this has been sent upstream, but they don't have a mailing list or project site
to link to.  oh well.

From e641584767c3c7cc1ff544805acc2562fc56cda9 Mon Sep 17 00:00:00 2001
From: Mike Frysinger <vapier@chromium.org>
Date: Mon, 17 Sep 2018 18:57:57 -0400
Subject: [PATCH 1/2] dc: add a --sandbox option

Other GNU projects (like sed & gawk) have a --sandbox flag whereby
access to files and system() are disabled.  This allows people to
run arbitrary scripts without worrying about them "escaping" and
executing arbitrary commands on the system.
---
 dc/dc.c     | 9 ++++++++-
 dc/dc.h     | 3 +++
 dc/misc.c   | 6 ++++++
 doc/dc.1    | 5 +++++
 doc/dc.texi | 4 ++++
 5 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/dc/dc.c b/dc/dc.c
index 6a2bb2639235..592a76be71da 100644
--- a/dc/dc.c
+++ b/dc/dc.c
@@ -54,6 +54,7 @@
 #endif
 
 const char *progname;	/* basename of program invocation */
+int dc_sandbox_enabled;	/* whether sandbox mode is enabled */
 
 static void
 bug_report_info DC_DECLVOID()
@@ -80,6 +81,7 @@ usage DC_DECLARG((f))
 Usage: %s [OPTION] [file ...]\n\
   -e, --expression=EXPR    evaluate expression\n\
   -f, --file=FILE          evaluate contents of file\n\
+  -S, --sandbox            disable the ! (system) command\n\
   -h, --help               display this help and exit\n\
   -V, --version            output version information and exit\n\
 \n\
@@ -252,6 +254,7 @@ main DC_DECLARG((argc, argv))
 	static struct option const long_opts[] = {
 		{"expression", required_argument, NULL, 'e'},
 		{"file", required_argument, NULL, 'f'},
+		{"sandbox", no_argument, NULL, 'S'},
 		{"help", no_argument, NULL, 'h'},
 		{"version", no_argument, NULL, 'V'},
 		{NULL, 0, NULL, 0}
@@ -260,12 +263,13 @@ main DC_DECLARG((argc, argv))
 	int c;
 
 	progname = r1bindex(*argv, '/');
+	dc_sandbox_enabled = 0;
 	dc_math_init();
 	dc_string_init();
 	dc_register_init();
 	dc_array_init();
 
-	while ((c = getopt_long(argc, argv, "hVe:f:", long_opts, (int *)0)) != EOF) {
+	while ((c = getopt_long(argc, argv, "hVe:f:S", long_opts, (int *)0)) != EOF) {
 		switch (c) {
 		case 'e':
 			{	dc_data string = dc_makestring(optarg, strlen(optarg));
@@ -279,6 +283,9 @@ main DC_DECLARG((argc, argv))
 			try_file(optarg);
 			did_eval = 1;
 			break;
+		case 'S':
+			dc_sandbox_enabled = 1;
+			break;
 		case 'h':
 			usage(stdout);
 			return flush_okay();
diff --git a/dc/dc.h b/dc/dc.h
index 6a871ad612a5..a148df467a92 100644
--- a/dc/dc.h
+++ b/dc/dc.h
@@ -76,4 +76,7 @@ typedef struct {
 /* This is dc's only global variable: */
 extern const char *progname;	/* basename of program invocation */
 
+/* Whether to run in sandbox mode. */
+extern int dc_sandbox_enabled;
+
 #endif /* not DC_DEFS_H */
diff --git a/dc/misc.c b/dc/misc.c
index cd23602fce32..115be90b03bf 100644
--- a/dc/misc.c
+++ b/dc/misc.c
@@ -131,6 +131,12 @@ dc_system DC_DECLARG((s))
 	char *tmpstr;
 	size_t len;
 
+	if (dc_sandbox_enabled) {
+		fprintf(stderr, "%s: ! command disabled in sandbox mode\n",
+				progname);
+		exit(EXIT_FAILURE);
+	}
+
 	p = strchr(s, '\n');
 	if (p != NULL) {
 		len = (size_t) (p - s);
diff --git a/doc/dc.1 b/doc/dc.1
index 1c666493e00a..7c4b6fffd616 100644
--- a/doc/dc.1
+++ b/doc/dc.1
@@ -84,6 +84,11 @@ to the set of commands to be run while processing the input.
 Add the commands contained in the file
 .I script-file
 to the set of commands to be run while processing the input.
+.TP
+.B -S
+.TP
+.B --sandbox
+Run in sandbox mode where access to \fB!\fR for the system function.
 .PP
 If any command-line parameters remain after processing the above,
 these parameters are interpreted as the names of input files to
-- 
2.17.1