From: Dmitry V. Levin <ldv@altlinux.org>
Date: Sat, 18 Mar 2017 22:14:16 +0000 (+0000)
Subject: test: rewrite skodic.c demo
X-Git-Tag: v4.17~151
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=ec5480453b6928a01fdbe7789bf40292a9369028;p=strace

test: rewrite skodic.c demo

* test/skodic.c: Rewrite this demo to simplify its use.
---

diff --git a/test/skodic.c b/test/skodic.c
index 4dcd9556..39d91641 100644
--- a/test/skodic.c
+++ b/test/skodic.c
@@ -1,36 +1,64 @@
-/* This demonstrates races: kernel may actually open other file then
- * you read at strace output. Create /tmp/delme with 10K of zeros and
- * 666 mode, then run this under strace. If you see open successfull
- * open of /etc/shadow, you know you've seen a race.
+/*
+ * This demonstrates races: kernel may actually open other file
+ * than you read in the strace output.
+ * If you see a successfull open of /etc/shadow,
+ * you know you've seen a race.
+ *
+ * $ gcc -Wall -O0 skodic.c -o skodic
+ * $ timeout 0.1 ../strace -yeopen -o'|grep "shadow.*= [0-9]"' ./skodic
  */
 
+#ifndef _GNU_SOURCE
+# define  _GNU_SOURCE 1
+#endif
+
+#include <errno.h>
+#include <error.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <sys/mman.h>
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <fcntl.h>
 
-int main(int argc, char *argv[])
+int
+main(void)
 {
-	char *c;
-	int fd;
+	FILE *fp = tmpfile();
+	if (!fp)
+		error(1, errno, "tmpfile");
+
+	int fd = fileno(fp);
+	size_t size = sysconf(_SC_PAGESIZE);
 
-	fd = open("/tmp/delme", O_RDWR);
-	c = mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
-	*c = 0;
+	if (ftruncate(fd, size))
+		error(1, errno, "ftruncate");
 
-	if (fork()) {
-		while (1) {
-			strcpy(c, "/etc/passwd");
-			strcpy(c, "/etc/shadow");
+	char *p = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (p == MAP_FAILED)
+		error(1, errno, "mmap");
+	fclose(fp);
+	fp = NULL;
+	fd = -1;
+
+	strcpy(p, "/etc/shadow");
+	if (open(p, 0) >= 0)
+		error(1, 0, p);
+
+	pid_t pid = fork();
+	if (pid < 0)
+		error(1, errno, "fork");
+
+	if (!pid) {
+		for (;;) {
+			strcpy(p, "/etc/passwd");
+			strcpy(p, "/etc/shadow");
 		}
 	} else {
-		while (1)
-			if ((fd = open(c, 0)) != -1)
+		for (;;) {
+			if ((fd = open(p, 0)) >= 0)
 				close(fd);
+		}
 	}
 
 	return 0;