--- /dev/null
+#include <linux/kernel.h>
+#include <linux/syscalls.h>
+#include <linux/fdtable.h>
+#include <linux/string.h>
+#include <linux/random.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/errno.h>
+#include <linux/cache.h>
+#include <linux/bug.h>
+#include <linux/err.h>
+#include <linux/kcmp.h>
+
+#include <asm/unistd.h>
+
+/*
+ * We don't expose the real in-memory order of objects for security reasons.
+ * But still the comparison results should be suitable for sorting. So we
+ * obfuscate kernel pointers values and compare the production instead.
+ *
+ * The obfuscation is done in two steps. First we xor the kernel pointer with
+ * a random value, which puts pointer into a new position in a reordered space.
+ * Secondly we multiply the xor production with a large odd random number to
+ * permute its bits even more (the odd multiplier guarantees that the product
+ * is unique ever after the high bits are truncated, since any odd number is
+ * relative prime to 2^n).
+ *
+ * Note also that the obfuscation itself is invisible to userspace and if needed
+ * it can be changed to an alternate scheme.
+ */
+static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
+
+static long kptr_obfuscate(long v, int type)
+{
+ return (v ^ cookies[type][0]) * cookies[type][1];
+}
+
+/*
+ * 0 - equal, i.e. v1 = v2
+ * 1 - less than, i.e. v1 < v2
+ * 2 - greater than, i.e. v1 > v2
+ * 3 - not equal but ordering unavailable (reserved for future)
+ */
+static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
+{
+ long ret;
+
+ ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);
+
+ return (ret < 0) | ((ret > 0) << 1);
+}
+
+/* The caller must have pinned the task */
+static struct file *
+get_file_raw_ptr(struct task_struct *task, unsigned int idx)
+{
+ struct file *file = NULL;
+
+ task_lock(task);
+ rcu_read_lock();
+
+ if (task->files)
+ file = fcheck_files(task->files, idx);
+
+ rcu_read_unlock();
+ task_unlock(task);
+
+ return file;
+}
+
+static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
+{
+ if (likely(m2 != m1))
+ mutex_unlock(m2);
+ mutex_unlock(m1);
+}
+
+static int kcmp_lock(struct mutex *m1, struct mutex *m2)
+{
+ int err;
+
+ if (m2 > m1)
+ swap(m1, m2);
+
+ err = mutex_lock_killable(m1);
+ if (!err && likely(m1 != m2)) {
+ err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
+ if (err)
+ mutex_unlock(m1);
+ }
+
+ return err;
+}
+
+SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
+ unsigned long, idx1, unsigned long, idx2)
+{
+ struct task_struct *task1, *task2;
+ int ret;
+
+ rcu_read_lock();
+
+ /*
+ * Tasks are looked up in caller's PID namespace only.
+ */
+ task1 = find_task_by_vpid(pid1);
+ task2 = find_task_by_vpid(pid2);
+ if (!task1 || !task2)
+ goto err_no_task;
+
+ get_task_struct(task1);
+ get_task_struct(task2);
+
+ rcu_read_unlock();
+
+ /*
+ * One should have enough rights to inspect task details.
+ */
+ ret = kcmp_lock(&task1->signal->cred_guard_mutex,
+ &task2->signal->cred_guard_mutex);
+ if (ret)
+ goto err;
+ if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
+ !ptrace_may_access(task2, PTRACE_MODE_READ)) {
+ ret = -EPERM;
+ goto err_unlock;
+ }
+
+ switch (type) {
+ case KCMP_FILE: {
+ struct file *filp1, *filp2;
+
+ filp1 = get_file_raw_ptr(task1, idx1);
+ filp2 = get_file_raw_ptr(task2, idx2);
+
+ if (filp1 && filp2)
+ ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
+ else
+ ret = -EBADF;
+ break;
+ }
+ case KCMP_VM:
+ ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
+ break;
+ case KCMP_FILES:
+ ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
+ break;
+ case KCMP_FS:
+ ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
+ break;
+ case KCMP_SIGHAND:
+ ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
+ break;
+ case KCMP_IO:
+ ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
+ break;
+ case KCMP_SYSVSEM:
+#ifdef CONFIG_SYSVIPC
+ ret = kcmp_ptr(task1->sysvsem.undo_list,
+ task2->sysvsem.undo_list,
+ KCMP_SYSVSEM);
+#else
+ ret = -EOPNOTSUPP;
+#endif
+ break;
+ default:
+ ret = -EINVAL;
+ break;
+ }
+
+err_unlock:
+ kcmp_unlock(&task1->signal->cred_guard_mutex,
+ &task2->signal->cred_guard_mutex);
+err:
+ put_task_struct(task1);
+ put_task_struct(task2);
+
+ return ret;
+
+err_no_task:
+ rcu_read_unlock();
+ return -ESRCH;
+}
+
+static __init int kcmp_cookies_init(void)
+{
+ int i;
+
+ get_random_bytes(cookies, sizeof(cookies));
+
+ for (i = 0; i < KCMP_TYPES; i++)
+ cookies[i][1] |= (~(~0UL >> 1) | 1);
+
+ return 0;
+}
+arch_initcall(kcmp_cookies_init);
--- /dev/null
+#define _GNU_SOURCE
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <limits.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <fcntl.h>
+
+#include <linux/unistd.h>
+#include <linux/kcmp.h>
+
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+static long sys_kcmp(int pid1, int pid2, int type, int fd1, int fd2)
+{
+ return syscall(__NR_kcmp, pid1, pid2, type, fd1, fd2);
+}
+
+int main(int argc, char **argv)
+{
+ const char kpath[] = "kcmp-test-file";
+ int pid1, pid2;
+ int fd1, fd2;
+ int status;
+
+ fd1 = open(kpath, O_RDWR | O_CREAT | O_TRUNC, 0644);
+ pid1 = getpid();
+
+ if (fd1 < 0) {
+ perror("Can't create file");
+ exit(1);
+ }
+
+ pid2 = fork();
+ if (pid2 < 0) {
+ perror("fork failed");
+ exit(1);
+ }
+
+ if (!pid2) {
+ int pid2 = getpid();
+ int ret;
+
+ fd2 = open(kpath, O_RDWR, 0644);
+ if (fd2 < 0) {
+ perror("Can't open file");
+ exit(1);
+ }
+
+ /* An example of output and arguments */
+ printf("pid1: %6d pid2: %6d FD: %2ld FILES: %2ld VM: %2ld "
+ "FS: %2ld SIGHAND: %2ld IO: %2ld SYSVSEM: %2ld "
+ "INV: %2ld\n",
+ pid1, pid2,
+ sys_kcmp(pid1, pid2, KCMP_FILE, fd1, fd2),
+ sys_kcmp(pid1, pid2, KCMP_FILES, 0, 0),
+ sys_kcmp(pid1, pid2, KCMP_VM, 0, 0),
+ sys_kcmp(pid1, pid2, KCMP_FS, 0, 0),
+ sys_kcmp(pid1, pid2, KCMP_SIGHAND, 0, 0),
+ sys_kcmp(pid1, pid2, KCMP_IO, 0, 0),
+ sys_kcmp(pid1, pid2, KCMP_SYSVSEM, 0, 0),
+
+ /* This one should fail */
+ sys_kcmp(pid1, pid2, KCMP_TYPES + 1, 0, 0));
+
+ /* This one should return same fd */
+ ret = sys_kcmp(pid1, pid2, KCMP_FILE, fd1, fd1);
+ if (ret) {
+ printf("FAIL: 0 expected but %d returned\n", ret);
+ ret = -1;
+ } else
+ printf("PASS: 0 returned as expected\n");
+
+ /* Compare with self */
+ ret = sys_kcmp(pid1, pid1, KCMP_VM, 0, 0);
+ if (ret) {
+ printf("FAIL: 0 expected but %li returned\n", ret);
+ ret = -1;
+ } else
+ printf("PASS: 0 returned as expected\n");
+
+ exit(ret);
+ }
+
+ waitpid(pid2, &status, P_ALL);
+
+ return 0;
+}
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