From ed8aefcec6235b5c70bf014699ccc3c4c6a43d80 Mon Sep 17 00:00:00 2001
From: Lars Wirzenius <liw@achmed>
Date: Tue, 27 Jan 2009 14:39:41 +0200
Subject: Wrote first version of corrupt.

---
 corrupt.c | 118 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 corrupt.c

(limited to 'corrupt.c')

diff --git a/corrupt.c b/corrupt.c
new file mode 100644
index 0000000..f8c5cd2
--- /dev/null
+++ b/corrupt.c
@@ -0,0 +1,118 @@
+/*
+ * Copyright (C) 2009  Lars Wirzenius
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+
+#define _LARGEFILE64_SOURCE
+
+#include <errno.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+
+/* Choose a random integer in [0..n-1]. */
+static long choose(long n)
+{
+    if (n == 0)
+        return 0;
+    
+    long buckets = (RAND_MAX + 1L) / n;
+    long max = buckets * n;
+    long r;
+    
+    do {
+        r = random();
+    } while (r >= max);
+    return r / buckets;
+}
+
+
+static int corrupt(const char *filename)
+{
+    int fd;
+    struct stat st;
+    
+    fd = open(filename, O_RDWR | O_LARGEFILE);
+    if (fd == -1) {
+        fprintf(stderr, "Can't open %s: %s\n", filename, strerror(errno));
+        return -1;
+    }
+
+    if (fstat(fd, &st) == -1) {
+        fprintf(stderr, "Can't find length of %s: %s", filename, 
+                strerror(errno));
+        return -1;
+    }
+
+    long offset = choose(st.st_size);
+    long bit = choose(CHAR_BIT);
+    printf("%s: offset=%ld bit=%ld\n", filename, offset, bit);
+    
+    if (lseek(fd, offset, SEEK_SET) == (off_t) -1) {
+        fprintf(stderr, "Can't seek in %s: %s", filename, strerror(errno));
+        return -1;
+    }
+    
+    unsigned char byte;
+    if (read(fd, &byte, 1) == -1) {
+        fprintf(stderr, "Can't read %s: %s", filename, strerror(errno));
+        return -1;
+    }
+    
+    /* This is where we toggle the bit. */
+    unsigned mask = 1 << bit; // All zeroes, except for the interesting bit.
+    unsigned interesting = byte & mask; // Only the interesting bit can be 1.
+    unsigned toggled = interesting ^ mask; // Interesting bit has been toggled.
+    unsigned others = byte & (~mask); // Interesting bit is 0.
+    byte = others | toggled;
+    
+    /* Now write it back. */
+    if (lseek(fd, offset, SEEK_SET) == (off_t) -1) {
+        fprintf(stderr, "Can't seek in %s: %s", filename, strerror(errno));
+        return -1;
+    }
+    if (write(fd, &byte, 1) != 1) {
+        fprintf(stderr, "Can't write %s: %s", filename, strerror(errno));
+        return -1;
+    }
+
+    if (close(fd) == -1) {
+        fprintf(stderr, "Can't close %s: %s", filename, strerror(errno));
+        return -1;
+    }
+    
+    return 0;
+}
+
+
+int main(int argc, char **argv)
+{
+    int exit_code;
+    
+    srandom(time(NULL) + getpid());
+    exit_code = 0;
+    for (int i = 1; i < argc; ++i)
+        if (corrupt(argv[i]) == -1)
+            exit_code = 1;
+    return exit_code;
+}
-- 
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