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       core - core dump file


       The  default  action  of  certain signals is to cause a process to terminate and produce a
       core dump file, a 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 produced:

       •  The  process  does  not  have permission to write the core file.  (By default, the core
          file is called core or, 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  not
          writable,  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 symbolic 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

       •  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)).   However,  RLIMIT_CORE  will be ignored if the system is
          configured to pipe core dumps to a program.

       •  The binary being executed by the process does not have read permission enabled.   (This
          is a security measure to ensure that an executable whose contents are not readable does
          not produce a—possibly readable—core dump containing an image of the executable.)

       •  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/kernel/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_COREDUMP 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/kernel/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 contain % 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  The  process or thread's comm value, which typically is the same as the executable
               filename (without path prefix, and truncated to a maximum of 15  characters),  but
               may  have  been  modified  to  be  something  different;  see  the  discussion  of
               /proc/pid/comm and /proc/pid/task/tid/comm in proc(5).
           %E  Pathname of executable, with slashes ('/') replaced  by  exclamation  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
           %u  Numeric real UID of dumped process.

       A single % at the end of the template is  dropped  from  the  core  filename,  as  is  the
       combination  of  a  %  followed by any character other than those listed above.  All other
       characters in the template become a literal part of the core filename.  The  template  may
       include  '/'  characters,  which  are  interpreted as delimiters for directory names.  The
       maximum size of the resulting core filename is 128 bytes (64 bytes before  Linux  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  Linux  2.4, Linux has also provided a more primitive method of controlling the name
       of the core dump file.  If the /proc/sys/kernel/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 ("suidsafe"), 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    Linux    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 interpreted as the command-line  for  a  user-space
       program (or script) that is to be executed.

       Since  Linux  5.3.0, the pipe template is split on spaces into an argument list before the
       template parameters are  expanded.   In  earlier  kernels,  the  template  parameters  are
       expanded  first  and  the resulting string is split on spaces into an argument list.  This
       means that in earlier kernels executable names added by the %e and %E template  parameters
       could  get  split  into  multiple  arguments.   So  the core dump handler needs to put the
       executable names as the last argument and ensure it joins all parts of the executable name
       using spaces.  Executable names with multiple spaces in them are not correctly represented
       in earlier kernels, meaning that the core dump handler needs to use mechanisms to find the
       executable name.

       Instead  of  being  written  to  a  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 pathname 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

       •  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 utilize 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  directory  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.

       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

       A  value  of  0  in  this  file  is special.  It indicates that unlimited processes may be
       captured in parallel, but that no waiting will take place (i.e., the collecting program is
       not guaranteed access to /proc/<crashing-PID>).  The default value for this file is 0.

   Controlling which mappings are written to the core dump
       Since  Linux  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
       CONFIG_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 virtual  DSO  (vdso(7))
       pages are always dumped, regardless of the coredump_filter value.

       A  child  process  created  via  fork(2)  inherits its parent's coredump_filter value; the
       coredump_filter value is preserved across an execve(2).

       It can be useful to set coredump_filter in the parent shell before running a program,  for

           $ echo 0x7 > /proc/self/coredump_filter
           $ ./some_program

       This  file is provided only if the kernel was built with the CONFIG_ELF_CORE configuration

   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-coredump %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   directory
       /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
       coredumpctl(1)  allow  a specified coredump file to be pulled from the systemd(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  (persistently) disable the systemd(1) mechanism that archives core dumps, restoring to
       something more like traditional Linux behavior, one can set an override for the systemd(1)
       mechanism, using something like:

           # echo "kernel.core_pattern=core.%p" > \
           # /lib/systemd/systemd-sysctl

       It  is  also possible to temporarily (i.e., until the next reboot) change the core_pattern
       setting using a command such as the following (which causes the names of core  dump  files
       to  include  the  executable  name as well as the number of the signal which triggered the
       core dump):

           # sysctl -w kernel.core_pattern="%e-%s.core"


       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
       filename,  unless  the  process ID was already included elsewhere in the filename via a %p
       specification in /proc/sys/kernel/core_pattern.  (This is primarily useful when  employing
       the  obsolete  LinuxThreads implementation, where each thread of a process has a different


       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
           # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s" > \
           # exit
           $ sleep 100
           ^\                     # type control-backslash
           Quit (core dumped)
           $ cat
           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

       main(int argc, char *argv[])
           ssize_t nread, tot;
           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]);

           /* Write output to file "" in that directory. */

           fp = fopen("", "w+");
           if (fp == NULL)

           /* Display command-line arguments given to core_pattern
              pipe program. */

           fprintf(fp, "argc=%d\n", argc);
           for (size_t j = 0; j < argc; j++)
               fprintf(fp, "argc[%zu]=<%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: %zd\n", tot);



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