CORE(5)                   (2020-11-01)                    CORE(5)

     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 termi-
          nated.  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 cre-
             ated 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 permission enabled.  (This is a security measure to
             ensure that an executable whose contents are not readable

     Page 1                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

             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 pro-
                  cess (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 charac-
                  ters), 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 (aq/aq) replaced
                  by exclamation marks (aq!aq) (since Linux 3.0).
              %g  Numeric real GID of dumped process.

     Page 2                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

              %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 charac-
          ters in the template become a literal part of the core file-
          name.  The template may include aq/aq characters, 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/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 path-
          name (starting with a leading aq/aq 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 char-
          acter 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.

     Page 3                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

          Since kernel 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 argu-
          ment 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 % spec-
             ifiers 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 crash-
             ing process.

          *  The process runs in the initial namespaces (PID, mount,
             user, and so on) and not in the namespaces of the crash-
             ing 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.

     Page 4                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

          *  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 pro-
          gram, 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 ker-
          nel 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
          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 follow-
          ing meanings:

              bit 0
                   Dump anonymous private mappings.
              bit 1
                   Dump anonymous shared mappings.
              bit 2
                   Dump file-backed private mappings.
              bit 3

     Page 5                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

                   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 pre-
          served 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 con-
          figured for systemd-coredump(8), typically as lz4(1) com-
          pressed files in the directory /var/lib/systemd/coredump/.
          One can list the core dumps that have been recorded by

     Page 6                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

          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 tradi-
          tional 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 multi-
          threaded 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

     Page 7                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

          primarily useful when employing the obsolete LinuxThreads
          implementation, where each thread of a process has a differ-
          ent 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 dq|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%sdq > \
                  /proc/sys/kernel/core_pattern
              # exit
              $ sleep 100
              ha\                     # 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

     Page 8                        Linux             (printed 5/26/22)

     CORE(5)                   (2020-11-01)                    CORE(5)

                 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 (int 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: %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)

     COLOPHON
          This page is part of release 5.10 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/.

     Page 9                        Linux             (printed 5/26/22)