[PATCH 2/2] Use getrandom on try_tempname_len [BZ #15813]

[Adhemerval Zanella]

Remove the usage of random-bits.h for _LIBC and use getrandom instead.

Also, the fallback relies on UB (the LCG might use unitialized value
from the stack variable) so initialize the initial state using some ASLR
entropy.

The fallback will be always used on Linux with kernels older than 3.17,
so it also adds some extra entropy based on the clock for the linear
congruential generator (LCG).

Checked on x86_64-linux-gnu.
---
 sysdeps/posix/tempname.c | 41 +++++++++++++++++++++++-----------------
 1 file changed, 24 insertions(+), 17 deletions(-)

diff --git a/sysdeps/posix/tempname.c b/sysdeps/posix/tempname.c
index 9219ee66af..1ef5536814 100644
--- a/sysdeps/posix/tempname.c
+++ b/sysdeps/posix/tempname.c
@@ -60,27 +60,31 @@
 # define __mkdir mkdir
 # define __open open
 # define __lxstat64(version, file, buf) lstat (file, buf)
+# define __getrandom getrandom
 #endif
 
-#ifdef _LIBC
-# include <random-bits.h>
-# define RANDOM_BITS(Var) ((Var) = random_bits ())
-typedef uint32_t random_value;
-# define RANDOM_VALUE_MAX UINT32_MAX
-# define BASE_62_DIGITS 5 /* 62**5 < UINT32_MAX */
-# define BASE_62_POWER (62 * 62 * 62 * 62 * 62) /* 2**BASE_62_DIGITS */
-#else
 /* Use getrandom if it works, falling back on a 64-bit linear
    congruential generator that starts with whatever Var's value
    happens to be.  */
-# define RANDOM_BITS(Var) \
-    ((void) (getrandom (&(Var), sizeof (Var), 0) == sizeof (Var) \
-             || ((Var) = 2862933555777941757 * (Var) + 3037000493)))
 typedef uint_fast64_t random_value;
-# define RANDOM_VALUE_MAX UINT_FAST64_MAX
-# define BASE_62_DIGITS 10 /* 62**10 < UINT_FAST64_MAX */
-# define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)
+#define RANDOM_VALUE_MAX UINT_FAST64_MAX
+#define BASE_62_DIGITS 10 /* 62**10 < UINT_FAST64_MAX */
+#define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)
+
+static random_value
+random_bits (random_value var)
+{
+  random_value r;
+  if (__getrandom (&r, sizeof (r), 0) == sizeof (r))
+    return r;
+#if _LIBC
+  /* Add some more entropy if getrandom is not supported.  */
+  struct __timespec64 tv;
+  __clock_gettime64 (CLOCK_MONOTONIC, &tv);
+  var = var ^ tv.tv_nsec;
 #endif
+  return 2862933555777941757 * var + 3037000493;
+}
 
 #if _LIBC
 /* Return nonzero if DIR is an existent directory.  */
@@ -250,8 +254,11 @@ try_tempname_len (char *tmpl, int suffixlen, void *args,
   unsigned int attempts = ATTEMPTS_MIN;
 #endif
 
-  /* A random variable.  */
-  random_value v;
+  /* A random variable.  The initial value is used only the for fallback path
+     on 'random_bits' on 'getrandom' failure.  Its initial value tries to use
+     some entropy from the ASLR and ignore possible bits from the stack
+     alignment.  */
+  random_value v = ((uintptr_t) &v) / 16;
 
   /* How many random base-62 digits can currently be extracted from V.  */
   int vdigits = 0;
@@ -279,7 +286,7 @@ try_tempname_len (char *tmpl, int suffixlen, void *args,
           if (vdigits == 0)
             {
               do
-                RANDOM_BITS (v);
+                v = random_bits (v);
               while (unfair_min <= v);
 
               vdigits = BASE_62_DIGITS;
-- 
2.25.1