userland: add some random filesystem and random stuff

Some moved from C++, some moved from SO, some I just made up.

userland/linux/open_o_tmpfile.c

@@ -0,0 +1,39 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#c */
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+int main(void) {
+    char buf[]  = { 'a', 'b', 'c', 'd' };
+    char buf2[]  = { 'e', 'f', 'g', 'h' };
+    int f, ret;
+    size_t off;
+
+    /* Create the temporary file and write to it. */
+    f = open(".", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR);
+    ret = write(f, buf, sizeof(buf));
+
+    /* Interactivelly check if anything changed on directory. It hasn't. */
+    /*puts("hit enter to continue");*/
+    /*getchar();*/
+
+    /* Read from the temporary file, and assert that
+     * we read the same as we wrote. */
+    lseek(f, 0, SEEK_SET);
+    off = 0;
+    while ((ret = read(f, buf2 + off, sizeof(buf) - off))) {
+        off += ret;
+    }
+    close(f);
+    assert(!memcmp(buf, buf2, sizeof(buf)));
+
+    /* Assert that the file is gone now that we removed it. */
+
+    return EXIT_SUCCESS;
+}