Fix infinite recursion with VERILATOR_BIN (#7496) (#7497)

Fixes #7496.

bin/verilator

@@ -27,6 +27,21 @@ use vars qw($Debug @Opt_Verilator_Sw);
 autoflush STDOUT 1;
 autoflush STDERR 1;
 
+# Guard against unbounded recursive re-entry. Hierarchical / --build flows
+# legitimately re-enter the wrapper a few levels deep (wrapper -> verilator_bin
+# -> make -> wrapper -> ...); a misconfigured VERILATOR_BIN, VERILATOR_GDB or
+# VERILATOR_VALGRIND that points back at this wrapper, however, recurses
+# without bound and exhausts the OS PID table. Use VERILATOR_RUNNING as a
+# depth counter and cap it well above any real nesting.
+my $verilator_running_max = 16;
+my $verilator_running_depth = ($ENV{VERILATOR_RUNNING} || 0) + 1;
+if ($verilator_running_depth > $verilator_running_max) {
+    die "%Error: verilator: re-entered $verilator_running_depth levels deep"
+        . " via \$VERILATOR_RUNNING; check that VERILATOR_BIN, VERILATOR_GDB"
+        . " and VERILATOR_VALGRIND do not resolve back to this wrapper script.\n";
+}
+$ENV{VERILATOR_RUNNING} = $verilator_running_depth;
+
 $Debug = 0;
 my $opt_aslr;
 my $opt_gdb;

docs/CONTRIBUTORS

@@ -165,6 +165,7 @@ Liam Braun
 Luca Colagrande
 Ludwig Rogiers
 Lukasz Dalek
+M2kar
 Maarten De Braekeleer
 Maciej Sobkowski
 Marcel Chang

test_regress/t/t_flag_verilator_running.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('dist')
+
+# Real-world repro: a misconfigured VERILATOR_BIN that points back at the
+# Perl wrapper used to fork-bomb the host. The wrapper now caps re-entry
+# depth via $VERILATOR_RUNNING and aborts past the cap.
+os.environ['VERILATOR_BIN'] = os.environ["VERILATOR_ROOT"] + "/bin/verilator"
+
+# --no-unlimited-stack avoids the ulimit_stack_unlimited() backtick branch,
+# which would otherwise double the recursion fanout. Linear re-entry through
+# run() is enough to exercise the depth-cap abort.
+test.run(fails=True,
+         cmd=[os.environ["VERILATOR_ROOT"] + "/bin/verilator",
+              "--no-unlimited-stack", "--version"],
+         logfile=test.run_log_filename)
+
+test.file_grep(test.run_log_filename,
+               r'%Error: verilator: re-entered \d+ levels deep via \$VERILATOR_RUNNING')
+
+test.passes()