lib/randutils: use getrandom(2) when it is available

System call getrandom(2) is relatively new, available since kernel 3.17 but
not supported by glibc 2.24.  That in mind autotools is made to check
availability of this function and keep old code as fallback.  It is
reasonable assume it will take years before the syscall(2) and fallback are
unproblematic to remove.

One might ask why bother using getrandom(2).  Main reason is to avoid
unnecessary system calls to achieve exactly same end result.  That
demonstrated with 'strace -c ./mcookie' showing 36 calls before, and 32
after this change.  Secondly the getrandom(2) function got to kernel with
promise it can be used to avoid file descriptor run down, and since uuidd
uses random_get_bytes() it should fulfill it's promise here.

Reference: http://man7.org/linux/man-pages/man2/getrandom.2.html
Reference: https://lwn.net/Articles/606141/
Reviewed-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

diff --git a/configure.ac b/configure.ac
index e0240b322e15fefeaba3769112959615f2082055..d301b3da2fd9abececdc0a8c7e8b478d2c6dd404 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -385,6 +385,7 @@ AC_CHECK_FUNCS([ \
        getdtablesize \
        getexecname \
        getmntinfo \
+       getrandom \
        getrlimit \
        getsgnam \
        inotify_init \

diff --git a/lib/randutils.c b/lib/randutils.c
index b93b9ba4701caa93e9b58637bbd0a796c47c7069..8defe744c6f6beec5eedaf0a9fe7b7061e2fc86e 100644 (file)
--- a/lib/randutils.c
+++ b/lib/randutils.c
@@ -25,6 +25,25 @@
 #define THREAD_LOCAL static
 #endif
 
+#ifdef HAVE_GETRANDOM
+# include <linux/random.h>
+#elif defined (__linux__)
+# include <sys/syscall.h>
+# if !defined(SYS_getrandom) && defined(__NR_getrandom)
+   /* usable kernel-headers, but old glibc-headers */
+#  define SYS_getrandom __NR_getrandom
+# endif
+#endif
+
+#if !defined(HAVE_GETRANDOM) && defined(SYS_getrandom)
+/* libc without function, but we have syscal */
+static int getrandom(void *buf, size_t buflen, unsigned int flags)
+{
+       return (syscall(SYS_getrandom, buf, buflen, flags));
+}
+# define HAVE_GETRANDOM
+#endif
+
 #if defined(__linux__) && defined(__NR_gettid) && defined(HAVE_JRAND48)
 #define DO_JRAND_MIX
 THREAD_LOCAL unsigned short ul_jrand_seed[3];
@@ -35,20 +54,11 @@ int rand_get_number(int low_n, int high_n)
        return rand() % (high_n - low_n + 1) + low_n;
 }
 
-int random_get_fd(void)
+static void crank_random(void)
 {
-       int i, fd;
+       int i;
        struct timeval tv;
 
-       fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
-       if (fd == -1)
-               fd = open("/dev/random", O_RDONLY | O_NONBLOCK | O_CLOEXEC);
-       if (fd >= 0) {
-               i = fcntl(fd, F_GETFD);
-               if (i >= 0)
-                       fcntl(fd, F_SETFD, i | FD_CLOEXEC);
-       }
-
        gettimeofday(&tv, 0);
        srand((getpid() << 16) ^ getuid() ^ tv.tv_sec ^ tv.tv_usec);
 
@@ -61,9 +71,23 @@ int random_get_fd(void)
        gettimeofday(&tv, 0);
        for (i = (tv.tv_sec ^ tv.tv_usec) & 0x1F; i > 0; i--)
                rand();
-       return fd;
 }
 
+int random_get_fd(void)
+{
+       int i, fd;
+
+       fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
+       if (fd == -1)
+               fd = open("/dev/random", O_RDONLY | O_NONBLOCK | O_CLOEXEC);
+       if (fd >= 0) {
+               i = fcntl(fd, F_GETFD);
+               if (i >= 0)
+                       fcntl(fd, F_SETFD, i | FD_CLOEXEC);
+       }
+       crank_random();
+       return fd;
+}
 
 /*
  * Generate a stream of random nbytes into buf.
@@ -72,10 +96,19 @@ int random_get_fd(void)
  */
 void random_get_bytes(void *buf, size_t nbytes)
 {
-       size_t i, n = nbytes;
+       size_t i;
+       unsigned char *cp = (unsigned char *)buf;
+
+#ifdef HAVE_GETRANDOM
+       while (getrandom(buf, nbytes, 0) < 0) {
+               if (errno == EINTR)
+                       continue;
+               break;
+       }
+#else
+       size_t n = nbytes;
        int fd = random_get_fd();
        int lose_counter = 0;
-       unsigned char *cp = (unsigned char *) buf;
 
        if (fd >= 0) {
                while (n > 0) {
@@ -92,11 +125,12 @@ void random_get_bytes(void *buf, size_t nbytes)
 
                close(fd);
        }
-
+#endif
        /*
         * We do this all the time, but this is the only source of
         * randomness if /dev/random/urandom is out to lunch.
         */
+       crank_random();
        for (cp = buf, i = 0; i < nbytes; i++)
                *cp++ ^= (rand() >> 7) & 0xFF;
 
@@ -122,6 +156,9 @@ void random_get_bytes(void *buf, size_t nbytes)
  */
 const char *random_tell_source(void)
 {
+#ifdef HAVE_GETRANDOM
+       return _("getrandom() function");
+#else
        size_t i;
        static const char *random_sources[] = {
                "/dev/urandom",
@@ -132,7 +169,7 @@ const char *random_tell_source(void)
                if (!access(random_sources[i], R_OK))
                        return random_sources[i];
        }
-
+#endif
        return _("libc pseudo-random functions");
 }