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:

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

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:

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

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).

Before Linux 5.12, this system call is available only if the kernel is configured with CONFIG_CHECKPOINT_RESTORE, since the original pupose 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

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:

#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

clone(2), unshare(2)