kcmp(2)



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

NAME
       kcmp  -  compare  two processes to determine if they share a kernel re-
       source

SYNOPSIS
       #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
       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 descrip-
              tor  idx2  in  the  process pid2.  The existence of two file de-
              scriptors that refer to the same open file description can occur
              as a result of dup(2) (and similar) fork(2), or passing file de-
              scriptors via a domain socket (see unix(7)).

       KCMP_FILES
              Check whether the processes share the same set of open file  de-
              scriptors.   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  informa-
              tion  (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 dis-
              positions.  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 sem-
              aphore undo operations.  The arguments idx1  and  idx2  are  ig-
              nored.    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).

       KCMP_EPOLL_TFD (since Linux 4.13)
              Check whether the file descriptor idx1 of the  process  pid1  is
              present  in  the  epoll(7)  instance  described  by  idx2 of the
              process pid2.  The argument idx2 is a  pointer  to  a  structure
              where  the  target  file  is  described.  This structure has the
              form:

           struct kcmp_epoll_slot {
               __u32 efd;
               __u32 tfd;
               __u64 toff;
           };

       Within this structure, efd is an epoll file  descriptor  returned  from
       epoll_create(2),  tfd is a target file descriptor number, and toff is a
       target file offset counted from zero.  Several different targets may be
       registered  with the same file descriptor number and setting a specific
       offset helps to investigate each of them.

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

RETURN VALUE
       The return value of a successful call to kcmp() is simply the result of
       arithmetic comparison of kernel pointers (when the kernel compares  re-
       sources, 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 de-
       scriptors.

ERRORS
       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/ker-
              nel/yama/ptrace_scope is 2, limits kcmp()  to  child  processes;
              see ptrace(2).

       ESRCH  Process pid1 or pid2 does not exist.

       EFAULT The  epoll  slot  addressed by idx2 is outside of the user's ad-
              dress space.

       ENOENT The target file is not present in epoll(7) instance.

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

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

NOTES
       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  secu-
       rity reasons.

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

EXAMPLE
       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  out-
       put.  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
       clone(2), unshare(2)

COLOPHON
       This  page  is  part of release 4.16 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                             2017-09-15                           KCMP(2)

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