kcmp(2) System Calls Manual kcmp(2)
NAME
kcmp - compare two processes to determine if they share a kernel re-
source
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
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 mem-
ory, 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 descrip-
tors that refer to the same open file description can occur as a
result of dup(2) (and similar) fork(2), or passing file descrip-
tors 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 information
(i.e., file mode creation mask, working directory, and filesystem
root). The arguments idx1 and idx2 are ignored. See the discus-
sion 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 dispo-
sitions. The arguments idx1 and idx2 are ignored. See the dis-
cussion of the CLONE_SIGHAND flag in clone(2).
KCMP_SYSVSEM
Check whether the processes share the same list of System V sema-
phore 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
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 un-
available.
On error, -1 is returned, and errno is set to indicate the error.
kcmp() was designed to return values suitable for sorting. This is par-
ticularly handy if one needs to compare a large number of file descrip-
tors.
ERRORS
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/ker-
nel/yama/ptrace_scope is 2, limits kcmp() to child processes; see
ptrace(2).
ESRCH Process pid1 or pid2 does not exist.
STANDARDS
Linux.
HISTORY
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) fea-
ture. (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 also available if the kernel is configured with CONFIG_KCMP.
NOTES
See clone(2) for some background information on the shared resources re-
ferred 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:
$ ./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
clone(2), unshare(2)
Linux man-pages 6.9.1 2024-06-15 kcmp(2)
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