From cbf60bec6ff900e6759b439b782c8cef163b3046 Mon Sep 17 00:00:00 2001
From: Yorick van Pelt <yorick@yorickvanpelt.nl>
Date: Tue, 24 May 2022 16:26:40 +0200
Subject: [PATCH] configure.ac: check for sandbox-shell's FEATURE_SH_STANDALONE

See also: https://bugs.archlinux.org/task/73998. Busybox's
FEATURE_SH_STANDALONE feature causes other busybox applets to
leak into the sandbox, where system() calls will start preferring
them over tools in $PATH. On arch, this even includes `ar`.

Let's check for this evil feature and disallow using this as a
sandbox shell.
---
 configure.ac | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/configure.ac b/configure.ac
index 8a01c33ec..715c70de1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -294,6 +294,17 @@ esac
 AC_ARG_WITH(sandbox-shell, AS_HELP_STRING([--with-sandbox-shell=PATH],[path of a statically-linked shell to use as /bin/sh in sandboxes]),
   sandbox_shell=$withval)
 AC_SUBST(sandbox_shell)
+if ! test -z ${sandbox_shell+x}; then
+  AC_MSG_CHECKING([whether sandbox-shell has the standalone feature])
+  # busybox shell sometimes allows executing other busybox applets,
+  # even if they are not in the path, breaking our sandbox
+  if PATH= $sandbox_shell -c "busybox" 2>&1 | grep -qv "not found"; then
+    AC_MSG_RESULT(enabled)
+    AC_MSG_ERROR([Please disable busybox FEATURE_SH_STANDALONE])
+  else
+    AC_MSG_RESULT(disabled)
+  fi
+fi
 
 # Expand all variables in config.status.
 test "$prefix" = NONE && prefix=$ac_default_prefix