Merge pull request #25 from dsprenkels/issue_22

Fallback to old-style entropy count if ioctl returns ENOSYS

Author: dsprenkels

Makefile

@@ -1,4 +1,4 @@
-CFLAGS := -g -O2 -m64 -std=gnu99 \
+CFLAGS ?= -g -O2 -std=gnu99 \
 	-Wall -Wextra -Wshadow -Wpointer-arith -Wcast-qual -Wformat \
 	-Wformat-security -Werror=format-security -Wstrict-prototypes \
 	-D_FORTIFY_SOURCE=2 -fPIC -fno-strict-overflow

randombytes.c

@@ -138,11 +138,17 @@ static int randombytes_linux_wait_for_entropy(int device)
 
 	/* If the device has enough entropy already, we will want to return early */
 	retcode = randombytes_linux_read_entropy_ioctl(device, &entropy);
-	if (retcode != 0 && errno == ENOTTY) {
-		/* The ioctl call on /dev/urandom has failed due to a ENOTTY (i.e.
-		 * unsupported action). We will fall back to reading from
-		 * `/proc/sys/kernel/random/entropy_avail`. This is obviously less
-		 * ideal, but at this point it seems we have no better option. */
+	// printf("errno: %d (%s)\n", errno, strerror(errno));
+	if (retcode != 0 && (errno == ENOTTY || errno == ENOSYS)) {
+		// The ioctl call on /dev/urandom has failed due to a
+		//   - ENOTTY (unsupported action), or
+		//   - ENOSYS (invalid ioctl; this happens on MIPS, see #22).
+		//
+		// We will fall back to reading from
+		// `/proc/sys/kernel/random/entropy_avail`.  This less ideal,
+		// because it allocates a file descriptor, and it may not work
+		// in a chroot.  But at this point it seems we have no better
+		// options left.
 		strategy = PROC;
 		// Open the entropy count file
 		proc_file = fopen("/proc/sys/kernel/random/entropy_avail", "r");

randombytes_test.c

@@ -87,6 +87,15 @@ static void test_issue_17(void) {
 	assert(memcmp(buf1, buf2, sizeof(buf1)) != 0);
 }
 
+static void test_issue_22(void) {
+	uint8_t buf1[20] = {}, buf2[sizeof(buf1)] = {};
+	const int ret1 = randombytes(buf1, sizeof(buf1));
+	const int ret2 = randombytes(buf2, sizeof(buf2));
+	assert(ret1 == 0);
+	assert(ret2 == 0);
+	assert(memcmp(buf1, buf2, sizeof(buf1)) != 0);
+}
+
 // ======== Mock OS functions to simulate uncommon behavior ========
 
 #if defined(__linux__) && defined(SYS_getrandom)
@@ -112,6 +121,10 @@ int __wrap_ioctl(int fd, int code, int* ret) {
 		errno = ENOTTY;
 		return -1;
 	}
+	if (current_test == test_issue_17) {
+		errno = ENOSYS;
+		return -1;
+	}
 	return __real_ioctl(fd, code, ret);
 }
 #endif /* defined(__linux__) && !defined(SYS_getrandom) */
@@ -133,8 +146,10 @@ int main(void) {
 #endif /* defined(__linux__) && defined(SYS_getrandom) */
 #if defined(__linux__) && !defined(SYS_getrandom)
 	RUN_TEST(test_issue_17)
+	RUN_TEST(test_issue_22)
 #else
 	SKIP_TEST(test_issue_17)
+	SKIP_TEST(test_issue_22)
 #endif /* defined(__linux__) && !defined(SYS_getrandom) */
 	return 0;
 }