core - core dump file
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'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 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) 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.
- *
- 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.
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 ('/') 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.
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
'|' 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.
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.
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.
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
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.)
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
/* 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\n", argc);
for (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: %d\n", tot);
fclose(fp);
exit(EXIT_SUCCESS);
}
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)