# core

## 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).
               %F  PIDFD of dumped process (since Linux 6.16).
               %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);
           }
    


* * *