kcmp(2) - Linux manual page (original) (raw)

kcmp(2) System Calls Manual kcmp(2)

NAME top

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

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <linux/kcmp.h>** /* Definition of **KCMP_*** constants */
   **#include <sys/syscall.h>** /* Definition of **SYS_*** constants */
   **#include <unistd.h>**

   **int syscall(SYS_kcmp, pid_t** _pid1_**, pid_t** _pid2_**, int** _type_**,**
               **unsigned long** _idx1_**, unsigned long** _idx2_**);**

   _Note_: glibc provides no wrapper for **kcmp**(), necessitating the use
   of [syscall(2)](../man2/syscall.2.html).

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)](../man2/ptrace.2.html).

   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)](../man2/open.2.html)) 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)](../man2/dup.2.html) (and similar)
          [fork(2)](../man2/fork.2.html), or passing file descriptors via a domain socket
          (see [unix(7)](../man7/unix.7.html)).

   **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)](../man2/clone.2.html).

   **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)](../man2/clone.2.html).

   **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)](../man2/clone.2.html).

   **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)](../man2/clone.2.html).

   **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)](../man2/clone.2.html).

   **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)](../man2/clone.2.html).

   **KCMP_EPOLL_TFD** (since Linux 4.13)
          Check whether the file descriptor _idx1_ of the process _pid1_
          is present in the [epoll(7)](../man7/epoll.7.html) 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)](../man2/epoll%5Fcreate.2.html), _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
   occur if the processes are currently running.  One should stop the
   processes by sending **SIGSTOP** (see [signal(7)](../man7/signal.7.html)) 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](../man3/errno.3.html)_ is set to indicate the error.

   **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.

   **EFAULT** The epoll slot addressed by _idx2_ is outside of the user's
          address space.

   **EINVAL** _type_ is invalid.

   **ENOENT** The target file is not present in [epoll(7)](../man7/epoll.7.html) instance.

   **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/ptracescope_ is 2, limits **kcmp**() to
          child processes; see [ptrace(2)](../man2/ptrace.2.html).

   **ESRCH** Process _pid1_ or _pid2_ does not exist.

STANDARDS top

   Linux.

HISTORY top

   Linux 3.5.

   Before Linux 5.12, this system call is available only if the
   kernel is configured with **CONFIG_CHECKPOINT_RESTORE**, since the
   original purpose of the system call was 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)](../man5/proc.5.html) filesystem; this was deemed to be unsuitable for
   security reasons.)  Since Linux 5.12, this system call is also
   available if the kernel is configured with **CONFIG_KCMP**.

NOTES top

   See [clone(2)](../man2/clone.2.html) for some background information on the shared
   resources referred to on this page.

EXAMPLES 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 <err.h>
   #include <fcntl.h>
   #include <linux/kcmp.h>
   #include <stdint.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <sys/syscall.h>
   #include <sys/types.h>
   #include <sys/wait.h>
   #include <unistd.h>

   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, pid_t pid1, pid_t pid2, int fd_a, int fd_b)
   {
       printf("\t%s\n", msg);
       printf("\t\tkcmp(%jd, %jd, KCMP_FILE, %d, %d) ==> %s\n",
              (intmax_t) pid1, (intmax_t) pid2, fd_a, fd_b,
              (kcmp(pid1, pid2, KCMP_FILE, fd_a, fd_b) == 0) ?
                           "same" : "different");
   }

   int
   main(void)
   {
       int                fd1, fd2, fd3;
       static const char  pathname[] = "/tmp/kcmp.test";

       fd1 = open(pathname, O_CREAT | O_RDWR, 0600);
       if (fd1 == -1)
           err(EXIT_FAILURE, "open");

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

       switch (fork()) {
       case -1:
           err(EXIT_FAILURE, "fork");

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

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

           fd2 = open(pathname, O_CREAT | O_RDWR, 0600);
           if (fd2 == -1)
               err(EXIT_FAILURE, "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)
               err(EXIT_FAILURE, "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)](../man2/clone.2.html), [unshare(2)](../man2/unshare.2.html)

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Linux man-pages 6.10 2024-07-23 kcmp(2)

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