diff --git a/mk/shim.mk b/mk/shim.mk
index 32ffb8a..d14ad23 100644
--- a/mk/shim.mk
+++ b/mk/shim.mk
@@ -9,6 +9,15 @@ $(BUILD_DIR)/shim.o: src/core/shim.S | $(BUILD_DIR)
 $(BUILD_DIR)/shim.bin: $(BUILD_DIR)/shim.o
 	@echo "  OBJCOPY $@"
 	$(Q)$(OBJCOPY) -O binary $< $@
+	$(Q)magic=$$(od -An -N4 -tx1 $@ | tr -d '[:space:]'); \
+	case "$$magic" in \
+	cffaedfe|cefaedfe|feedface|feedfacf|cafebabe|bebafeca|cafebabf|bfbafeca) \
+	  echo "ERROR: $@ still has a Mach-O header (magic $$magic)."; \
+	  echo "       $(OBJCOPY) does not strip Mach-O containers in -O binary mode."; \
+	  echo "       Install GNU binutils (brew install binutils) and rebuild, or"; \
+	  echo "       set OBJCOPY=/opt/homebrew/opt/binutils/bin/objcopy."; \
+	  rm -f $@; exit 1;; \
+	esac
 
 $(BUILD_DIR)/shim_blob.h: $(BUILD_DIR)/shim.bin
 	@echo "  GEN     $@"
diff --git a/mk/tests.mk b/mk/tests.mk
index 2268273..cd86dc3 100644
--- a/mk/tests.mk
+++ b/mk/tests.mk
@@ -6,6 +6,7 @@
         test-glibc-coreutils test-perf \
         test-matrix test-matrix-elfuse-aarch64 test-matrix-qemu-aarch64 \
         test-full test-multi-vcpu test-rwx test-sysroot-rename \
+        test-proctitle-low-stack \
         test-sysroot-procfs-exec test-timeout-disable \
         test-sysroot-nofollow test-sysroot-chdir perf
 
@@ -17,6 +18,8 @@ test-hello: $(ELFUSE_BIN) $(TEST_HELLO_DEP)
 ## Run the unit test suite plus busybox applet validation
 check: $(ELFUSE_BIN) $(TEST_DEPS)
 	@bash tests/driver.sh -e $(ELFUSE_BIN) -d $(TEST_DIR) -v
+	@printf "\n$(BLUE)━━━ proctitle low-stack regression ━━━$(RESET)\n"
+	@$(MAKE) --no-print-directory test-proctitle-low-stack
 	@printf "\n$(BLUE)━━━ busybox applet validation ━━━$(RESET)\n"
 	@$(MAKE) --no-print-directory test-busybox
 	@printf "\n$(BLUE)━━━ sysroot procfs exec validation ━━━$(RESET)\n"
@@ -205,6 +208,14 @@ test-busybox: $(ELFUSE_BIN) $(BUSYBOX_DEPS)
 	fi
 	@bash tests/test-busybox.sh $(ELFUSE_BIN) $(BUSYBOX_BIN)
 
+## Run the low-stack argv rewrite regression on busybox startup
+test-proctitle-low-stack: $(ELFUSE_BIN) $(BUSYBOX_DEPS)
+	@if [ ! -x "$(BUSYBOX_BIN)" ]; then \
+		printf "$(RED)✗ Busybox not found.$(RESET) Set BUSYBOX_BIN=/path/to/busybox.\n"; \
+		exit 1; \
+	fi
+	@bash tests/test-proctitle-low-stack.sh $(ELFUSE_BIN) $(BUSYBOX_BIN)
+
 # ── Static binary integration tests ──────────────────────────────
 
 ifdef GUEST_STATIC_BINS
diff --git a/src/main.c b/src/main.c
index a4966ff..f82ca45 100644
--- a/src/main.c
+++ b/src/main.c
@@ -19,6 +19,7 @@
 #include <errno.h>
 #include <limits.h>
 #include <stdbool.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -75,6 +76,37 @@ static void cleanup_main_resources(guest_t *g,
 /* Build-time version string (generated by make into build/version.h) */
 #include "version.h"
 
+/* Verify the host CPU's DC ZVA granule matches the shim's hardcoded value.
+ *
+ * DCZID_EL0 is readable from EL0 without trapping, so guest libc reads the
+ * host's value directly and uses it as the stride for memset(0) loops. The
+ * shim emulates each trapped DC ZVA by zeroing exactly 64 bytes
+ * (src/core/shim.S). Apple Silicon M1..M4 report DCZID_EL0.BS=4 (64 bytes);
+ * any future host that advertises a different granule would cause silent
+ * partial-zero corruption of guest memory. Abort here so the mismatch
+ * surfaces at startup instead of as data corruption later.
+ */
+static int host_dc_zva_assert(void)
+{
+    uint64_t dczid;
+    __asm__ volatile("mrs %0, DCZID_EL0" : "=r"(dczid));
+    if (dczid & (1ULL << 4)) {
+        log_error(
+            "host CPU prohibits DC ZVA (DCZID_EL0.DZP=1); cannot run "
+            "guests that depend on it");
+        return -1;
+    }
+    unsigned bs = (unsigned) (dczid & 0xF);
+    if (bs != 4) {
+        log_error(
+            "host DCZID_EL0.BS=%u (%u-byte DC ZVA block) but the shim "
+            "emulates 64 bytes; update src/core/shim.S before running",
+            bs, 1u << (bs + 2));
+        return -1;
+    }
+    return 0;
+}
+
 int main(int argc, char **argv)
 {
     log_init();
@@ -114,6 +146,9 @@ int main(int argc, char **argv)
         }
     }
 
+    if (host_dc_zva_assert() < 0)
+        return 1;
+
     /* Parse elfuse options until the first guest argv element. */
     while (arg_start < argc && argv[arg_start][0] == '-') {
         if (!strcmp(argv[arg_start], "--verbose") ||
diff --git a/src/runtime/proctitle.c b/src/runtime/proctitle.c
index 4e296dc..61be8c2 100644
--- a/src/runtime/proctitle.c
+++ b/src/runtime/proctitle.c
@@ -15,68 +15,35 @@
 
 #include "runtime/proctitle.h"
 
-static char *runtime_find_argv_environ_end(int argc, char **argv, char **envp)
+/* Return the contiguous argv block size starting at argv[0].
+ *
+ * Stop at the first non-contiguous argv entry and exclude the environment block
+ * entirely. Rewriting through envp is unsafe on Apple Silicon because libc's
+ * optimized memset may zero in cache-line chunks and step past the top of the
+ * stack when argv/env reach the stack ceiling under a small RLIMIT_STACK.
+ */
+static size_t runtime_argv_block_size(int argc, char **argv)
 {
-    char *end = argv[0];
+    char *next = argv[0];
 
     for (int i = 0; i < argc; i++) {
-        if (!argv[i])
-            continue;
-
-        char *next = argv[i] + strlen(argv[i]) + 1;
-        if (next > end)
-            end = next;
-    }
-
-    for (int i = 0; envp[i]; i++) {
-        char *next = envp[i] + strlen(envp[i]) + 1;
-        if (next > end)
-            end = next;
-    }
-
-    return end;
-}
-
-static bool runtime_duplicate_environment(char ***out_envp)
-{
-    extern char **environ;
-    int env_count = 0;
-
-    while (environ[env_count])
-        env_count++;
-
-    char **new_environ =
-        (char **) malloc((size_t) (env_count + 1) * sizeof(char *));
-    if (!new_environ)
-        return false;
-
-    for (int i = 0; i < env_count; i++) {
-        new_environ[i] = strdup(environ[i]);
-        if (new_environ[i])
-            continue;
-
-        for (int j = 0; j < i; j++)
-            free(new_environ[j]);
-        free(new_environ);
-        return false;
+        if (!argv[i] || argv[i] != next)
+            break;
+        next = argv[i] + strlen(argv[i]) + 1;
     }
 
-    new_environ[env_count] = NULL;
-    *out_envp = new_environ;
-    return true;
+    return (size_t) (next - argv[0]);
 }
 
 void runtime_set_process_title(int argc, char **argv, const char *elf_path)
 {
-    extern char **environ;
-    char **new_environ = NULL;
     size_t avail;
     const char *arch = "aarch64";
     char title[256];
     char thread_name[64];
     size_t title_len;
 
-    if (argc <= 0 || !argv || !argv[0] || !elf_path || !environ)
+    if (argc <= 0 || !argv || !argv[0] || !elf_path)
         return;
 
     const char *slash = strrchr(elf_path, '/');
@@ -90,19 +57,23 @@ void runtime_set_process_title(int argc, char **argv, const char *elf_path)
     snprintf(thread_name, sizeof(thread_name), "%s (%s-linux)", bin, arch);
     pthread_setname_np(thread_name);
 
-    avail =
-        (size_t) (runtime_find_argv_environ_end(argc, argv, environ) - argv[0]);
+    avail = runtime_argv_block_size(argc, argv);
     if (avail == 0)
         return;
 
-    if (!runtime_duplicate_environment(&new_environ))
-        return;
-    environ = new_environ;
+    /* Write the argv block with explicit byte stores through a volatile
+     * destination. The libc memcpy/memset on Apple Silicon are free to use
+     * cache-line-aligned stp/DC ZVA stores; using single-byte STRB removes
+     * any chance of touching the byte past avail, which on a Linux-style
+     * initial stack is the first character of envp[0].
+     */
+    size_t copy = title_len < avail ? title_len : avail - 1;
+    volatile char *dst = (volatile char *) argv[0];
+    for (size_t i = 0; i < copy; i++)
+        dst[i] = title[i];
+    for (size_t i = copy; i < avail; i++)
+        dst[i] = '\0';
 
-    if (title_len < avail) {
-        memcpy(argv[0], title, title_len);
-        memset(argv[0] + title_len, '\0', avail - title_len);
-    }
     for (int i = 1; i < argc; i++)
         argv[i] = NULL;
 }
diff --git a/tests/lib/test-runner.sh b/tests/lib/test-runner.sh
index b5322e1..aa6ef6e 100644
--- a/tests/lib/test-runner.sh
+++ b/tests/lib/test-runner.sh
@@ -10,6 +10,41 @@
 : "${TEST_LABEL_WIDTH:=14}"
 : "${TEST_TIMEOUT:=10}"
 
+# Resolve a working `timeout` binary. macOS doesn't ship one, so fall back to
+# GNU coreutils' gtimeout. Wrap as a function so callers keep using the bare
+# name `timeout`. Resolution order: TIMEOUT_BIN env override, `timeout` on
+# PATH, `gtimeout` on PATH, then Homebrew's stable opt symlinks for ARM and
+# Intel macOS (the install prefix differs between the two).
+if [ -n "${TIMEOUT_BIN:-}" ]; then
+    timeout()
+    {
+        "$TIMEOUT_BIN" "$@"
+    }
+elif ! command -v timeout > /dev/null 2>&1; then
+    _timeout_bin=
+    if command -v gtimeout > /dev/null 2>&1; then
+        _timeout_bin=gtimeout
+    else
+        for _candidate in /opt/homebrew/opt/coreutils/bin/gtimeout \
+            /usr/local/opt/coreutils/bin/gtimeout; do
+            if [ -x "$_candidate" ]; then
+                _timeout_bin="$_candidate"
+                break
+            fi
+        done
+    fi
+    if [ -n "$_timeout_bin" ]; then
+        # shellcheck disable=SC2317  # Invoked indirectly via `timeout` callers.
+        eval "timeout() { \"$_timeout_bin\" \"\$@\"; }"
+    else
+        echo "test-runner: no 'timeout' or 'gtimeout' in PATH." >&2
+        echo "  Install GNU coreutils (brew install coreutils), put gtimeout" >&2
+        echo "  on PATH, or set TIMEOUT_BIN=/path/to/timeout." >&2
+        exit 127
+    fi
+    unset _timeout_bin _candidate
+fi
+
 if [ -t 1 ]; then
     # Use ANSI-C quoting so the variables hold real ESC bytes, not the literal
     # 4-char "\033" sequence. Without this, callers that pass colors as printf
diff --git a/tests/test-busybox.sh b/tests/test-busybox.sh
index e5dbe07..9969912 100755
--- a/tests/test-busybox.sh
+++ b/tests/test-busybox.sh
@@ -37,9 +37,50 @@ test_tool_path()
     printf "%s" "$1"
 }
 
+# Probe which applets this busybox binary actually carries. The Debian
+# busybox-static drops a handful of applets (e.g. comm) compared to a
+# full build, and tests for them must skip rather than fail. Hard-fail
+# the whole suite if the probe itself fails so a broken elfuse/busybox
+# does not silently degrade to "all SKIP".
+if ! _bb_list=$(timeout "$TEST_TIMEOUT" "$ELFUSE" "$BB" --list 2>&1); then
+    printf "test-busybox: probing '%s --list' under elfuse failed:\n%s\n" \
+        "$BB" "$_bb_list" >&2
+    exit 1
+fi
+BB_APPLETS=" $(printf '%s\n' "$_bb_list" | tr '\n' ' ') "
+# Sanity: a usable busybox should expose at least one of these common
+# applets. A reduced build may legitimately omit sh, so accept any of
+# the small universal set; only fail if --list produced nothing usable.
+case "$BB_APPLETS" in
+    *" sh "* | *" echo "* | *" cat "* | *" ls "* | *" true "*) ;;
+    *)
+        printf "test-busybox: applet list from '%s --list' looks empty or malformed:\n%s\n" \
+            "$BB" "$_bb_list" >&2
+        exit 1
+        ;;
+esac
+unset _bb_list
+
+# Override: skip if the requested applet isn't compiled into this busybox.
+# shellcheck disable=SC2329  # Invoked indirectly by tests/lib/test-runner.sh.
+test_skip_missing_tool()
+{
+    local tool="$1"
+    case "$BB_APPLETS" in
+        *" $tool "*) return 1 ;;
+    esac
+    run_skip "$tool" "applet not in this busybox build"
+    return 0
+}
+
 run_nc_http_check()
 {
     local applet="nc" output rc server_pid port_file port
+
+    if test_skip_missing_tool "$applet"; then
+        return
+    fi
+
     port_file=$(mktemp "${TMPDIR}/nc-http-port.XXXXXX") || {
         test_report skip "$applet" " (failed to create port file)"
         skip=$((skip + 1))
diff --git a/tests/test-proctitle-low-stack.sh b/tests/test-proctitle-low-stack.sh
new file mode 100644
index 0000000..276dc70
--- /dev/null
+++ b/tests/test-proctitle-low-stack.sh
@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# test-proctitle-low-stack.sh — Regress Apple Silicon argv/env stack overwrite
+#
+# Copyright 2026 elfuse contributors
+# SPDX-License-Identifier: Apache-2.0
+#
+# Usage: tests/test-proctitle-low-stack.sh <elfuse-binary> <busybox-binary>
+
+set -euo pipefail
+
+ELFUSE="${1:?Usage: $0 <elfuse-binary> <busybox-binary>}"
+BB="${2:?Usage: $0 <elfuse-binary> <busybox-binary>}"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+# shellcheck disable=SC2034  # Consumed by tests/lib/test-runner.sh.
+TEST_TIMEOUT="${TEST_TIMEOUT:-10}"
+# shellcheck source=tests/lib/test-runner.sh
+source "$SCRIPT_DIR/lib/test-runner.sh"
+
+output=
+if output="$(
+    # shellcheck disable=SC2016  # Positional params are expanded by the child shell.
+    timeout "$TEST_TIMEOUT" sh -c '
+        current_stack=$(ulimit -S -s)
+        case "$current_stack" in
+            unlimited) ulimit -S -s 8192 ;;
+            "" | *[!0-9]*) ;;
+            *)
+                if [ "$current_stack" -gt 8192 ]; then
+                    ulimit -S -s 8192
+                fi
+                ;;
+        esac
+        exec "$1" "$2" echo hello
+    ' sh "$ELFUSE" "$BB"
+)"; then
+    :
+else
+    rc=$?
+    if [ "$rc" -eq 124 ]; then
+        printf "test-proctitle-low-stack: elfuse hung under low stack (timeout after %ss)\n" \
+            "$TEST_TIMEOUT" >&2
+        exit 1
+    fi
+    printf "test-proctitle-low-stack: elfuse failed under low stack (rc=%d)\n" \
+        "$rc" >&2
+    exit "$rc"
+fi
+
+if [ "$output" != "hello" ]; then
+    printf "test-proctitle-low-stack: unexpected output under low stack: %s\n" \
+        "$output" >&2
+    exit 1
+fi
+
+printf "test-proctitle-low-stack: PASS\n"