1CORE(5) Linux Programmer's Manual CORE(5)
2
6 core - core dump file
7
9 The default action of certain signals is to cause a process to termi‐
10 nate and produce a core dump file, a disk file containing an image of
11 the process's memory at the time of termination. This image can be
12 used in a debugger (e.g., gdb(1)) to inspect the state of the program
13 at the time that it terminated. A list of the signals which cause a
14 process to dump core can be found in signal(7).
15 A process can set its soft RLIMIT_CORE resource limit to place an upper
17 limit on the size of the core dump file that will be produced if it
18 receives a "core dump" signal; see getrlimit(2) for details.
19 There are various circumstances in which a core dump file is not pro‐
21 duced:
22 * The process does not have permission to write the core file. (By
24 default, the core file is called core or core.pid, where pid is the
25 ID of the process that dumped core, and is created in the current
26 working directory. See below for details on naming.) Writing the
27 core file fails if the directory in which it is to be created is
28 nonwritable, or if a file with the same name exists and is not
29 writable or is not a regular file (e.g., it is a directory or a sym‐
30 bolic link).
31 * A (writable, regular) file with the same name as would be used for
33 the core dump already exists, but there is more than one hard link
34 to that file.
35 * The filesystem where the core dump file would be created is full; or
37 has run out of inodes; or is mounted read-only; or the user has
38 reached their quota for the filesystem.
39 * The directory in which the core dump file is to be created does not
41 exist.
42 * The RLIMIT_CORE (core file size) or RLIMIT_FSIZE (file size)
44 resource limits for the process are set to zero; see getrlimit(2)
45 and the documentation of the shell's ulimit command (limit in
46 csh(1)).
47 * The binary being executed by the process does not have read permis‐
49 sion enabled.
50 * The process is executing a set-user-ID (set-group-ID) program that
52 is owned by a user (group) other than the real user (group) ID of
53 the process, or the process is executing a program that has file
54 capabilities (see capabilities(7)). (However, see the description
55 of the prctl(2) PR_SET_DUMPABLE operation, and the description of
56 the /proc/sys/fs/suid_dumpable file in proc(5).)
57 * /proc/sys/kernel/core_pattern is empty and /proc/sys/ker‐
59 nel/core_uses_pid contains the value 0. (These files are described
60 below.) Note that if /proc/sys/kernel/core_pattern is empty and
61 /proc/sys/kernel/core_uses_pid contains the value 1, core dump files
62 will have names of the form .pid, and such files are hidden unless
63 one uses the ls(1) -a option.
64 * (Since Linux 3.7) The kernel was configured without the CONFIG_CORE‐
66 DUMP option.
67 In addition, a core dump may exclude part of the address space of the
69 process if the madvise(2) MADV_DONTDUMP flag was employed.
70 On systems that employ systemd(1) as the init framework, core dumps may
72 instead be placed in a location determined by systemd(1). See below
73 for further details.
74 Naming of core dump files
76 By default, a core dump file is named core, but the /proc/sys/ker‐
77 nel/core_pattern file (since Linux 2.6 and 2.4.21) can be set to define
78 a template that is used to name core dump files. The template can con‐
79 tain % specifiers which are substituted by the following values when a
80 core file is created:
81 %% a single % character
83 %c core file size soft resource limit of crashing process (since
84 Linux 2.6.24)
85 %d dump mode—same as value returned by prctl(2) PR_GET_DUMPABLE
86 (since Linux 3.7)
87 %e executable filename (without path prefix)
88 %E pathname of executable, with slashes ('/') replaced by exclama‐
89 tion marks ('!') (since Linux 3.0).
90 %g (numeric) real GID of dumped process
91 %h hostname (same as nodename returned by uname(2))
92 %i TID of thread that triggered core dump, as seen in the PID
93 namespace in which the thread resides (since Linux 3.18)
94 %I TID of thread that triggered core dump, as seen in the initial
95 PID namespace (since Linux 3.18)
96 %p PID of dumped process, as seen in the PID namespace in which
97 the process resides
98 %P PID of dumped process, as seen in the initial PID namespace
99 (since Linux 3.12)
100 %s number of signal causing dump
101 %t time of dump, expressed as seconds since the Epoch, 1970-01-01
102 00:00:00 +0000 (UTC)
103 %u (numeric) real UID of dumped process
104 A single % at the end of the template is dropped from the core file‐
106 name, as is the combination of a % followed by any character other than
107 those listed above. All other characters in the template become a lit‐
108 eral part of the core filename. The template may include '/' charac‐
109 ters, which are interpreted as delimiters for directory names. The
110 maximum size of the resulting core filename is 128 bytes (64 bytes in
111 kernels before 2.6.19). The default value in this file is "core". For
112 backward compatibility, if /proc/sys/kernel/core_pattern does not
113 include %p and /proc/sys/kernel/core_uses_pid (see below) is nonzero,
114 then .PID will be appended to the core filename.
115 Paths are interpreted according to the settings that are active for the
117 crashing process. That means the crashing process's mount namespace
118 (see mount_namespaces(7)), its current working directory (found via
119 getcwd(2)), and its root directory (see chroot(2)).
120 Since version 2.4, Linux has also provided a more primitive method of
122 controlling the name of the core dump file. If the /proc/sys/ker‐
123 nel/core_uses_pid file contains the value 0, then a core dump file is
124 simply named core. If this file contains a nonzero value, then the
125 core dump file includes the process ID in a name of the form core.PID.
126 Since Linux 3.6, if /proc/sys/fs/suid_dumpable is set to 2 ("suid‐
128 safe"), the pattern must be either an absolute pathname (starting with
129 a leading '/' character) or a pipe, as defined below.
130 Piping core dumps to a program
132 Since kernel 2.6.19, Linux supports an alternate syntax for the
133 /proc/sys/kernel/core_pattern file. If the first character of this
134 file is a pipe symbol (|), then the remainder of the line is inter‐
135 preted as the command-line for a user-space program (or script) that is
136 to be executed. Instead of being written to a disk file, the core dump
137 is given as standard input to the program. Note the following points:
138 * The program must be specified using an absolute pathname (or a path‐
140 name relative to the root directory, /), and must immediately follow
141 the '|' character.
142 * The command-line arguments can include any of the % specifiers
144 listed above. For example, to pass the PID of the process that is
145 being dumped, specify %p in an argument.
146 * The process created to run the program runs as user and group root.
148 * Running as root does not confer any exceptional security bypasses.
150 Namely, LSMs (e.g., SELinux) are still active and may prevent the
151 handler from accessing details about the crashed process via
152 /proc/[pid].
153 * The program pathname is interpreted with respect to the initial
155 mount namespace as it is always executed there. It is not affected
156 by the settings (e.g., root directory, mount namespace, current
157 working directory) of the crashing process.
158 * The process runs in the initial namespaces (PID, mount, user, and so
160 on) and not in the namespaces of the crashing process. One can uti‐
161 lize specifiers such as %P to find the right /proc/[pid] directory
162 and probe/enter the crashing process's namespaces if needed.
163 * The process starts with its current working directory as the root
165 directory. If desired, it is possible change to the working direc‐
166 tory of the dumping process by employing the value provided by the
167 %P specifier to change to the location of the dumping process via
168 /proc/[pid]/cwd.
169 * Command-line arguments can be supplied to the program (since Linux
171 2.6.24), delimited by white space (up to a total line length of 128
172 bytes).
173 * The RLIMIT_CORE limit is not enforced for core dumps that are piped
175 to a program via this mechanism.
176 /proc/sys/kernel/core_pipe_limit
178 When collecting core dumps via a pipe to a user-space program, it can
179 be useful for the collecting program to gather data about the crashing
180 process from that process's /proc/[pid] directory. In order to do this
181 safely, the kernel must wait for the program collecting the core dump
182 to exit, so as not to remove the crashing process's /proc/[pid] files
183 prematurely. This in turn creates the possibility that a misbehaving
184 collecting program can block the reaping of a crashed process by simply
185 never exiting.
186 Since Linux 2.6.32, the /proc/sys/kernel/core_pipe_limit can be used to
188 defend against this possibility. The value in this file defines how
189 many concurrent crashing processes may be piped to user-space programs
190 in parallel. If this value is exceeded, then those crashing processes
191 above this value are noted in the kernel log and their core dumps are
192 skipped.
193 A value of 0 in this file is special. It indicates that unlimited pro‐
195 cesses may be captured in parallel, but that no waiting will take place
196 (i.e., the collecting program is not guaranteed access to /proc/<crash‐
197 ing-PID>). The default value for this file is 0.
198 Controlling which mappings are written to the core dump
200 Since kernel 2.6.23, the Linux-specific /proc/[pid]/coredump_filter
201 file can be used to control which memory segments are written to the
202 core dump file in the event that a core dump is performed for the
203 process with the corresponding process ID.
204 The value in the file is a bit mask of memory mapping types (see
206 mmap(2)). If a bit is set in the mask, then memory mappings of the
207 corresponding type are dumped; otherwise they are not dumped. The bits
208 in this file have the following meanings:
209 bit 0 Dump anonymous private mappings.
211 bit 1 Dump anonymous shared mappings.
212 bit 2 Dump file-backed private mappings.
213 bit 3 Dump file-backed shared mappings.
214 bit 4 (since Linux 2.6.24)
215 Dump ELF headers.
216 bit 5 (since Linux 2.6.28)
217 Dump private huge pages.
218 bit 6 (since Linux 2.6.28)
219 Dump shared huge pages.
220 bit 7 (since Linux 4.4)
221 Dump private DAX pages.
222 bit 8 (since Linux 4.4)
223 Dump shared DAX pages.
224 By default, the following bits are set: 0, 1, 4 (if the CON‐
226 FIG_CORE_DUMP_DEFAULT_ELF_HEADERS kernel configuration option is
227 enabled), and 5. This default can be modified at boot time using the
228 coredump_filter boot option.
229 The value of this file is displayed in hexadecimal. (The default value
231 is thus displayed as 33.)
232 Memory-mapped I/O pages such as frame buffer are never dumped, and vir‐
234 tual DSO (vdso(7)) pages are always dumped, regardless of the core‐
235 dump_filter value.
236 A child process created via fork(2) inherits its parent's coredump_fil‐
238 ter value; the coredump_filter value is preserved across an execve(2).
239 It can be useful to set coredump_filter in the parent shell before run‐
241 ning a program, for example:
242 $ echo 0x7 > /proc/self/coredump_filter
244 $ ./some_program
245 This file is provided only if the kernel was built with the CON‐
247 FIG_ELF_CORE configuration option.
248 Core dumps and systemd
250 On systems using the systemd(1) init framework, core dumps may be
251 placed in a location determined by systemd(1). To do this, systemd(1)
252 employs the core_pattern feature that allows piping core dumps to a
253 program. One can verify this by checking whether core dumps are being
254 piped to the systemd-coredump(8) program:
255 $ cat /proc/sys/kernel/core_pattern
257 |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %e
258 In this case, core dumps will be placed in the location configured for
260 systemd-coredump(8), typically as lz4(1) compressed files in the direc‐
261 tory /var/lib/systemd/coredump/. One can list the core dumps that have
262 been recorded by systemd-coredump(8) using coredumpctl(1):
263 $ coredumpctl list | tail -5
265 Wed 2017-10-11 22:25:30 CEST 2748 1000 1000 3 present /usr/bin/sleep
266 Thu 2017-10-12 06:29:10 CEST 2716 1000 1000 3 present /usr/bin/sleep
267 Thu 2017-10-12 06:30:50 CEST 2767 1000 1000 3 present /usr/bin/sleep
268 Thu 2017-10-12 06:37:40 CEST 2918 1000 1000 3 present /usr/bin/cat
269 Thu 2017-10-12 08:13:07 CEST 2955 1000 1000 3 present /usr/bin/cat
270 The information shown for each core dump includes the date and time of
272 the dump, the PID, UID, and GID of the dumping process, the signal
273 number that caused the core dump, and the pathname of the executable
274 that was being run by the dumped process. Various options to core‐
275 dumpctl(1) allow a specified coredump file to be pulled from the sys‐
276 temd(1) location into a specified file. For example, to extract the
277 core dump for PID 2955 shown above to a file named core in the current
278 directory, one could use:
279 $ coredumpctl dump 2955 -o core
281 For more extensive details, see the coredumpctl(1) manual page.
283 To disable the systemd(1) mechanism that archives core dumps, restoring
285 to something more like traditional Linux behavior, one can set an over‐
286 ride for the systemd(1) mechanism, using something like:
287 # echo "kernel.core_pattern=core.%p" > /etc/sysctl.d/50-coredump.conf
289 # /lib/systemd/systemd-sysctl
290
292 The gdb(1) gcore command can be used to obtain a core dump of a running
293 process.
294 In Linux versions up to and including 2.6.27, if a multithreaded
296 process (or, more precisely, a process that shares its memory with
297 another process by being created with the CLONE_VM flag of clone(2))
298 dumps core, then the process ID is always appended to the core file‐
299 name, unless the process ID was already included elsewhere in the file‐
300 name via a %p specification in /proc/sys/kernel/core_pattern. (This is
301 primarily useful when employing the obsolete LinuxThreads implementa‐
302 tion, where each thread of a process has a different PID.)
303
305 The program below can be used to demonstrate the use of the pipe syntax
306 in the /proc/sys/kernel/core_pattern file. The following shell session
307 demonstrates the use of this program (compiled to create an executable
308 named core_pattern_pipe_test):
309 $ cc -o core_pattern_pipe_test core_pattern_pipe_test.c
311 $ su
312 Password:
313 # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s" > \
314 /proc/sys/kernel/core_pattern
315 # exit
316 $ sleep 100
317 ^\ # type control-backslash
318 Quit (core dumped)
319 $ cat core.info
320 argc=5
321 argc[0]=</home/mtk/core_pattern_pipe_test>
322 argc[1]=<20575>
323 argc[2]=<UID=1000>
324 argc[3]=<GID=100>
325 argc[4]=<sig=3>
326 Total bytes in core dump: 282624
327 Program source
329 /* core_pattern_pipe_test.c */
331 #define _GNU_SOURCE
333 #include <sys/stat.h>
334 #include <fcntl.h>
335 #include <limits.h>
336 #include <stdio.h>
337 #include <stdlib.h>
338 #include <unistd.h>
339 #define BUF_SIZE 1024
341 int
343 main(int argc, char *argv[])
344 {
345 int tot, j;
346 ssize_t nread;
347 char buf[BUF_SIZE];
348 FILE *fp;
349 char cwd[PATH_MAX];
350 /* Change our current working directory to that of the
352 crashing process */
353 snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
355 chdir(cwd);
356 /* Write output to file "core.info" in that directory */
358 fp = fopen("core.info", "w+");
360 if (fp == NULL)
361 exit(EXIT_FAILURE);
362 /* Display command-line arguments given to core_pattern
364 pipe program */
365 fprintf(fp, "argc=%d\n", argc);
367 for (j = 0; j < argc; j++)
368 fprintf(fp, "argc[%d]=<%s>\n", j, argv[j]);
369 /* Count bytes in standard input (the core dump) */
371 tot = 0;
373 while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
374 tot += nread;
375 fprintf(fp, "Total bytes in core dump: %d\n", tot);
376 fclose(fp);
378 exit(EXIT_SUCCESS);
379 }
380
382 bash(1), coredumpctl(1), gdb(1), getrlimit(2), mmap(2), prctl(2),
383 sigaction(2), elf(5), proc(5), pthreads(7), signal(7), systemd-core‐
384 dump(8)
385
387 This page is part of release 5.02 of the Linux man-pages project. A
388 description of the project, information about reporting bugs, and the
389 latest version of this page, can be found at
390 https://www.kernel.org/doc/man-pages/.
391Linux 2019-03-06 CORE(5)