NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

KCMP(2)                   Linux Programmer's Manual                  KCMP(2)

NAME         top

       kcmp  -  compare  two  processes  to determine if they share a kernel
       resource

SYNOPSIS         top

       #include <linux/kcmp.h>

       int kcmp(pid_t pid1, pid_t pid2, int type,
                unsigned long idx1, unsigned long idx2);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION         top

       The kcmp() system call can be used to check whether the two processes
       identified by pid1 and pid2 share a kernel resource such as virtual
       memory, file descriptors, and so on.

       Permission to employ kcmp() is governed by ptrace access mode
       PTRACE_MODE_READ_REALCREDS checks against both pid1 and pid2; see
       ptrace(2).

       The type argument specifies which resource is to be compared in the
       two processes.  It has one of the following values:

       KCMP_FILE
              Check whether a file descriptor idx1 in the process pid1
              refers to the same open file description (see open(2)) as file
              descriptor idx2 in the process pid2.  The existence of two
              file descriptors that refer to the same open file description
              can occur as a result of dup(2) (and similar) fork(2), or
              passing file descriptors via a domain socket (see unix(7)).

       KCMP_FILES
              Check whether the processes share the same set of open file
              descriptors.  The arguments idx1 and idx2 are ignored.  See
              the discussion of the CLONE_FILES flag in clone(2).

       KCMP_FS
              Check whether the processes share the same filesystem
              information (i.e., file mode creation mask, working directory,
              and filesystem root).  The arguments idx1 and idx2 are
              ignored.  See the discussion of the CLONE_FS flag in clone(2).

       KCMP_IO
              Check whether the processes share I/O context.  The arguments
              idx1 and idx2 are ignored.  See the discussion of the CLONE_IO
              flag in clone(2).

       KCMP_SIGHAND
              Check whether the processes share the same table of signal
              dispositions.  The arguments idx1 and idx2 are ignored.  See
              the discussion of the CLONE_SIGHAND flag in clone(2).

       KCMP_SYSVSEM
              Check whether the processes share the same list of System V
              semaphore undo operations.  The arguments idx1 and idx2 are
              ignored.  See the discussion of the CLONE_SYSVSEM flag in
              clone(2).

       KCMP_VM
              Check whether the processes share the same address space.  The
              arguments idx1 and idx2 are ignored.  See the discussion of
              the CLONE_VM flag in clone(2).

       Note the kcmp() is not protected against false positives which may
       occur if the processes are currently running.  One should stop the
       processes by sending SIGSTOP (see signal(7)) prior to inspection with
       this system call to obtain meaningful results.

RETURN VALUE         top

       The return value of a successful call to kcmp() is simply the result
       of arithmetic comparison of kernel pointers (when the kernel compares
       resources, it uses their memory addresses).

       The easiest way to explain is to consider an example.  Suppose that
       v1 and v2 are the addresses of appropriate resources, then the return
       value is one of the following:

           0   v1 is equal to v2; in other words, the two processes share
               the resource.

           1   v1 is less than v2.

           2   v1 is greater than v2.

           3   v1 is not equal to v2, but ordering information is
               unavailable.

       On error, -1 is returned, and errno is set appropriately.

       kcmp() was designed to return values suitable for sorting.  This is
       particularly handy if one needs to compare a large number of file
       descriptors.

ERRORS         top

       EBADF  type is KCMP_FILE and fd1 or fd2 is not an open file
              descriptor.

       EINVAL type is invalid.

       EPERM  Insufficient permission to inspect process resources.  The
              CAP_SYS_PTRACE capability is required to inspect processes
              that you do not own.  Other ptrace limitations may also apply,
              such as CONFIG_SECURITY_YAMA, which, when
              /proc/sys/kernel/yama/ptrace_scope is 2, limits kcmp() to
              child processes; see ptrace(2).

       ESRCH  Process pid1 or pid2 does not exist.

VERSIONS         top

       The kcmp() system call first appeared in Linux 3.5.

CONFORMING TO         top

       kcmp() is Linux-specific and should not be used in programs intended
       to be portable.

NOTES         top

       Glibc does not provide a wrapper for this system call; call it using
       syscall(2).

       This system call is available only if the kernel was configured with
       CONFIG_CHECKPOINT_RESTORE.  The main use of the system call is for
       the checkpoint/restore in user space (CRIU) feature.  The alternative
       to this system call would have been to expose suitable process
       information via the proc(5) filesystem; this was deemed to be
       unsuitable for security reasons.

       See clone(2) for some background information on the shared resources
       referred to on this page.

EXAMPLE         top

       The program below uses kcmp() to test whether pairs of file
       descriptors refer to the same open file description.  The program
       tests different cases for the file descriptor pairs, as described in
       the program output.  An example run of the program is as follows:

           $ ./a.out
           Parent PID is 1144
           Parent opened file on FD 3

           PID of child of fork() is 1145
                Compare duplicate FDs from different processes:
                     kcmp(1145, 1144, KCMP_FILE, 3, 3) ==> same
           Child opened file on FD 4
                Compare FDs from distinct open()s in same process:
                     kcmp(1145, 1145, KCMP_FILE, 3, 4) ==> different
           Child duplicated FD 3 to create FD 5
                Compare duplicated FDs in same process:
                     kcmp(1145, 1145, KCMP_FILE, 3, 5) ==> same

   Program source

       #define _GNU_SOURCE
       #include <sys/syscall.h>
       #include <sys/wait.h>
       #include <sys/stat.h>
       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <fcntl.h>
       #include <linux/kcmp.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static int
       kcmp(pid_t pid1, pid_t pid2, int type,
            unsigned long idx1, unsigned long idx2)
       {
           return syscall(SYS_kcmp, pid1, pid2, type, idx1, idx2);
       }

       static void
       test_kcmp(char *msg, id_t pid1, pid_t pid2, int fd_a, int fd_b)
       {
           printf("\t%s\n", msg);
           printf("\t\tkcmp(%ld, %ld, KCMP_FILE, %d, %d) ==> %s\n",
                   (long) pid1, (long) pid2, fd_a, fd_b,
                   (kcmp(pid1, pid2, KCMP_FILE, fd_a, fd_b) == 0) ?
                               "same" : "different");
       }

       int
       main(int argc, char *argv[])
       {
           int fd1, fd2, fd3;
           char pathname[] = "/tmp/kcmp.test";

           fd1 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
           if (fd1 == -1)
               errExit("open");

           printf("Parent PID is %ld\n", (long) getpid());
           printf("Parent opened file on FD %d\n\n", fd1);

           switch (fork()) {
           case -1:
               errExit("fork");

           case 0:
               printf("PID of child of fork() is %ld\n", (long) getpid());

               test_kcmp("Compare duplicate FDs from different processes:",
                       getpid(), getppid(), fd1, fd1);

               fd2 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
               if (fd2 == -1)
                   errExit("open");
               printf("Child opened file on FD %d\n", fd2);

               test_kcmp("Compare FDs from distinct open()s in same process:",
                       getpid(), getpid(), fd1, fd2);

               fd3 = dup(fd1);
               if (fd3 == -1)
                   errExit("dup");
               printf("Child duplicated FD %d to create FD %d\n", fd1, fd3);

               test_kcmp("Compare duplicated FDs in same process:",
                       getpid(), getpid(), fd1, fd3);
               break;

           default:
               wait(NULL);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       clone(2), unshare(2)

COLOPHON         top

       This page is part of release 4.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2017-03-13                          KCMP(2)

Pages that refer to this page: clone(2)open(2)ptrace(2)syscalls(2)unshare(2)capabilities(7)