Tweak urandom_test for Android devices without getrandom.

Some Android devices in our builder pool are so old that they lack
getrandom. This change attempts to make them happy.

Change-Id: I5eea04f1b1dc599852e3b8448ad829bea05b9fe9
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/57527
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: Adam Langley <agl@google.com>

crypto/fipsmodule/rand/urandom_test.cc

diff --git a/crypto/fipsmodule/rand/urandom_test.cc b/crypto/fipsmodule/rand/urandom_test.cc
index c3f8240..b1732cf 100644
--- a/crypto/fipsmodule/rand/urandom_test.cc
+++ b/crypto/fipsmodule/rand/urandom_test.cc
@@ -36,6 +36,7 @@
 #include <sys/user.h>
 
 #include "fork_detect.h"
+#include "getrandom_fillin.h"
 
 #if !defined(PTRACE_O_EXITKILL)
 #define PTRACE_O_EXITKILL (1 << 20)
@@ -776,6 +777,12 @@
 TEST(URandomTest, Test) {
   char buf[256];
 
+  // Some Android systems lack getrandom.
+  uint8_t scratch[1];
+  const bool has_getrandom =
+      (syscall(__NR_getrandom, scratch, sizeof(scratch), GRND_NONBLOCK) != -1 ||
+       errno != ENOSYS);
+
 #define TRACE_FLAG(flag)                                         \
   snprintf(buf, sizeof(buf), #flag ": %d", (flags & flag) != 0); \
   SCOPED_TRACE(buf);
@@ -788,6 +795,10 @@
       continue;
     }
 
+    if (!has_getrandom && !(flags & NO_GETRANDOM)) {
+        continue;
+    }
+
     TRACE_FLAG(NO_GETRANDOM);
     TRACE_FLAG(NO_URANDOM);
     TRACE_FLAG(GETRANDOM_NOT_READY);