Merge bitcoin/bitcoin#31826: random: Check GetRNDRRS is supported in InitHardwareRand to avoid infinite loop

09b150b In `InitHardwareRand`, do trail test for `RNDRRS` by `VerifyRNDRRS` (Eval EXEC)

Pull request description:

  This PR want to fix #31817 by added a maximum retry limit (`max_retries`) to the `GetRNDRRS` function to prevent it from entering an infinite loop if the hardware random number generator fails to return a valid random number. This change improves stability and ensures that the function terminates after a predefined number of retries.

ACKs for top commit:
  achow101:
    ACK 09b150b
  sipa:
    utACK 09b150b

Tree-SHA512: 5626b6b182a55d344a3aba11b782259ecc6bbec513771d50077874c5f70934750e68add8f63aa0bf69c6b7b313112940a85508af5447622c703cc5e92439ab4a

Author: achow101

src/random.cpp

@@ -192,20 +192,24 @@ uint64_t GetRdSeed() noexcept
 #elif defined(__aarch64__) && defined(HWCAP2_RNG)
 
 bool g_rndr_supported = false;
+bool g_rndrrs_supported = false;
 
 void InitHardwareRand()
 {
     if (getauxval(AT_HWCAP2) & HWCAP2_RNG) {
         g_rndr_supported = true;
+        g_rndrrs_supported = VerifyRNDRRS();
     }
 }
 
 void ReportHardwareRand()
 {
     // This must be done in a separate function, as InitHardwareRand() may be indirectly called
     // from global constructors, before logging is initialized.
-    if (g_rndr_supported) {
+    if (g_rndr_supported && g_rndrrs_supported) {
         LogPrintf("Using RNDR and RNDRRS as additional entropy sources\n");
+    } else if (g_rndr_supported) {
+        LogPrintf("Using RNDR as an additional entropy source\n");
     }
 }
 
@@ -227,24 +231,43 @@ uint64_t GetRNDR() noexcept
     return r1;
 }
 
-/** Read 64 bits of entropy using rndrrs.
- *
+// Helper function to retrieve random value using RNDRRS
+bool GetRNDRRSInternal(uint64_t &r1) noexcept
+{
+    uint8_t ok = 0;
+    __asm__ volatile("mrs %0, s3_3_c2_c4_1; cset %w1, ne;"
+                     : "=r"(r1), "=r"(ok)::"cc");
+    return ok != 0;
+}
+
+
+/** Read 64 bits of entropy using RNDRRS.
  * Must only be called when RNDRRS is supported.
  */
 uint64_t GetRNDRRS() noexcept
 {
-    uint8_t ok = 0;
     uint64_t r1;
-    do {
-        // https://developer.arm.com/documentation/ddi0601/2022-12/AArch64-Registers/RNDRRS--Reseeded-Random-Number
-        __asm__ volatile("mrs %0, s3_3_c2_c4_1; cset %w1, ne;"
-                         : "=r"(r1), "=r"(ok)::"cc");
-        if (ok) break;
+    while (!GetRNDRRSInternal(r1)) {
         __asm__ volatile("yield");
-    } while (true);
+    }
     return r1;
 }
 
+/** Verify if RNDRRS is supported and functional.
+ * Return true if it works within the retry limit.
+ */
+bool VerifyRNDRRS() noexcept
+{
+    uint64_t test;
+    for (int retry = 0; retry < 10; ++retry) {
+        if (GetRNDRRSInternal(test)) {
+            return true;
+        }
+        __asm__ volatile("yield");
+    }
+    return false;
+}
+
 #else
 /* Access to other hardware random number generators could be added here later,
  * assuming it is sufficiently fast (in the order of a few hundred CPU cycles).
@@ -295,7 +318,7 @@ void SeedHardwareSlow(CSHA512& hasher) noexcept {
         return;
     }
 #elif defined(__aarch64__) && defined(HWCAP2_RNG)
-    if (g_rndr_supported) {
+    if (g_rndrrs_supported) {
         for (int i = 0; i < 4; ++i) {
             uint64_t out = GetRNDRRS();
             hasher.Write((const unsigned char*)&out, sizeof(out));