kcmp(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | 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>       /* 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).

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).

       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 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 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) 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/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

       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) filesystem; this was
       deemed to be unsuitable for security reasons.)  Since Linux 5.12,
       this system call is made available unconditionally.

       See clone(2) 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 <sys/syscall.h>
       #include <sys/wait.h>
       #include <sys/stat.h>
       #include <stdint.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, 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(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 %jd\n", (intmax_t) 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 %jd\n", (intmax_t) 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 5.13 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                          2021-03-22                        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)