Index: libc/src/assert/CMakeLists.txt
===================================================================
--- libc/src/assert/CMakeLists.txt
+++ libc/src/assert/CMakeLists.txt
@@ -10,4 +10,11 @@
     # later.
     sys_syscall_h
     linux_syscall_h
+    # TOOD: Remove these as they are dependencies of abort
+    sigaction
+    sigfillset
+    sigdelset
+    sigprocmask
+    signal
+    __errno_location
 )
Index: libc/src/signal/linux/CMakeLists.txt
===================================================================
--- libc/src/signal/linux/CMakeLists.txt
+++ libc/src/signal/linux/CMakeLists.txt
@@ -95,6 +95,8 @@
   DEPENDS
     sigaction
     signal_h
+    # TODO: Remove this it is a dependency of sigaction
+    __errno_location
 )
 
 add_entrypoint_object(
Index: libc/src/stdlib/CMakeLists.txt
===================================================================
--- libc/src/stdlib/CMakeLists.txt
+++ libc/src/stdlib/CMakeLists.txt
@@ -12,4 +12,10 @@
     raise
     _Exit
     stdlib_h
+    sigfillset
+    sigdelset
+    sigprocmask
+    signal
+    # TODO: Remove this, dependency of signal via sigaction
+    __errno_location
 )
Index: libc/src/stdlib/abort.cpp
===================================================================
--- libc/src/stdlib/abort.cpp
+++ libc/src/stdlib/abort.cpp
@@ -8,21 +8,43 @@
 
 #include "src/__support/common.h"
 #include "src/signal/raise.h"
+#include "src/signal/sigdelset.h"
+#include "src/signal/sigfillset.h"
+#include "src/signal/signal.h"
+#include "src/signal/sigprocmask.h"
 #include "src/stdlib/_Exit.h"
 
 #include "src/stdlib/abort.h"
 
 namespace __llvm_libc {
 
+// Currently the signal implementation doesn't guarantee that SIGABRT's handler
+// or the signal mask as a whole is not modified from other threads.
 void LLVM_LIBC_ENTRYPOINT(abort)() {
-  // TODO: When sigprocmask and sigaction land:
-  // Unblock SIGABRT, raise it, if it was ignored or the handler returned,
-  // change its action to SIG_DFL, raise it again.
-  // TODO: When C11 mutexes land:
-  // Aquire recursive mutex (in case the current signal handler for SIGABRT
-  // itself calls abort we don't want to deadlock on the same thread trying
-  // to aquire it's own mutex.)
+  // Block all blockable signals except SIGABRT, so this function doesn't get
+  // interrupted by any async signals. This also unblocks SIGABRT.
+  sigset_t set;
+  __llvm_libc::sigfillset(&set);
+  __llvm_libc::sigdelset(&set, SIGABRT);
+  __llvm_libc::sigprocmask(SIG_SETMASK, &set, nullptr);
+
+  // If the current SIGABRT handler calls abort(), we should make sure it
+  // it does not infinitely call itself.
+  static bool called;
+  if (!called) {
+    called = true;
+    __llvm_libc::raise(SIGABRT);
+    // Reset the signal mask in case the user handler changed it.
+    __llvm_libc::sigprocmask(SIG_SETMASK, &set, nullptr);
+  }
+
+  // If the user handler returned, we re-raise SIGABRT with the default handler
+  // to attempt to die with SIGABRT.
+  __llvm_libc::signal(SIGABRT, SIG_DFL);
   __llvm_libc::raise(SIGABRT);
+
+  // If the program still hasn't been killed by SIGABRT we must guarantee that
+  // this function never returns.
   __llvm_libc::raise(SIGKILL);
   __llvm_libc::_Exit(127);
 }
Index: libc/test/src/assert/CMakeLists.txt
===================================================================
--- libc/test/src/assert/CMakeLists.txt
+++ libc/test/src/assert/CMakeLists.txt
@@ -12,4 +12,10 @@
     abort
     raise
     _Exit
+    sigaction
+    sigfillset
+    sigdelset
+    sigprocmask
+    signal
+    __errno_location
 )
Index: libc/test/src/assert/assert_test.cpp
===================================================================
--- libc/test/src/assert/assert_test.cpp
+++ libc/test/src/assert/assert_test.cpp
@@ -7,21 +7,21 @@
 //===----------------------------------------------------------------------===//
 
 #undef NDEBUG
+#include "include/signal.h"
 #include "src/assert/assert.h"
 #include "utils/UnitTest/Test.h"
 
 extern "C" int close(int);
 
 TEST(Assert, Enabled) {
-  // -1 matches against any signal, which is necessary for now until
-  // __llvm_libc::abort() unblocks SIGABRT. Close standard error for the
-  // child process so we don't print the assertion failure message.
+  // Close standard error for the child process so we don't print the assertion
+  // failure message.
   EXPECT_DEATH(
       [] {
         close(2);
         assert(0);
       },
-      -1);
+      SIGABRT);
 }
 
 #define NDEBUG
Index: libc/test/src/stdlib/CMakeLists.txt
===================================================================
--- libc/test/src/stdlib/CMakeLists.txt
+++ libc/test/src/stdlib/CMakeLists.txt
@@ -23,4 +23,13 @@
     abort
     _Exit
     raise
+    # TODO: Remove depencies of abort
+    sigaction
+    sigemptyset
+    sigaddset
+    sigfillset
+    sigdelset
+    sigprocmask
+    signal
+    __errno_location
 )
Index: libc/test/src/stdlib/abort_test.cpp
===================================================================
--- libc/test/src/stdlib/abort_test.cpp
+++ libc/test/src/stdlib/abort_test.cpp
@@ -8,11 +8,47 @@
 
 #include "include/signal.h"
 #include "include/stdlib.h"
+#include "src/signal/sigaddset.h"
+#include "src/signal/sigemptyset.h"
+#include "src/signal/signal.h"
+#include "src/signal/sigprocmask.h"
 #include "src/stdlib/abort.h"
 #include "utils/UnitTest/Test.h"
 
-TEST(Stdlib, abort) {
-  // -1 matches against any signal, which is necessary for now until
-  // __llvm_libc::abort() unblocks SIGABRT.
-  EXPECT_DEATH([] { __llvm_libc::abort(); }, -1);
+TEST(Abort, Basic) {
+  EXPECT_DEATH([] { __llvm_libc::abort(); }, SIGABRT);
+}
+
+TEST(Abort, Blocked) {
+  __llvm_libc::signal(SIGABRT, SIG_IGN);
+  EXPECT_DEATH([] { __llvm_libc::abort(); }, SIGABRT);
+}
+
+TEST(Abort, RecursiveHandler) {
+  __llvm_libc::signal(
+      SIGABRT, +[](int) { __llvm_libc::abort(); });
+  EXPECT_DEATH([] { __llvm_libc::abort(); }, SIGABRT);
+}
+
+TEST(Abort, BlockFromHandler) {
+  __llvm_libc::signal(
+      SIGABRT, +[](int) {
+        sigset_t set;
+        __llvm_libc::sigemptyset(&set);
+        __llvm_libc::sigaddset(&set, SIGABRT);
+        __llvm_libc::sigprocmask(SIG_BLOCK, &set, nullptr);
+      });
+  EXPECT_DEATH([] { __llvm_libc::abort(); }, SIGABRT);
+}
+
+TEST(Abort, BlockFromHandlerRecursive) {
+  __llvm_libc::signal(
+      SIGABRT, +[](int) {
+        sigset_t set;
+        __llvm_libc::sigemptyset(&set);
+        __llvm_libc::sigaddset(&set, SIGABRT);
+        __llvm_libc::sigprocmask(SIG_BLOCK, &set, nullptr);
+        __llvm_libc::abort();
+      });
+  EXPECT_DEATH([] { __llvm_libc::abort(); }, SIGABRT);
 }