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KCMP(2) Linux Programmer's Manual KCMP(2)

NAME

     kcmp  -  compare  two  processes  to  determine  if they share a kernel
     resource

SYNOPSIS

     #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; 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
            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  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 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 dis-
            positions.  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 sem-
            aphore  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 pro-
     cesses 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:

         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
     descriptors.

ERRORS

     EBADF  type is KCMP_FILE and fd1 or fd2 is not an open file descriptor.

     EINVAL type is invalid.

     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.

     EFAULT The  epoll  slot  addressed  by  idx2  is  outside of the user's
            address space.

     ENOENT The target file is not present in epoll(7) instance.

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).

     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  secu-
     rity reasons.

     See  clone(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 dif-
         ferent 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); }