Patch

0001-Update-PRNG.patch

From c796842c4aaaca1b640a1fd0e03c9d8e85f571c8 Mon Sep 17 00:00:00 2001
From: rfl890 <[email protected]>
Date: Wed, 24 Jan 2024 18:14:16 -0500
Subject: [PATCH] Update PRNG

---
 src/lib_math.c |  2 +-
 src/lj_prng.c  | 43 ++++++++++++++++++++++---------------------
 2 files changed, 23 insertions(+), 22 deletions(-)

diff --git a/src/lib_math.c b/src/lib_math.c
index 4539f804..5ca674f6 100644
--- a/src/lib_math.c
+++ b/src/lib_math.c
@@ -194,7 +194,7 @@ LJLIB_CF(math_randomseed)
 LUALIB_API int luaopen_math(lua_State *L)
 {
   PRNGState *rs = (PRNGState *)lua_newuserdata(L, sizeof(PRNGState));
-  lj_prng_seed_fixed(rs);
+  lj_prng_seed_secure(rs);
   LJ_LIB_REG(L, LUA_MATHLIBNAME, math);
   return 1;
 }
diff --git a/src/lj_prng.c b/src/lj_prng.c
index 326b41e6..1697a304 100644
--- a/src/lj_prng.c
+++ b/src/lj_prng.c
@@ -17,10 +17,7 @@
 
 /* -- PRNG step function -------------------------------------------------- */
 
-/* This implements a Tausworthe PRNG with period 2^223. Based on:
-**   Tables of maximally-equidistributed combined LFSR generators,
-**   Pierre L'Ecuyer, 1991, table 3, 1st entry.
-** Full-period ME-CF generator with L=64, J=4, k=223, N1=49.
+/* This implements an Xoshiro256** PRNG with period 2^256.
 **
 ** Important note: This PRNG is NOT suitable for cryptographic use!
 **
@@ -31,33 +28,37 @@
 ** the difficulty for various attacks on the VM.
 */
 
-/* Update generator i and compute a running xor of all states. */
-#define TW223_GEN(rs, z, r, i, k, q, s) \
-  z = rs->u[i]; \
-  z = (((z<<q)^z) >> (k-s)) ^ ((z&((uint64_t)(int64_t)-1 << (64-k)))<<s); \
-  r ^= z; rs->u[i] = z;
+static LJ_AINLINE uint64_t rotl(const uint64_t x, int k) {
+	return (x << k) | (x >> (64 - k));
+}
+
+static uint64_t xs256(uint64_t *s) {
+  const uint64_t result = rotl(s[1] * 5, 7) * 9;
+
+	const uint64_t t = s[1] << 17;
+
+	s[2] ^= s[0];
+	s[3] ^= s[1];
+	s[1] ^= s[2];
+	s[0] ^= s[3];
 
-#define TW223_STEP(rs, z, r) \
-  TW223_GEN(rs, z, r, 0, 63, 31, 18) \
-  TW223_GEN(rs, z, r, 1, 58, 19, 28) \
-  TW223_GEN(rs, z, r, 2, 55, 24,  7) \
-  TW223_GEN(rs, z, r, 3, 47, 21,  8)
+	s[2] ^= t;
+
+	s[3] = rotl(s[3], 45);
+
+	return result;
+}
 
 /* PRNG step function with uint64_t result. */
 LJ_NOINLINE uint64_t LJ_FASTCALL lj_prng_u64(PRNGState *rs)
 {
-  uint64_t z, r = 0;
-  TW223_STEP(rs, z, r)
-  return r;
+  return xs256(rs->u);
 }
 
 /* PRNG step function with double in uint64_t result. */
 LJ_NOINLINE uint64_t LJ_FASTCALL lj_prng_u64d(PRNGState *rs)
 {
-  uint64_t z, r = 0;
-  TW223_STEP(rs, z, r)
-  /* Returns a double bit pattern in the range 1.0 <= d < 2.0. */
-  return (r & U64x(000fffff,ffffffff)) | U64x(3ff00000,00000000);
+  return (xs256(rs->u) & U64x(000fffff,ffffffff)) | U64x(3ff00000,00000000);
 }
 
 /* Condition seed: ensure k[i] MSB of u[i] are non-zero. */
-- 
2.43.0.windows.1