
 
   // 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))) {
+  uintptr_t old_brk = reinterpret_cast<uintptr_t>(__bionic_brk);
+  if ((increment > 0 && static_cast<uintptr_t>(increment) > (UINTPTR_MAX - old_brk)) ||
+      (increment < 0 && static_cast<uintptr_t>(-increment) > old_brk)) {
     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/tests/unistd_test.cpp b/tests/unistd_test.cpp
index 3f31861..6aa259a 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -56,21 +56,82 @@
   ASSERT_EQ(ENOMEM, errno);
 }
 
+#if defined(__GLIBC__)
+#define SBRK_MIN INTPTR_MIN
+#define SBRK_MAX INTPTR_MAX
+#else
+#define SBRK_MIN PTRDIFF_MIN
+#define SBRK_MAX PTRDIFF_MAX
+#endif
+
 TEST(unistd, sbrk_ENOMEM) {
-  intptr_t current_brk = reinterpret_cast<intptr_t>(get_brk());
+#if defined(__BIONIC__) && !defined(__LP64__)
+  // There is no way to guarantee that all overflow conditions can be tested
+  // without manipulating the underlying values of the current break.
+  extern void* __bionic_brk;
+
+  class ScopedBrk {
+  public:
+    ScopedBrk() : saved_brk_(__bionic_brk) {}
+    virtual ~ScopedBrk() { __bionic_brk = saved_brk_; }
+
+  private:
+    void* saved_brk_;
+  };
+
+  ScopedBrk scope_brk;
+
+  // Set the current break to a point that will cause an overflow.
+  __bionic_brk = reinterpret_cast<void*>(static_cast<uintptr_t>(PTRDIFF_MAX) + 2);
 
   // 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);
+  // Set the current break to a point that will cause an overflow.
+  __bionic_brk = reinterpret_cast<void*>(static_cast<uintptr_t>(PTRDIFF_MAX));
 
-#if defined(__BIONIC__)
-  // 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);
+
+  __bionic_brk = reinterpret_cast<void*>(static_cast<uintptr_t>(PTRDIFF_MAX) - 1);
+
+  ASSERT_EQ(reinterpret_cast<void*>(-1), sbrk(PTRDIFF_MIN + 1));
+  ASSERT_EQ(ENOMEM, errno);
+#else
+  class ScopedBrk {
+  public:
+    ScopedBrk() : saved_brk_(get_brk()) {}
+    virtual ~ScopedBrk() { brk(saved_brk_); }
+
+  private:
+    void* saved_brk_;
+  };
+
+  ScopedBrk scope_brk;
+
+  uintptr_t cur_brk = reinterpret_cast<uintptr_t>(get_brk());
+  if (cur_brk < static_cast<uintptr_t>(-(SBRK_MIN+1))) {
+    // Do the overflow test for a max negative increment.
+    ASSERT_EQ(reinterpret_cast<void*>(-1), sbrk(SBRK_MIN));
+#if defined(__BIONIC__)
+    // GLIBC does not set errno in overflow case.
+    ASSERT_EQ(ENOMEM, errno);
+#endif
+  }
+
+  uintptr_t overflow_brk = static_cast<uintptr_t>(SBRK_MAX) + 2;
+  if (cur_brk < overflow_brk) {
+    // Try and move the value to PTRDIFF_MAX + 2.
+    cur_brk = reinterpret_cast<uintptr_t>(sbrk(overflow_brk));
+  }
+  if (cur_brk >= overflow_brk) {
+    ASSERT_EQ(reinterpret_cast<void*>(-1), sbrk(SBRK_MAX));
+#if defined(__BIONIC__)
+    // GLIBC does not set errno in overflow case.
+    ASSERT_EQ(ENOMEM, errno);
+#endif
+  }
 #endif
 }