core

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 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)).

       *  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 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 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 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 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/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 (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 5.13 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/.



Linux                              2021-03-22                            CORE(5)