From dca237d41e42bf82ca9871ca87666bcf73bce6cf Mon Sep 17 00:00:00 2001
From: Marius Halden <marius.h@lden.org>
Date: Fri, 4 Nov 2016 14:30:03 +0100
Subject: Allow setting fd through argv[1] and set O_CLOEXEC if file does not
 start with #!

---
 runfd.c | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/runfd.c b/runfd.c
index 4e986fe..3a3260e 100644
--- a/runfd.c
+++ b/runfd.c
@@ -1,7 +1,10 @@
 #include <stdio.h>
+#include <stdlib.h>
 #include <err.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <errno.h>
+#include <string.h>
 
 /**
  * NOTE: requires fdescfs to be mounted to run interpreted files (starting with #!)
@@ -10,10 +13,33 @@
 char *args[] = { "batchrun", NULL };
 
 int
-main()
+main(int argc, char **argv)
 {
-	//fcntl(3, F_SETFD, FD_CLOEXEC);
+	char buf[2];
+	ssize_t ret;
+	int tries = 0;
+	int fd = 3;
 
-	fexecve(3, args, NULL);
+	if (argc > 1) {
+		fd = atoi(argv[1]);
+	}
+
+retry:
+	if (tries++ >= 3)
+		errx(1, "Failed to read file header");
+
+	ret = pread(fd, buf, sizeof(buf), 0);
+	if (ret == 2) {
+		if (strncmp(buf, "#!", 2) != 0) {
+			fcntl(3, F_SETFD, FD_CLOEXEC);
+		}
+	} else if (ret == -1) {
+		if (errno == EINTR)
+			goto retry;
+	} else if (ret != 0) {
+		goto retry;
+	}
+
+	fexecve(fd, args, NULL);
 	err(1, "fexecve()");
 }
-- 
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