Section (5) 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 disk file containing an image of the process_zsingle_quotesz_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
orcore.pid
, wherepid
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 nonwritable, 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) orRLIMIT_FSIZE
(file size) resource limits for the process are set to zero; see getrlimit(2) and the documentation of the shell_zsingle_quotesz_sulimit
command (limit
in csh(1)). -
The binary being executed by the process does not have read permission enabled.
-
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
executable filename (without path prefix)
- %E
pathname of executable, with slashes (_zsingle_quotesz_/_zsingle_quotesz_) replaced by exclamation marks (_zsingle_quotesz_!_zsingle_quotesz_) (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 _zsingle_quotesz_/_zsingle_quotesz_ 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_zsingle_quotesz_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 _zsingle_quotesz_/_zsingle_quotesz_ 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 disk 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 _zsingle_quotesz_|_zsingle_quotesz_ 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_zsingle_quotesz_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_zsingle_quotesz_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_zsingle_quotesz_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_zsingle_quotesz_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 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
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.)
EXAMPLE
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[]) { int tot, j; ssize_t nread; 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 , argc); for (j = 0; j < argc; j++) fprintf(fp, argc[%d]=<%s> , 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: %d , 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 5.04 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/.
Copyright (c) 2006, 2008 by Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |