1KCMP(2)                    Linux Programmer's Manual                   KCMP(2)
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NAME

6       kcmp  -  compare  two  processes  to  determine  if they share a kernel
7       resource
8

SYNOPSIS

10       #include <linux/kcmp.h>
11
12       int kcmp(pid_t pid1, pid_t pid2, int type,
13                unsigned long idx1, unsigned long idx2);
14
15       Note: There is no glibc wrapper for this system call; see NOTES.
16

DESCRIPTION

18       The kcmp() system call can be used to check whether the  two  processes
19       identified  by  pid1  and  pid2 share a kernel resource such as virtual
20       memory, file descriptors, and so on.
21
22       Permission  to  employ  kcmp()  is  governed  by  ptrace  access   mode
23       PTRACE_MODE_READ_REALCREDS  checks  against  both  pid1  and  pid2; see
24       ptrace(2).
25
26       The type argument specifies which resource is to be compared in the two
27       processes.  It has one of the following values:
28
29       KCMP_FILE
30              Check  whether a file descriptor idx1 in the process pid1 refers
31              to the same open file description (see open(2)) as file descrip‐
32              tor  idx2  in  the  process  pid2.   The  existence  of two file
33              descriptors that refer to the same  open  file  description  can
34              occur  as  a  result of dup(2) (and similar) fork(2), or passing
35              file descriptors via a domain socket (see unix(7)).
36
37       KCMP_FILES
38              Check whether the processes share the  same  set  of  open  file
39              descriptors.   The arguments idx1 and idx2 are ignored.  See the
40              discussion of the CLONE_FILES flag in clone(2).
41
42       KCMP_FS
43              Check whether the processes share the same  filesystem  informa‐
44              tion  (i.e.,  file  mode  creation  mask, working directory, and
45              filesystem root).  The arguments idx1 and idx2 are ignored.  See
46              the discussion of the CLONE_FS flag in clone(2).
47
48       KCMP_IO
49              Check  whether  the  processes share I/O context.  The arguments
50              idx1 and idx2 are ignored.  See the discussion of  the  CLONE_IO
51              flag in clone(2).
52
53       KCMP_SIGHAND
54              Check  whether the processes share the same table of signal dis‐
55              positions.  The arguments idx1 and idx2 are  ignored.   See  the
56              discussion of the CLONE_SIGHAND flag in clone(2).
57
58       KCMP_SYSVSEM
59              Check whether the processes share the same list of System V sem‐
60              aphore  undo  operations.   The  arguments  idx1  and  idx2  are
61              ignored.   See  the  discussion  of  the  CLONE_SYSVSEM  flag in
62              clone(2).
63
64       KCMP_VM
65              Check whether the processes share the same address  space.   The
66              arguments  idx1 and idx2 are ignored.  See the discussion of the
67              CLONE_VM flag in clone(2).
68
69       KCMP_EPOLL_TFD (since Linux 4.13)
70              Check whether the file descriptor idx1 of the  process  pid1  is
71              present  in  the  epoll(7)  instance  described  by  idx2 of the
72              process pid2.  The argument idx2 is a  pointer  to  a  structure
73              where  the  target  file  is  described.  This structure has the
74              form:
75
76           struct kcmp_epoll_slot {
77               __u32 efd;
78               __u32 tfd;
79               __u64 toff;
80           };
81
82       Within this structure, efd is an epoll file  descriptor  returned  from
83       epoll_create(2),  tfd is a target file descriptor number, and toff is a
84       target file offset counted from zero.  Several different targets may be
85       registered  with the same file descriptor number and setting a specific
86       offset helps to investigate each of them.
87
88       Note the kcmp() is not protected  against  false  positives  which  may
89       occur if the processes are currently running.  One should stop the pro‐
90       cesses by sending SIGSTOP (see signal(7)) prior to inspection with this
91       system call to obtain meaningful results.
92

RETURN VALUE

94       The return value of a successful call to kcmp() is simply the result of
95       arithmetic comparison of kernel  pointers  (when  the  kernel  compares
96       resources, it uses their memory addresses).
97
98       The  easiest way to explain is to consider an example.  Suppose that v1
99       and v2 are the addresses of  appropriate  resources,  then  the  return
100       value is one of the following:
101
102           0   v1  is equal to v2; in other words, the two processes share the
103               resource.
104
105           1   v1 is less than v2.
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107           2   v1 is greater than v2.
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109           3   v1 is not equal to v2, but ordering information is unavailable.
110
111       On error, -1 is returned, and errno is set appropriately.
112
113       kcmp() was designed to return values suitable  for  sorting.   This  is
114       particularly  handy  if  one  needs  to  compare a large number of file
115       descriptors.
116

ERRORS

118       EBADF  type is KCMP_FILE and fd1 or fd2 is not an open file descriptor.
119
120       EINVAL type is invalid.
121
122       EPERM  Insufficient  permission  to  inspect  process  resources.   The
123              CAP_SYS_PTRACE  capability is required to inspect processes that
124              you do not own.  Other ptrace limitations may also  apply,  such
125              as     CONFIG_SECURITY_YAMA,    which,    when    /proc/sys/ker‐
126              nel/yama/ptrace_scope is 2, limits kcmp()  to  child  processes;
127              see ptrace(2).
128
129       ESRCH  Process pid1 or pid2 does not exist.
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131       EFAULT The  epoll  slot  addressed  by  idx2  is  outside of the user's
132              address space.
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134       ENOENT The target file is not present in epoll(7) instance.
135

VERSIONS

137       The kcmp() system call first appeared in Linux 3.5.
138

CONFORMING TO

140       kcmp() is Linux-specific and should not be used in programs intended to
141       be portable.
142

NOTES

144       Glibc  does  not  provide a wrapper for this system call; call it using
145       syscall(2).
146
147       This system call is available only if the kernel  was  configured  with
148       CONFIG_CHECKPOINT_RESTORE.   The main use of the system call is for the
149       checkpoint/restore in user space (CRIU) feature.   The  alternative  to
150       this system call would have been to expose suitable process information
151       via the proc(5) filesystem; this was deemed to be unsuitable for  secu‐
152       rity reasons.
153
154       See  clone(2)  for  some background information on the shared resources
155       referred to on this page.
156

EXAMPLE

158       The program below uses kcmp() to test whether pairs of file descriptors
159       refer  to  the same open file description.  The program tests different
160       cases for the file descriptor pairs, as described in the  program  out‐
161       put.  An example run of the program is as follows:
162
163           $ ./a.out
164           Parent PID is 1144
165           Parent opened file on FD 3
166
167           PID of child of fork() is 1145
168                Compare duplicate FDs from different processes:
169                     kcmp(1145, 1144, KCMP_FILE, 3, 3) ==> same
170           Child opened file on FD 4
171                Compare FDs from distinct open()s in same process:
172                     kcmp(1145, 1145, KCMP_FILE, 3, 4) ==> different
173           Child duplicated FD 3 to create FD 5
174                Compare duplicated FDs in same process:
175                     kcmp(1145, 1145, KCMP_FILE, 3, 5) ==> same
176
177   Program source
178
179       #define _GNU_SOURCE
180       #include <sys/syscall.h>
181       #include <sys/wait.h>
182       #include <sys/stat.h>
183       #include <stdlib.h>
184       #include <stdio.h>
185       #include <unistd.h>
186       #include <fcntl.h>
187       #include <linux/kcmp.h>
188
189       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
190                               } while (0)
191
192       static int
193       kcmp(pid_t pid1, pid_t pid2, int type,
194            unsigned long idx1, unsigned long idx2)
195       {
196           return syscall(SYS_kcmp, pid1, pid2, type, idx1, idx2);
197       }
198
199       static void
200       test_kcmp(char *msg, id_t pid1, pid_t pid2, int fd_a, int fd_b)
201       {
202           printf("\t%s\n", msg);
203           printf("\t\tkcmp(%ld, %ld, KCMP_FILE, %d, %d) ==> %s\n",
204                   (long) pid1, (long) pid2, fd_a, fd_b,
205                   (kcmp(pid1, pid2, KCMP_FILE, fd_a, fd_b) == 0) ?
206                               "same" : "different");
207       }
208
209       int
210       main(int argc, char *argv[])
211       {
212           int fd1, fd2, fd3;
213           char pathname[] = "/tmp/kcmp.test";
214
215           fd1 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
216           if (fd1 == -1)
217               errExit("open");
218
219           printf("Parent PID is %ld\n", (long) getpid());
220           printf("Parent opened file on FD %d\n\n", fd1);
221
222           switch (fork()) {
223           case -1:
224               errExit("fork");
225
226           case 0:
227               printf("PID of child of fork() is %ld\n", (long) getpid());
228
229               test_kcmp("Compare duplicate FDs from different processes:",
230                       getpid(), getppid(), fd1, fd1);
231
232               fd2 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
233               if (fd2 == -1)
234                   errExit("open");
235               printf("Child opened file on FD %d\n", fd2);
236
237               test_kcmp("Compare FDs from distinct open()s in same process:",
238                       getpid(), getpid(), fd1, fd2);
239
240               fd3 = dup(fd1);
241               if (fd3 == -1)
242                   errExit("dup");
243               printf("Child duplicated FD %d to create FD %d\n", fd1, fd3);
244
245               test_kcmp("Compare duplicated FDs in same process:",
246                       getpid(), getpid(), fd1, fd3);
247               break;
248
249           default:
250               wait(NULL);
251           }
252
253           exit(EXIT_SUCCESS);
254       }
255

SEE ALSO

257       clone(2), unshare(2)
258

COLOPHON

260       This  page  is  part of release 4.15 of the Linux man-pages project.  A
261       description of the project, information about reporting bugs,  and  the
262       latest     version     of     this    page,    can    be    found    at
263       https://www.kernel.org/doc/man-pages/.
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267Linux                             2017-09-15                           KCMP(2)
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