| KCMP(2) | Linux Programmer's Manual | KCMP(2) |
kcmp - compare two processes to determine if they share a kernel resource
#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.
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:
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.
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 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.
The kcmp() system call first appeared in Linux 3.5.
kcmp() is Linux-specific and should not be used in programs intended to be portable.
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.
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
#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);
}
This page is part of release 5.10 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/.
| 2020-11-01 | Linux |