CORE(5) Linux Programmer's Manual CORE(5)
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 will fail 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).)
* (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.
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 ('/') 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. 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).
/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 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.
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
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\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), gdb(1), getrlimit(2), mmap(2), prctl(2), sigaction(2),
elf(5), proc(5), pthreads(7), signal(7)
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latest version of this page, can be found at
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Linux 2017-03-13 CORE(5)
Pages that refer to this page: getrlimit(2), madvise(2), prctl(2), sigaction(2), coredump.conf(5), elf(5), proc(5), signal(7), systemd-coredump(8)