core(5) File Formats 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 re-
ceives a "core dump" signal; see getrlimit(2) for details.
There are various circumstances in which a core dump file is not pro-
duced:
• The process does not have permission to write the core file. (By de-
fault, 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 work-
ing 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 doc-
umentation 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 permis-
sion enabled. (This is a security measure to ensure that an exe-
cutable 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 capabil-
ities (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/ker-
nel/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_CORE-
DUMP 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/ker-
nel/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 con-
tain % 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 exclama-
tion 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 name-
space 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 com-
patibility, 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 con-
trolling the name of the core dump file. If the /proc/sys/ker-
nel/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 exe-
cuted.
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 ker-
nels 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 multi-
ple 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 path-
name 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 di-
rectory) 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 uti-
lize 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 di-
rectory. 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 prema-
turely. This in turn creates the possibility that a misbehaving col-
lecting 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 cor-
responding 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 CON-
FIG_CORE_DUMP_DEFAULT_ELF_HEADERS kernel configuration option is en-
abled), 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 vir-
tual DSO (vdso(7)) pages are always dumped, regardless of the core-
dump_filter value.
A child process created via fork(2) inherits its parent's coredump_fil-
ter value; the coredump_filter value is preserved across an execve(2).
It can be useful to set coredump_filter in the parent shell before run-
ning a program, for example:
$ echo 0x7 > /proc/self/coredump_filter
$ ./some_program
This file is provided only if the kernel was built with the CON-
FIG_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 direc-
tory /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 num-
ber 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) loca-
tion 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 spec-
ification 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);
}
SEE ALSO
bash(1), coredumpctl(1), gdb(1), getrlimit(2), mmap(2), prctl(2), sigac-
tion(2), elf(5), proc(5), pthreads(7), signal(7), systemd-coredump(8)
Linux man-pages 6.9.1 2024-06-15 core(5)
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