commit f430293d842031f2afc3013f156e1018065e480e

Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>

Date:   Tue Jan 12 09:17:09 2021 -0300

    posix: consume less entropy on tempname

    The first getrandom is used only for __GT_NOCREATE, which is inherently

    insecure and can use the entropy as a small improvement.  On the

    second and later attempts it might help against DoS attacks.

    It sync with gnulib commit 854fbb81d91f7a0f2b463e7ace2499dee2f380f2.

    Checked on x86_64-linux-gnu.

diff --git a/sysdeps/posix/tempname.c b/sysdeps/posix/tempname.c

index fcab9b26364021e4..3435c4bf75a01f42 100644

--- a/sysdeps/posix/tempname.c

+++ b/sysdeps/posix/tempname.c

@@ -22,6 +22,7 @@

 #include <sys/types.h>

 #include <assert.h>

+#include <stdbool.h>

 #include <errno.h>

@@ -79,11 +80,11 @@ typedef uint_fast64_t random_value;

 #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.  */

@@ -271,6 +272,13 @@ try_tempname_len (char *tmpl, int suffixlen, void *args,

   /* 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

@@ -294,7 +302,10 @@ try_tempname_len (char *tmpl, int suffixlen, void *args,

           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;