+2021-01-11 Paul Eggert <eggert@cs.ucla.edu>
+
+ tempname: consume less entropy
+ Derived from a glibc patch proposed by Adhemerval Zanella in:
+ https://sourceware.org/pipermail/libc-alpha/2021-January/121302.html
+ * lib/tempname.c: Include stdbool.h.
+ (random_bits): New arg use_getrandom.
+ (try_tempname_len): Skip getrandom on the first try,
+ unless __GT_NOCREATE.
+ * modules/tempname (Depends-on): Add stdbool.
+
2021-01-10 Bruno Haible <bruno@clisp.org>
lchmod-tests: Fix link error.
#include <sys/types.h>
#include <assert.h>
+#include <stdbool.h>
#include <errno.h>
#define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)
static random_value
-random_bits (random_value var)
+random_bits (random_value var, bool use_getrandom)
{
random_value r;
/* Without GRND_NONBLOCK it can be blocked for minutes on some systems. */
- if (__getrandom (&r, sizeof r, GRND_NONBLOCK) == sizeof r)
+ if (use_getrandom && __getrandom (&r, sizeof r, GRND_NONBLOCK) == sizeof r)
return r;
#if _LIBC || (defined CLOCK_MONOTONIC && HAVE_CLOCK_GETTIME)
/* Add entropy if getrandom did not work. */
/* How many random base-62 digits can currently be extracted from V. */
int vdigits = 0;
+ /* Whether to consume entropy when acquiring random bits. On the
+ first try it's worth the entropy cost with __GT_NOCREATE, which
+ is inherently insecure and can use the entropy to make it a bit
+ less secure. On the (rare) second and later attempts it might
+ help against DoS attacks. */
+ bool use_getrandom = tryfunc == try_nocreate;
+
/* Least unfair value for V. If V is less than this, V can generate
BASE_62_DIGITS digits fairly. Otherwise it might be biased. */
random_value const unfair_min
if (vdigits == 0)
{
do
- v = random_bits (v);
+ {
+ v = random_bits (v, use_getrandom);
+ use_getrandom = true;
+ }
while (unfair_min <= v);
vdigits = BASE_62_DIGITS;