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man:core

CORE(5) Linux Programmer's Manual CORE(5)

NAME

     core - core dump file

DESCRIPTION

     The  default  action of certain signals is to cause a process to termi-
     nate and produce a core dump file, a disk file containing an  image  of
     the  process's  memory  at  the time of termination.  This image can be
     used in a debugger (e.g., gdb(1)) to inspect the state of  the  program
     at  the  time  that it terminated.  A list of the signals which cause a
     process to dump core can be found in signal(7).
     A process can set its soft RLIMIT_CORE resource limit to place an upper
     limit  on  the  size  of the core dump file that will be produced if it
     receives a "core dump" signal; see getrlimit(2) for details.
     There are various circumstances in which a core dump file is  not  pro-
     duced:
  • The process does not have permission to write the core file. (By

default, the core file is called core or core.pid, where pid is the

        ID  of  the  process that dumped core, and is created in the current
        working directory.  See below for details on naming.)   Writing  the
        core  file  fails  if  the directory in which it is to be created is
        nonwritable, or if a file with the  same  name  exists  and  is  not
        writable or is not a regular file (e.g., it is a directory or a sym-
        bolic link).
  • A (writable, regular) file with the same name as would be used for

the core dump already exists, but there is more than one hard link

        to that file.
  • The filesystem where the core dump file would be created is full; or

has run out of inodes; or is mounted read-only; or the user has

        reached their quota for the filesystem.
  • The directory in which the core dump file is to be created does not

exist.

  • The RLIMIT_CORE (core file size) or RLIMIT_FSIZE (file size)

resource limits for the process are set to zero; see getrlimit(2)

        and  the  documentation  of  the  shell's  ulimit  command (limit in
        csh(1)).
  • The binary being executed by the process does not have read permis-

sion enabled.

  • The process is executing a set-user-ID (set-group-ID) program that

is owned by a user (group) other than the real user (group) ID of

        the  process,  or  the  process is executing a program that has file
        capabilities (see capabilities(7)).  (However, see  the  description
        of  the  prctl(2)  PR_SET_DUMPABLE operation, and the description of
        the /proc/sys/fs/suid_dumpable file in proc(5).)
  • /proc/sys/kernel/core_pattern is empty and /proc/sys/ker-

nel/core_uses_pid contains the value 0. (These files are described

        below.)  Note that if  /proc/sys/kernel/core_pattern  is  empty  and
        /proc/sys/kernel/core_uses_pid contains the value 1, core dump files
        will have names of the form .pid, and such files are  hidden  unless
        one uses the ls(1) -a option.
  • (Since Linux 3.7) The kernel was configured without the CONFIG_CORE-

DUMP option.

     In addition, a core dump may exclude part of the address space  of  the
     process if the madvise(2) MADV_DONTDUMP flag was employed.
     On systems that employ systemd(1) as the init framework, core dumps may
     instead be placed in a location determined by  systemd(1).   See  below
     for further details.
 Naming of core dump files
     By  default,  a  core  dump  file is named core, but the /proc/sys/ker-
     nel/core_pattern file (since Linux 2.6 and 2.4.21) can be set to define
     a template that is used to name core dump files.  The template can con-
     tain % specifiers which are substituted by the following values when  a
     core file is created:
         %%  a single % character
         %c  core  file  size soft resource limit of crashing process (since
             Linux 2.6.24)
         %d  dump mode--same as value returned by  prctl(2)  PR_GET_DUMPABLE
             (since Linux 3.7)
         %e  executable filename (without path prefix)
         %E  pathname of executable, with slashes ('/') replaced by exclama-
             tion marks ('!') (since Linux 3.0).
         %g  (numeric) real GID of dumped process
         %h  hostname (same as nodename returned by uname(2))
         %i  TID of thread that triggered core dump,  as  seen  in  the  PID
             namespace in which the thread resides (since Linux 3.18)
         %I  TID  of thread that triggered core dump, as seen in the initial
             PID namespace (since Linux 3.18)
         %p  PID of dumped process, as seen in the PID  namespace  in  which
             the process resides
         %P  PID  of  dumped  process,  as seen in the initial PID namespace
             (since Linux 3.12)
         %s  number of signal causing dump
         %t  time of dump, expressed as seconds since the Epoch,  1970-01-01
             00:00:00 +0000 (UTC)
         %u  (numeric) real UID of dumped process
     A  single  %  at the end of the template is dropped from the core file-
     name, as is the combination of a % followed by any character other than
     those listed above.  All other characters in the template become a lit-
     eral part of the core filename.  The template may include  '/'  charac-
     ters,  which  are  interpreted  as delimiters for directory names.  The
     maximum size of the resulting core filename is 128 bytes (64  bytes  in
     kernels before 2.6.19).  The default value in this file is "core".  For
     backward  compatibility,  if  /proc/sys/kernel/core_pattern  does   not
     include  %p  and /proc/sys/kernel/core_uses_pid (see below) is nonzero,
     then .PID will be appended to the core filename.
     Paths are interpreted according to the settings that are active for the
     crashing  process.   That  means the crashing process's mount namespace
     (see mount_namespaces(7)), its current  working  directory  (found  via
     getcwd(2)), and its root directory (see chroot(2)).
     Since  version  2.4, Linux has also provided a more primitive method of
     controlling the name of the core  dump  file.   If  the  /proc/sys/ker-
     nel/core_uses_pid  file  contains the value 0, then a core dump file is
     simply named core.  If this file contains a  nonzero  value,  then  the
     core  dump file includes the process ID in a name of the form core.PID.
     Since Linux 3.6, if /proc/sys/fs/suid_dumpable  is  set  to  2  ("suid-
     safe"),  the pattern must be either an absolute pathname (starting with
     a leading '/' character) or a pipe, as defined below.
 Piping core dumps to a program
     Since kernel  2.6.19,  Linux  supports  an  alternate  syntax  for  the
     /proc/sys/kernel/core_pattern  file.   If  the  first character of this
     file is a pipe symbol (|), then the remainder of  the  line  is  inter-
     preted as the command-line for a user-space program (or script) that is
     to be executed.  Instead of being written to a disk file, the core dump
     is given as standard input to the program.  Note the following points:
  • The program must be specified using an absolute pathname (or a path-

name relative to the root directory, /), and must immediately follow

        the '|' character.
  • The command-line arguments can include any of the % specifiers

listed above. For example, to pass the PID of the process that is

        being dumped, specify %p in an argument.
  • The process created to run the program runs as user and group root.
  • Running as root does not confer any exceptional security bypasses.

Namely, LSMs (e.g., SELinux) are still active and may prevent the

        handler  from  accessing  details  about  the  crashed  process  via
        /proc/[pid].
  • The program pathname is interpreted with respect to the initial

mount namespace as it is always executed there. It is not affected

        by  the  settings  (e.g.,  root  directory, mount namespace, current
        working directory) of the crashing process.
  • The process runs in the initial namespaces (PID, mount, user, and so

on) and not in the namespaces of the crashing process. One can uti-

        lize specifiers such as %P to find the right  /proc/[pid]  directory
        and probe/enter the crashing process's namespaces if needed.
  • The process starts with its current working directory as the root

directory. If desired, it is possible change to the working direc-

        tory  of  the dumping process by employing the value provided by the
        %P specifier to change to the location of the  dumping  process  via
        /proc/[pid]/cwd.
  • Command-line arguments can be supplied to the program (since Linux

2.6.24), delimited by white space (up to a total line length of 128

        bytes).
  • The RLIMIT_CORE limit is not enforced for core dumps that are piped

to a program via this mechanism.

 /proc/sys/kernel/core_pipe_limit
     When collecting core dumps via a pipe to a user-space program,  it  can
     be  useful for the collecting program to gather data about the crashing
     process from that process's /proc/[pid] directory.  In order to do this
     safely,  the  kernel must wait for the program collecting the core dump
     to exit, so as not to remove the crashing process's  /proc/[pid]  files
     prematurely.   This  in turn creates the possibility that a misbehaving
     collecting program can block the reaping of a crashed process by simply
     never exiting.
     Since Linux 2.6.32, the /proc/sys/kernel/core_pipe_limit can be used to
     defend against this possibility.  The value in this  file  defines  how
     many  concurrent crashing processes may be piped to user-space programs
     in parallel.  If this value is exceeded, then those crashing  processes
     above  this  value are noted in the kernel log and their core dumps are
     skipped.
     A value of 0 in this file is special.  It indicates that unlimited pro-
     cesses may be captured in parallel, but that no waiting will take place
     (i.e., the collecting program is not guaranteed access to /proc/<crash-
     ing-PID>).  The default value for this file is 0.
 Controlling which mappings are written to the core dump
     Since  kernel  2.6.23,  the  Linux-specific /proc/[pid]/coredump_filter
     file can be used to control which memory segments are  written  to  the
     core  dump  file  in  the  event  that a core dump is performed for the
     process with the corresponding process ID.
     The value in the file is a  bit  mask  of  memory  mapping  types  (see
     mmap(2)).   If  a  bit  is set in the mask, then memory mappings of the
     corresponding type are dumped; otherwise they are not dumped.  The bits
     in this file have the following meanings:
         bit 0  Dump anonymous private mappings.
         bit 1  Dump anonymous shared mappings.
         bit 2  Dump file-backed private mappings.
         bit 3  Dump file-backed shared mappings.
         bit 4 (since Linux 2.6.24)
                Dump ELF headers.
         bit 5 (since Linux 2.6.28)
                Dump private huge pages.
         bit 6 (since Linux 2.6.28)
                Dump shared huge pages.
         bit 7 (since Linux 4.4)
                Dump private DAX pages.
         bit 8 (since Linux 4.4)
                Dump shared DAX pages.
     By  default,  the  following  bits  are  set:  0,  1,  4  (if  the CON-
     FIG_CORE_DUMP_DEFAULT_ELF_HEADERS  kernel   configuration   option   is
     enabled),  and  5.  This default can be modified at boot time using the
     coredump_filter boot option.
     The value of this file is displayed in hexadecimal.  (The default value
     is thus displayed as 33.)
     Memory-mapped I/O pages such as frame buffer are never dumped, and vir-
     tual DSO pages are always dumped,  regardless  of  the  coredump_filter
     value.
     A child process created via fork(2) inherits its parent's coredump_fil-
     ter value; the coredump_filter value is preserved across an  execve(2).
     It can be useful to set coredump_filter in the parent shell before run-
     ning a program, for example:
         $ echo 0x7 > /proc/self/coredump_filter $ ./some_program
     This file is provided only if  the  kernel  was  built  with  the  CON-
     FIG_ELF_CORE configuration option.
 Core dumps and systemd
     On  systems  using  the  systemd(1)  init  framework, core dumps may be
     placed in a location determined by systemd(1).  To do this,  systemd(1)
     employs  the  core_pattern  feature  that allows piping core dumps to a
     program.  One can verify this by checking whether core dumps are  being
     piped to the systemd-coredump(8) program:
         $ cat /proc/sys/kernel/core_pattern |/usr/lib/systemd/systemd-core-
         dump %P %u %g %s %t %c %e
     In this case, core dumps will be placed in the location configured  for
     systemd-coredump(8), typically as lz4(1) compressed files in the direc-
     tory /var/lib/systemd/coredump/.  One can list the core dumps that have
     been recorded by systemd-coredump(8) using coredumpctl(1):
       $  coredumpctl list | tail -5 Wed 2017-10-11 22:25:30 CEST  2748 1000
       1000 3 present  /usr/bin/sleep Thu  2017-10-12  06:29:10  CEST   2716
       1000  1000  3  present   /usr/bin/sleep  Thu 2017-10-12 06:30:50 CEST
       2767 1000 1000 3 present  /usr/bin/sleep Thu 2017-10-12 06:37:40 CEST
       2918  1000  1000 3 present  /usr/bin/cat Thu 2017-10-12 08:13:07 CEST
       2955 1000 1000 3 present  /usr/bin/cat
     The information shown for each core dump includes the date and time  of
     the  dump,  the  PID,  UID, and GID  of the dumping process, the signal
     number that caused the core dump, and the pathname  of  the  executable
     that  was  being  run  by the dumped process.  Various options to core-
     dumpctl(1) allow a specified coredump file to be pulled from  the  sys-
     temd(1)  location  into  a specified file.  For example, to extract the
     core dump for PID 2955 shown above to a file named core in the  current
     directory, one could use:
         $ coredumpctl dump 2955 -o core
     For more extensive details, see the coredumpctl(1) manual page.
     To disable the systemd(1) mechanism that archives core dumps, restoring
     to something more like traditional Linux behavior, one can set an over-
     ride for the systemd(1) mechanism, using something like:
       # echo "kernel.core_pattern=core.%p" > /etc/sysctl.d/50-coredump.conf
       # /lib/systemd/systemd-sysctl

NOTES

     The gdb(1) gcore command can be used to obtain a core dump of a running
     process.
     In  Linux  versions  up  to  and  including  2.6.27, if a multithreaded
     process (or, more precisely, a process  that  shares  its  memory  with
     another  process  by  being created with the CLONE_VM flag of clone(2))
     dumps core, then the process ID is always appended to  the  core  file-
     name, unless the process ID was already included elsewhere in the file-
     name via a %p specification in /proc/sys/kernel/core_pattern.  (This is
     primarily  useful  when employing the obsolete LinuxThreads implementa-
     tion, where each thread of a process has a different PID.)

EXAMPLE

     The program below can be used to demonstrate the use of the pipe syntax
     in the /proc/sys/kernel/core_pattern file.  The following shell session
     demonstrates the use of this program (compiled to create an  executable
     named core_pattern_pipe_test):
         $  cc -o core_pattern_pipe_test core_pattern_pipe_test.c $ su Pass-
         word: # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s"
         >  \       /proc/sys/kernel/core_pattern  #  exit  $  sleep  100 ^\
         # type control-backslash Quit (core dumped) $ cat core.info  argc=5
         argc[0]=</home/mtk/core_pattern_pipe_test>          argc[1]=<20575>
         argc[2]=<UID=1000> argc[3]=<GID=100> argc[4]=<sig=3> Total bytes in
         core dump: 282624
 Program source
      /* core_pattern_pipe_test.c */
     #define  _GNU_SOURCE  #include <sys/stat.h> #include <fcntl.h> #include
     <limits.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h>
     #define BUF_SIZE 1024
     int main(int argc, char *argv[]) {
         int tot, j;
         ssize_t nread;
         char buf[BUF_SIZE];
         FILE *fp;
         char cwd[PATH_MAX];
         /* Change our current working directory to that of the
            crashing process */
         snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
         chdir(cwd);
         /* Write output to file "core.info" in that directory */
         fp = fopen("core.info", "w+");
         if (fp == NULL)
             exit(EXIT_FAILURE);
         /* Display command-line arguments given to core_pattern
            pipe program */
         fprintf(fp, "argc=%d\n", argc);
         for (j = 0; j < argc; j++)
             fprintf(fp, "argc[%d]=<%s>\n", j, argv[j]);
         /* Count bytes in standard input (the core dump) */
         tot = 0;
         while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
             tot += nread;
         fprintf(fp, "Total bytes in core dump: %d\n", tot);
         fclose(fp);
         exit(EXIT_SUCCESS); }

SEE ALSO

     bash(1),  coredumpctl(1),  gdb(1),  getrlimit(2),  mmap(2),   prctl(2),
     sigaction(2),  elf(5),  proc(5),  pthreads(7), signal(7), systemd-core-
     dump(8)

COLOPHON

     This page is part of release 4.16 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 2017-09-15 CORE(5)

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