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KCMP(2)                    Linux Programmer's ManualKCMP(2)NAME       kcmp  -  compare  two processes to determine if they share a kernel re-       sourceSYNOPSIS       #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.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; seeptrace(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 (seeopen(2)) as file descrip-              tor  idx2  in  the  process pid2.  The existence of two file de-              scriptors that refer to the same open file description can occur              as a result ofdup(2) (and similar)fork(2), or passing file de-              scriptors via a domain socket (seeunix(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 inclone(2).       KCMP_FS              Check whether the processes share the same  filesystem  informa-              tion  (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 inclone(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 inclone(2).       KCMP_SIGHAND              Check  whether the processes share the same table of signal dis-              positions.  The arguments idx1 and idx2 are  ignored.   See  the              discussion of the CLONE_SIGHAND flag inclone(2).       KCMP_SYSVSEM              Check whether the processes share the same list of System V sem-              aphore undo operations.  The arguments idx1  and  idx2  are  ig-              nored.    See  the  discussion  of  the  CLONE_SYSVSEM  flag  inclone(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 inclone(2).       KCMP_EPOLL_TFD (since Linux 4.13)              Check whether the file descriptor idx1 of the  process  pid1  is              present  in  theepoll(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  fromepoll_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  oc-       cur  if  the processes are currently running.  One should stop the pro-       cesses by sending SIGSTOP (seesignal(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 unavailable.       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 de-       scriptors.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  ad-              dress space.       EINVAL type is invalid.       ENOENT The target file is not present inepoll(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;              seeptrace(2).       ESRCH  Process pid1 or pid2 does not exist.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 usingsyscall(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 theproc(5) filesystem; this was deemed to be unsuitable for  secu-       rity reasons.       Seeclone(2)  for  some background information on the shared resources       referred to on this page.EXAMPLE       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  out-       put.  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 <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, id_t pid1, pid_t pid2, int fd_a, int fd_b)       {           printf("\t%s\n", msg);           printf("\t\tkcmp(%ld, %ld, KCMP_FILE, %d, %d) ==> %s\n",                   (long) pid1, (long) 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 %ld\n", (long) 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 %ld\n", (long) 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 ALSOclone(2),unshare(2)COLOPHON       This  page  is  part of release 5.05 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                             2019-10-10KCMP(2)