We don't really need uniform distribution for mktemp(3) so use

arc4random_buf() to avoid lots of arc4random() calls with a getpid()
syscall for each one.  We fetch 32 bytes of random data at a time
which can handle up to 16 Xs.  16 Xs should be enough for anyone.
Requested and OK deraadt@

diff --git a/lib/libc/stdio/mktemp.c b/lib/libc/stdio/mktemp.c
index 61db06a..cb154c4 100644 (file)
--- a/lib/libc/stdio/mktemp.c
+++ b/lib/libc/stdio/mktemp.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: mktemp.c,v 1.32 2013/03/12 16:47:11 guenther Exp $ */
+/*     $OpenBSD: mktemp.c,v 1.33 2014/05/06 22:55:27 millert Exp $ */
 /*
  * Copyright (c) 1996-1998, 2008 Theo de Raadt
  * Copyright (c) 1997, 2008-2009 Todd C. Miller
@@ -35,12 +35,16 @@
 #define NUM_CHARS      (sizeof(TEMPCHARS) - 1)
 #define MIN_X          6
 
+#ifndef nitems
+#define nitems(_a)     (sizeof((_a)) / sizeof((_a)[0]))
+#endif
+
 static int
 mktemp_internal(char *path, int slen, int mode)
 {
        char *start, *cp, *ep;
        const char *tempchars = TEMPCHARS;
-       unsigned int r, tries;
+       unsigned int tries;
        struct stat sb;
        size_t len;
        int fd;
@@ -61,10 +65,19 @@ mktemp_internal(char *path, int slen, int mode)
 
        tries = INT_MAX;
        do {
-               for (cp = start; cp != ep; cp++) {
-                       r = arc4random_uniform(NUM_CHARS);
-                       *cp = tempchars[r];
-               }
+               cp = start;
+               do {
+                       unsigned short rbuf[16];
+                       unsigned int i;
+
+                       /*
+                        * Avoid lots of arc4random() calls by using
+                        * a buffer sized for up to 16 Xs at a time.
+                        */
+                       arc4random_buf(rbuf, sizeof(rbuf));
+                       for (i = 0; i < nitems(rbuf) && cp != ep; i++)
+                               *cp++ = tempchars[rbuf[i] % NUM_CHARS];
+               } while (cp != ep);
 
                switch (mode) {
                case MKTEMP_NAME: