From 7868b86e48d053a4ddec90c4562bf26022b8be16 Mon Sep 17 00:00:00 2001
From: snow flurry <snow@datagirl.xyz>
Date: Tue, 17 Nov 2020 13:45:15 -0800
Subject: [PATCH] corrupt: upload the corruption bits

... These were supposed to be uploaded a while ago
---
 src/corrupt.c | 113 ++++++++++++++++++++++++++++++++++++++++++++++++++
 src/corrupt.h |  10 +++++
 2 files changed, 123 insertions(+)
 create mode 100644 src/corrupt.c
 create mode 100644 src/corrupt.h

diff --git a/src/corrupt.c b/src/corrupt.c
new file mode 100644
index 0000000..2be7b33
--- /dev/null
+++ b/src/corrupt.c
@@ -0,0 +1,113 @@
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "config.h"
+#include "corrupt.h"
+#include "debug.h"
+#include "dosfs.h"
+
+static uint32_t xs_rand(uint32_t);
+static uint32_t xs_randb(uint32_t *, uint32_t, uint32_t);
+
+/*
+ * Finds the rule (if any) that matches the given file. If no rule
+ * matches, the function returns NULL.
+ */
+config_t *
+match_rule(config_t *conf, dosfile_t *file)
+{
+    config_t *cur;
+    int match;
+
+    for (cur = conf; cur != NULL; cur = cur->next) {
+        match = 0;
+        match += ((cur->filename[0] == '*' && cur->filename[1] == 0) ||
+            (!strncmp(cur->filename, file->fname, 8)));
+
+        match += ((cur->ext[0] == '*' && cur->ext[1] == 0) ||
+            (!strncmp(cur->ext, file->fext, 3)));
+
+        if (match >= 2)
+            return cur;
+    } 
+
+    return NULL;
+}
+
+/*
+ * Corrupts the given dosfile_t according to conf.
+ */
+int
+corrupt_file(dosfs_t *fsd, dosfile_t *file, config_t *conf, uint32_t *state)
+{
+    uint32_t off;
+    int real_off;
+    unsigned char c;
+    struct conf_opts_t opts;
+
+    opts = conf->opts;
+    for (off = conf->start; (off < conf->end) && (off < file->ent.size);
+        off += xs_randb(state, conf->skip_a, conf->skip_b)) {
+        real_off = get_byte_offset(fsd, file, off);
+        if (real_off < 0) {
+            DPRINTF(("get_byte_offset returned %s", strerror(real_off * -1)));
+            return 0;
+        }
+
+        if (pread(fsd->ifd, &c, 1, real_off) == -1) {
+            perror("file read failed");
+            return 1;
+        }
+
+        /*
+         * The fun part! Originally the plan was to check for each
+         * function (i.e., "is it zero or not") before running the
+         * operations, but zero shift, add, and xor should be just as
+         * efficient as a bunch of if statements. Might be flawed
+         * logic, let me know if it is!
+         */
+        c = c >> opts.shr;
+        c = c << opts.shl;
+        c += opts.add;
+        c ^= opts.bit_xor;
+
+        if (pwrite(fsd->ifd, &c, 1, real_off) == -1) {
+            perror("file write failed");
+            return 1;
+        }
+    }
+
+    return 0;
+}
+
+/*
+ * Generates a random number within two bounds.
+ */
+static uint32_t
+xs_randb(uint32_t *state, uint32_t lower, uint32_t upper)
+{
+    *state = xs_rand(*state);
+    /* if we're already between, just roll with it */
+    if (*state <= upper && *state >= lower) {
+        return *state;
+    }
+    return ((*state % (upper - lower)) + lower);
+}
+
+/*
+ * Implement xorshift for randomness
+ */
+static uint32_t
+xs_rand(uint32_t in)
+{
+    uint32_t out;
+
+    out = in;
+    out ^= in << 13;
+    out ^= out >> 17;
+    out ^= out << 5;
+    return out;
+}
diff --git a/src/corrupt.h b/src/corrupt.h
new file mode 100644
index 0000000..6bdebf8
--- /dev/null
+++ b/src/corrupt.h
@@ -0,0 +1,10 @@
+#ifndef _CORRUPT_H
+#define _CORRUPT_H
+
+#include "config.h"
+#include "dosfs.h"
+
+config_t *match_rule(config_t *, dosfile_t *);
+int corrupt_file(dosfs_t *, dosfile_t *, config_t *, uint32_t *);
+
+#endif /* !_CORRUPT_H */