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NAME

       core - core dump file

DESCRIPTION

       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 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 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 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)).
          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 (UTC).
           %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 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 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.

   /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 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 example:

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

       This file is provided only if the kernel was built with the CONFIG_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-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" > \
                          /etc/sysctl.d/50-coredump.conf
           # /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"

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 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 PID.)

EXAMPLES

       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
           Password:
           # 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[])
       {
           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]);
           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 (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);

           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-coredump(8)