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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 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 sig‐
       nals 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)).

       *  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 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 ('/') 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 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_name‐
       spaces(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 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 interpreted as the command-line for a user-space program (or script) that is to be executed.

       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 pa‐
       rameters 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 en‐
       sure 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 collect‐
       ing 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 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 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/sys‐
       temd/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 cre‐
       ated  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  implemen‐
       tation, 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 (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)

Linux                                                                          2021-03-22                                                                        CORE(5)