core(5) - Linux manual page (original) (raw)

core(5) File Formats Manual core(5)

NAME top

   core - core dump file

DESCRIPTION top

   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)](../man1/gdb.1.html)) 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)](../man7/signal.7.html).

   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)](../man2/getrlimit.2.html) 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)](../man2/getrlimit.2.html) 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)](../man7/capabilities.7.html)).
      (However, see the description of the [prctl(2)](../man2/prctl.2.html) **PR_SET_DUMPABLE**
      operation, and the description of the
      _/proc/sys/fs/suiddumpable_ file in [proc(5)](../man5/proc.5.html).)

   •  _/proc/sys/kernel/corepattern_ is empty and
      _/proc/sys/kernel/coreusespid_ contains the value 0.  (These
      files are described below.)  Note that if
      _/proc/sys/kernel/corepattern_ is empty and
      _/proc/sys/kernel/coreusespid_ 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)](../man1/ls.1.html) _-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)](../man2/madvise.2.html) **MADV_DONTDUMP** flag was employed.

   On systems that employ [systemd(1)](../man1/systemd.1.html) as the _init_ framework, core
   dumps may instead be placed in a location determined by
   [systemd(1)](../man1/systemd.1.html).  See below for further details.

Naming of core dump files By default, a core dump file is named core, but the /proc/sys/kernel/corepattern 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)](../man2/prctl.2.html)
           **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)](../man5/proc.5.html).
       %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)](../man2/uname.2.html)).
       %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/corepattern_ does not include _%p_ and
   _/proc/sys/kernel/coreusespid_ (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)](../man7/mount%5Fnamespaces.7.html)), its current working directory
   (found via [getcwd(2)](../man2/getcwd.2.html)), and its root directory (see [chroot(2)](../man2/chroot.2.html)).

   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/coreusespid_ 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/suiddumpable_ 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/corepattern 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/corepipelimit_ 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/coredumpfilter 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)](../man2/mmap.2.html)).  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 _coredumpfilter_ 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)](../man7/vdso.7.html)) pages are always dumped, regardless of the
   _coredumpfilter_ value.

   A child process created via [fork(2)](../man2/fork.2.html) inherits its parent's
   _coredumpfilter_ value; the _coredumpfilter_ value is preserved
   across an [execve(2)](../man2/execve.2.html).

   It can be useful to set _coredumpfilter_ 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 corepattern 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)](../man8/systemd-coredump.8.html), 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)](../man8/systemd-coredump.8.html) using
   [coredumpctl(1)](../man1/coredumpctl.1.html):

   $ **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)](../man1/coredumpctl.1.html) allow a specified coredump file to be
   pulled from the [systemd(1)](../man1/systemd.1.html) 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)](../man1/coredumpctl.1.html) manual page.

   To (persistently) disable the [systemd(1)](../man1/systemd.1.html) mechanism that archives
   core dumps, restoring to something more like traditional Linux
   behavior, one can set an override for the [systemd(1)](../man1/systemd.1.html) 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 _corepattern_ 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 top

   The [gdb(1)](../man1/gdb.1.html) _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)](../man2/clone.2.html)) 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/corepattern_.  (This is primarily useful when
   employing the obsolete LinuxThreads implementation, where each
   thread of a process has a different PID.)

EXAMPLES top

   The program below can be used to demonstrate the use of the pipe
   syntax in the _/proc/sys/kernel/corepattern_ file.  The following
   shell session demonstrates the use of this program (compiled to
   create an executable named _corepatternpipetest_):

       $ **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);
   }

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Linux man-pages 6.10 2024-06-15 core(5)