commit d062c8afba962a54c82798c7e478f4e8719e4f95
author Adam Langley <agl@google.com> Fri Jan 30 15:30:44 2015 -0800
committer Adam Langley <agl@google.com> Mon Feb 02 19:51:14 2015 +0000
tree 17c3ffb628c26099d636cf56322238f19bbf911e
parent 2b2d66d4094e5c25789ce68310d86192c377c63b

Probe for NEON support if getauxval is missing.

Android didn't have getauxval until Jelly Bean (4.1). This means that
BoringSSL running on older Androids won't be able to detect NEON
support. (This is moot for Chromium because Chromium calls
android_getCpuFeatures and sets the NEON flag itself, but other users of
BoringSSL on Android probably won't do that.)

This change mirrors a little of what upstream does and tries running a
NEON instruction with a handler for SIGILL installed.

Change-Id: I853b85c37ffb049b240582d71fcf07adedc37a30
Reviewed-on: https://boringssl-review.googlesource.com/3190
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

crypto/cpu-arm.c

diff --git a/crypto/cpu-arm.c b/crypto/cpu-arm.c
index 96392d8..12098bd 100644
--- a/crypto/cpu-arm.c
+++ b/crypto/cpu-arm.c
@@ -17,7 +17,8 @@
 #if defined(OPENSSL_ARM) || defined(OPENSSL_AARCH64)
 
 #include <inttypes.h>
-#include <stdio.h>
+#include <setjmp.h>
+#include <signal.h>
 
 #include "arm_arch.h"
 
@@ -35,7 +36,11 @@
   return (OPENSSL_armcap_P & ARMV7_NEON) != 0;
 }
 
+static char g_set_neon_called = 0;
+
 void CRYPTO_set_NEON_capable(char neon_capable) {
+  g_set_neon_called = 1;
+
   if (neon_capable) {
     OPENSSL_armcap_P |= ARMV7_NEON;
   } else {
@@ -56,8 +61,66 @@
   }
 }
 
+static sigjmp_buf sigill_jmp;
+
+static void sigill_handler(int signal) {
+  siglongjmp(sigill_jmp, signal);
+}
+
+void CRYPTO_arm_neon_probe();
+
+// probe_for_NEON returns 1 if a NEON instruction runs successfully. Because
+// getauxval doesn't exist on Android until Jelly Bean, supporting NEON on
+// older devices requires this.
+static int probe_for_NEON() {
+  int supported = 0;
+
+#if !defined(OPENSSL_NO_ASM)
+  sigset_t sigmask;
+  sigfillset(&sigmask);
+  sigdelset(&sigmask, SIGILL);
+  sigdelset(&sigmask, SIGTRAP);
+  sigdelset(&sigmask, SIGFPE);
+  sigdelset(&sigmask, SIGBUS);
+  sigdelset(&sigmask, SIGSEGV);
+
+  struct sigaction sigill_original_action, sigill_action;
+  memset(&sigill_action, 0, sizeof(sigill_action));
+  sigill_action.sa_handler = sigill_handler;
+  sigill_action.sa_mask = sigmask;
+
+  sigset_t original_sigmask;
+  sigprocmask(SIG_SETMASK, &sigmask, &original_sigmask);
+  sigaction(SIGILL, &sigill_action, &sigill_original_action);
+
+
+  if (sigsetjmp(sigill_jmp, 1 /* save signals */) == 0) {
+    // This function cannot be inline asm because GCC will refuse to compile
+    // inline NEON instructions unless building with -mfpu=neon, which would
+    // defeat the point of probing for support at runtime.
+    CRYPTO_arm_neon_probe();
+    supported = 1;
+  }
+  // Note that Android up to and including Lollipop doesn't restore the signal
+  // mask correctly after returning from a sigsetjmp. So that would need to be
+  // set again here if more probes were added.
+  // See https://android-review.googlesource.com/#/c/127624/
+
+  sigaction(SIGILL, &sigill_original_action, NULL);
+  sigprocmask(SIG_SETMASK, &original_sigmask, NULL);
+#endif
+
+  return supported;
+}
+
 void OPENSSL_cpuid_setup(void) {
   if (getauxval == NULL) {
+    // |CRYPTO_is_NEON_capable| can be true even if |CRYPTO_set_NEON_capable|
+    // has never been called if the code was compiled with NEON support enabled
+    // (e.g. -mfpu=neon).
+    if (!g_set_neon_called && !CRYPTO_is_NEON_capable() && probe_for_NEON()) {
+      OPENSSL_armcap_P |= ARMV7_NEON;
+    }
     return;
   }