I wrote:
> Can you try the following patch on aarch64 and report back?

Actually, the last patch was not quite right.  C/C++ makes it rather
difficult to avoid edge cases in arithmetic.  Can you try this one
instead?

    Thanks,
      Mark

--8<---------------cut here---------------start------------->8---
diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 693fa70c8..63540ddfc 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -26,6 +26,7 @@
 #include <errno.h>
 #include <stdio.h>
 #include <cstring>
+#include <stdint.h>
 
 #include <pwd.h>
 #include <grp.h>
@@ -2008,11 +2009,11 @@ void DerivationGoal::startBuilder()
        char stack[32 * 1024];
        int flags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | 
SIGCHLD;
        if (!fixedOutput) flags |= CLONE_NEWNET;
-#ifdef __aarch64__
-           pid = clone(childEntry, stack + sizeof(stack) - 16, flags, this);
-#else
-           pid = clone(childEntry, stack + sizeof(stack) - 8, flags, this);
-#endif
+       /* Ensure proper alignment on the stack.  On aarch64, it has to be 16
+          bytes.  */
+       pid = clone(childEntry,
+                   (char *)(((uintptr_t)stack + sizeof(stack) - 8) & 
~(uintptr_t)0xf),
+                   flags, this);
        if (pid == -1)
            throw SysError("cloning builder process");
     } else
--8<---------------cut here---------------end--------------->8---

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