From ad0958b816f28e53d9bda4486e969ec3ca63538a Mon Sep 17 00:00:00 2001
From: Alejandro Colomar <alx@kernel.org>
Date: Wed, 19 Jun 2024 19:54:16 +0200
Subject: [PATCH] lib/csrand.c: Fix the lower part of the domain of
 csrand_uniform()

I accidentally broke this code during an un-optimization.  We need to
start from a random value of the width of the limit, that is, 32 bits.

Thanks to Jason for pointing to his similar code in the kernel, which
made me see my mistake.

Fixes: 2a61122b5e8f ("Unoptimize the higher part of the domain of csrand_uniform()")
Closes: <https://github.com/shadow-maint/shadow/issues/1015>
Reported-by: Michael Brunnbauer <https://github.com/michaelbrunnbauer>
Link: <https://git.zx2c4.com/linux-rng/tree/drivers/char/random.c#n535>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Link: <https://github.com/shadow-maint/shadow/pull/638>
Link: <https://github.com/shadow-maint/shadow/issues/634>
Link: <https://github.com/shadow-maint/shadow/pull/624>
Tested-by: Michael Brunnbauer <https://github.com/michaelbrunnbauer>
Reviewed-by: Michael Brunnbauer <https://github.com/michaelbrunnbauer>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Cherry-picked-from: 4119a2dce564 ("lib/csrand.c: Fix the lower part of the domain of csrand_uniform()")
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Link: <https://github.com/shadow-maint/shadow/pull/1025>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

Conflict: N/A
Reference: https://github.com/shadow-maint/shadow/commit/ad0958b816f28e53d9bda4486e969ec3ca63538a

---
 lib/csrand.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/lib/csrand.c b/lib/csrand.c
index e85eaa8a..16bcccf0 100644
--- a/lib/csrand.c
+++ b/lib/csrand.c
@@ -22,6 +22,7 @@
 #include "shadowlog.h"
 
 
+static uint32_t csrand32(void);
 static uint32_t csrand_uniform32(uint32_t n);
 static unsigned long csrand_uniform_slow(unsigned long n);
 
@@ -96,6 +97,13 @@ csrand_interval(unsigned long min, unsigned long max)
 }
 
 
+static uint32_t
+csrand32(void)
+{
+	return csrand();
+}
+
+
 /*
  * Fast Random Integer Generation in an Interval
  * ACM Transactions on Modeling and Computer Simulation 29 (1), 2019
@@ -108,12 +116,12 @@ csrand_uniform32(uint32_t n)
 	uint64_t  r, mult;
 
 	if (n == 0)
-		return csrand();
+		return csrand32();
 
 	bound = -n % n;  // analogous to `2^32 % n`, since `x % y == (x-y) % y`
 
 	do {
-		r = csrand();
+		r = csrand32();
 		mult = r * n;
 		rem = mult;  // analogous to `mult % 2^32`
 	} while (rem < bound);  // p = (2^32 % n) / 2^32;  W.C.: n=2^31+1, p=0.5
-- 
2.46.0

