Merge #18443: lockedpool: avoid sensitive data in core files (FreeBSD)

f85203097f lockedpool: avoid sensitive data in core files (FreeBSD) (Vasil Dimov)

Pull request description:

  This is a followup to
  23991ee53 / https://github.com/bitcoin/bitcoin/pull/15600
  to also use madvise(2) on FreeBSD to avoid sensitive data allocated
  with secure_allocator ending up in core files in addition to preventing
  it from going to the swap.

ACKs for top commit:
  sipa:
    ACK f85203097f if someone verifies this works as intended on *BSD.
  laanwj:
    ACK f85203097f
  practicalswift:
    Code-review ACK f85203097f assuming a reviewer with FreeBSD access verifies that the PR goal is achieved :)

Tree-SHA512: 2e6d4ab6a9fbe18732c8ba530eacc17f58128c97140758b80c905b5b838922a2bcaa5f9abc45ab69d5a1a2baa0cba322f006048b60a877228e089c7e64dadd2a

src/support/lockedpool.cpp

@@ -253,8 +253,10 @@ void *PosixLockedPageAllocator::AllocateLocked(size_t len, bool *lockingSuccess)
     }
     if (addr) {
         *lockingSuccess = mlock(addr, len) == 0;
-#ifdef MADV_DONTDUMP
+#if defined(MADV_DONTDUMP) // Linux
         madvise(addr, len, MADV_DONTDUMP);
+#elif defined(MADV_NOCORE) // FreeBSD
+        madvise(addr, len, MADV_NOCORE);
 #endif
     }
     return addr;