1EXECVE(2) Linux Programmer's Manual EXECVE(2)
2
6 execve - execute program
7
9 #include <unistd.h>
10 int execve(const char *filename, char *const argv[],
12 char *const envp[]);
13
15 execve() executes the program pointed to by filename. This causes the
16 program that is currently being run by the calling process to be
17 replaced with a new program, with newly initialized stack, heap, and
18 (initialized and uninitialized) data segments.
19 filename must be either a binary executable, or a script starting with
21 a line of the form:
22 #! interpreter [optional-arg]
24 For details of the latter case, see "Interpreter scripts" below.
26 argv is an array of argument strings passed to the new program. By
28 convention, the first of these strings (i.e., argv[0]) should contain
29 the filename associated with the file being executed. envp is an array
30 of strings, conventionally of the form key=value, which are passed as
31 environment to the new program. The argv and envp arrays must each
32 include a null pointer at the end of the array.
33 The argument vector and environment can be accessed by the called pro‐
35 gram's main function, when it is defined as:
36 int main(int argc, char *argv[], char *envp[])
38 Note, however, that the use of a third argument to the main function is
40 not specified in POSIX.1; according to POSIX.1, the environment should
41 be accessed via the external variable environ(7).
42 execve() does not return on success, and the text, initialized data,
44 uninitialized data (bss), and stack of the calling process are over‐
45 written according to the contents of the newly loaded program.
46 If the current program is being ptraced, a SIGTRAP signal is sent to it
48 after a successful execve().
49 If the set-user-ID bit is set on the program file pointed to by file‐
51 name, then the effective user ID of the calling process is changed to
52 that of the owner of the program file. Similarly, when the set-group-
53 ID bit of the program file is set the effective group ID of the calling
54 process is set to the group of the program file.
55 The aforementioned transformations of the effective IDs are not per‐
57 formed (i.e., the set-user-ID and set-group-ID bits are ignored) if any
58 of the following is true:
59 * the no_new_privs attribute is set for the calling thread (see
61 prctl(2));
62 * the underlying filesystem is mounted nosuid (the MS_NOSUID flag for
64 mount(2)); or
65 * the calling process is being ptraced.
67 The capabilities of the program file (see capabilities(7)) are also
69 ignored if any of the above are true.
70 The effective user ID of the process is copied to the saved set-user-
72 ID; similarly, the effective group ID is copied to the saved set-group-
73 ID. This copying takes place after any effective ID changes that occur
74 because of the set-user-ID and set-group-ID mode bits.
75 The process's real UID and real GID, as well its supplementary group
77 IDs, are unchanged by a call to execve().
78 If the executable is an a.out dynamically linked binary executable con‐
80 taining shared-library stubs, the Linux dynamic linker ld.so(8) is
81 called at the start of execution to bring needed shared objects into
82 memory and link the executable with them.
83 If the executable is a dynamically linked ELF executable, the inter‐
85 preter named in the PT_INTERP segment is used to load the needed shared
86 objects. This interpreter is typically /lib/ld-linux.so.2 for binaries
87 linked with glibc (see ld-linux.so(8)).
88 All process attributes are preserved during an execve(), except the
90 following:
91 * The dispositions of any signals that are being caught are reset to
93 the default (signal(7)).
94 * Any alternate signal stack is not preserved (sigaltstack(2)).
96 * Memory mappings are not preserved (mmap(2)).
98 * Attached System V shared memory segments are detached (shmat(2)).
100 * POSIX shared memory regions are unmapped (shm_open(3)).
102 * Open POSIX message queue descriptors are closed (mq_overview(7)).
104 * Any open POSIX named semaphores are closed (sem_overview(7)).
106 * POSIX timers are not preserved (timer_create(2)).
108 * Any open directory streams are closed (opendir(3)).
110 * Memory locks are not preserved (mlock(2), mlockall(2)).
112 * Exit handlers are not preserved (atexit(3), on_exit(3)).
114 * The floating-point environment is reset to the default (see
116 fenv(3)).
117 The process attributes in the preceding list are all specified in
119 POSIX.1. The following Linux-specific process attributes are also not
120 preserved during an execve():
121 * The prctl(2) PR_SET_DUMPABLE flag is set, unless a set-user-ID or
123 set-group ID program is being executed, in which case it is cleared.
124 * The prctl(2) PR_SET_KEEPCAPS flag is cleared.
126 * (Since Linux 2.4.36 / 2.6.23) If a set-user-ID or set-group-ID pro‐
128 gram is being executed, then the parent death signal set by prctl(2)
129 PR_SET_PDEATHSIG flag is cleared.
130 * The process name, as set by prctl(2) PR_SET_NAME (and displayed by
132 ps -o comm), is reset to the name of the new executable file.
133 * The SECBIT_KEEP_CAPS securebits flag is cleared. See capabili‐
135 ties(7).
136 * The termination signal is reset to SIGCHLD (see clone(2)).
138 * The file descriptor table is unshared, undoing the effect of the
140 CLONE_FILES flag of clone(2).
141 Note the following further points:
143 * All threads other than the calling thread are destroyed during an
145 execve(). Mutexes, condition variables, and other pthreads objects
146 are not preserved.
147 * The equivalent of setlocale(LC_ALL, "C") is executed at program
149 start-up.
150 * POSIX.1 specifies that the dispositions of any signals that are
152 ignored or set to the default are left unchanged. POSIX.1 specifies
153 one exception: if SIGCHLD is being ignored, then an implementation
154 may leave the disposition unchanged or reset it to the default;
155 Linux does the former.
156 * Any outstanding asynchronous I/O operations are canceled
158 (aio_read(3), aio_write(3)).
159 * For the handling of capabilities during execve(), see capabili‐
161 ties(7).
162 * By default, file descriptors remain open across an execve(). File
164 descriptors that are marked close-on-exec are closed; see the
165 description of FD_CLOEXEC in fcntl(2). (If a file descriptor is
166 closed, this will cause the release of all record locks obtained on
167 the underlying file by this process. See fcntl(2) for details.)
168 POSIX.1 says that if file descriptors 0, 1, and 2 would otherwise be
169 closed after a successful execve(), and the process would gain priv‐
170 ilege because the set-user-ID or set-group_ID mode bit was set on
171 the executed file, then the system may open an unspecified file for
172 each of these file descriptors. As a general principle, no portable
173 program, whether privileged or not, can assume that these three file
174 descriptors will remain closed across an execve().
175 Interpreter scripts
177 An interpreter script is a text file that has execute permission
178 enabled and whose first line is of the form:
179 #! interpreter [optional-arg]
181 The interpreter must be a valid pathname for an executable file. If
183 the filename argument of execve() specifies an interpreter script, then
184 interpreter will be invoked with the following arguments:
185 interpreter [optional-arg] filename arg...
187 where arg... is the series of words pointed to by the argv argument of
189 execve(), starting at argv[1].
190 For portable use, optional-arg should either be absent, or be specified
192 as a single word (i.e., it should not contain white space); see NOTES
193 below.
194 Since Linux 2.6.28, the kernel permits the interpreter of a script to
196 itself be a script. This permission is recursive, up to a limit of
197 four recursions, so that the interpreter may be a script which is
198 interpreted by a script, and so on.
199 Limits on size of arguments and environment
201 Most UNIX implementations impose some limit on the total size of the
202 command-line argument (argv) and environment (envp) strings that may be
203 passed to a new program. POSIX.1 allows an implementation to advertise
204 this limit using the ARG_MAX constant (either defined in <limits.h> or
205 available at run time using the call sysconf(_SC_ARG_MAX)).
206 On Linux prior to kernel 2.6.23, the memory used to store the environ‐
208 ment and argument strings was limited to 32 pages (defined by the ker‐
209 nel constant MAX_ARG_PAGES). On architectures with a 4-kB page size,
210 this yields a maximum size of 128 kB.
211 On kernel 2.6.23 and later, most architectures support a size limit
213 derived from the soft RLIMIT_STACK resource limit (see getrlimit(2))
214 that is in force at the time of the execve() call. (Architectures with
215 no memory management unit are excepted: they maintain the limit that
216 was in effect before kernel 2.6.23.) This change allows programs to
217 have a much larger argument and/or environment list. For these archi‐
218 tectures, the total size is limited to 1/4 of the allowed stack size.
219 (Imposing the 1/4-limit ensures that the new program always has some
220 stack space.) Since Linux 2.6.25, the kernel places a floor of 32
221 pages on this size limit, so that, even when RLIMIT_STACK is set very
222 low, applications are guaranteed to have at least as much argument and
223 environment space as was provided by Linux 2.6.23 and earlier. (This
224 guarantee was not provided in Linux 2.6.23 and 2.6.24.) Additionally,
225 the limit per string is 32 pages (the kernel constant MAX_ARG_STRLEN),
226 and the maximum number of strings is 0x7FFFFFFF.
227
229 On success, execve() does not return, on error -1 is returned, and
230 errno is set appropriately.
231
233 E2BIG The total number of bytes in the environment (envp) and argument
234 list (argv) is too large.
235 EACCES Search permission is denied on a component of the path prefix of
237 filename or the name of a script interpreter. (See also
238 path_resolution(7).)
239 EACCES The file or a script interpreter is not a regular file.
241 EACCES Execute permission is denied for the file or a script or ELF
243 interpreter.
244 EACCES The filesystem is mounted noexec.
246 EAGAIN (since Linux 3.1)
248 Having changed its real UID using one of the set*uid() calls,
249 the caller was—and is now still—above its RLIMIT_NPROC resource
250 limit (see setrlimit(2)). For a more detailed explanation of
251 this error, see NOTES.
252 EFAULT filename or one of the pointers in the vectors argv or envp
254 points outside your accessible address space.
255 EINVAL An ELF executable had more than one PT_INTERP segment (i.e.,
257 tried to name more than one interpreter).
258 EIO An I/O error occurred.
260 EISDIR An ELF interpreter was a directory.
262 ELIBBAD
264 An ELF interpreter was not in a recognized format.
265 ELOOP Too many symbolic links were encountered in resolving filename
267 or the name of a script or ELF interpreter.
268 ELOOP The maximum recursion limit was reached during recursive script
270 interpretation (see "Interpreter scripts", above). Before Linux
271 3.8, the error produced for this case was ENOEXEC.
272 EMFILE The per-process limit on the number of open file descriptors has
274 been reached.
275 ENAMETOOLONG
277 filename is too long.
278 ENFILE The system-wide limit on the total number of open files has been
280 reached.
281 ENOENT The file filename or a script or ELF interpreter does not exist,
283 or a shared library needed for the file or interpreter cannot be
284 found.
285 ENOEXEC
287 An executable is not in a recognized format, is for the wrong
288 architecture, or has some other format error that means it can‐
289 not be executed.
290 ENOMEM Insufficient kernel memory was available.
292 ENOTDIR
294 A component of the path prefix of filename or a script or ELF
295 interpreter is not a directory.
296 EPERM The filesystem is mounted nosuid, the user is not the superuser,
298 and the file has the set-user-ID or set-group-ID bit set.
299 EPERM The process is being traced, the user is not the superuser and
301 the file has the set-user-ID or set-group-ID bit set.
302 EPERM A "capability-dumb" applications would not obtain the full set
304 of permitted capabilities granted by the executable file. See
305 capabilities(7).
306 ETXTBSY
308 The specified executable was open for writing by one or more
309 processes.
310
312 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. POSIX does not document the
313 #! behavior, but it exists (with some variations) on other UNIX sys‐
314 tems.
315
317 One sometimes sees execve() (and the related functions described in
318 exec(3)) described as "executing a new process" (or similar). This is
319 a highly misleading description: there is no new process; many
320 attributes of the calling process remain unchanged (in particular, its
321 PID). All that execve(2) does is arrange for an existing process (the
322 calling process) to execute a new program.
323 Set-user-ID and set-group-ID processes can not be ptrace(2)d.
325 The result of mounting a filesystem nosuid varies across Linux kernel
327 versions: some will refuse execution of set-user-ID and set-group-ID
328 executables when this would give the user powers she did not have
329 already (and return EPERM), some will just ignore the set-user-ID and
330 set-group-ID bits and exec() successfully.
331 On Linux, argv and envp can be specified as NULL. In both cases, this
333 has the same effect as specifying the argument as a pointer to a list
334 containing a single null pointer. Do not take advantage of this non‐
335 standard and nonportable misfeature! On many other UNIX systems, spec‐
336 ifying argv as NULL will result in an error (EFAULT). Some other UNIX
337 systems treat the envp==NULL case the same as Linux.
338 POSIX.1 says that values returned by sysconf(3) should be invariant
340 over the lifetime of a process. However, since Linux 2.6.23, if the
341 RLIMIT_STACK resource limit changes, then the value reported by
342 _SC_ARG_MAX will also change, to reflect the fact that the limit on
343 space for holding command-line arguments and environment variables has
344 changed.
345 In most cases where execve() fails, control returns to the original
347 executable image, and the caller of execve() can then handle the error.
348 However, in (rare) cases (typically caused by resource exhaustion),
349 failure may occur past the point of no return: the original executable
350 image has been torn down, but the new image could not be completely
351 built. In such cases, the kernel kills the process with a SIGKILL sig‐
352 nal.
353 Interpreter scripts
355 A maximum line length of 127 characters is allowed for the first line
356 in an interpreter script.
357 The semantics of the optional-arg argument of an interpreter script
359 vary across implementations. On Linux, the entire string following the
360 interpreter name is passed as a single argument to the interpreter, and
361 this string can include white space. However, behavior differs on some
362 other systems. Some systems use the first white space to terminate
363 optional-arg. On some systems, an interpreter script can have multiple
364 arguments, and white spaces in optional-arg are used to delimit the
365 arguments.
366 Linux ignores the set-user-ID and set-group-ID bits on scripts.
368 execve() and EAGAIN
370 A more detailed explanation of the EAGAIN error that can occur (since
371 Linux 3.1) when calling execve() is as follows.
372 The EAGAIN error can occur when a preceding call to setuid(2),
374 setreuid(2), or setresuid(2) caused the real user ID of the process to
375 change, and that change caused the process to exceed its RLIMIT_NPROC
376 resource limit (i.e., the number of processes belonging to the new real
377 UID exceeds the resource limit). From Linux 2.6.0 to 3.0, this caused
378 the set*uid() call to fail. (Prior to 2.6, the resource limit was not
379 imposed on processes that changed their user IDs.)
380 Since Linux 3.1, the scenario just described no longer causes the
382 set*uid() call to fail, because it too often led to security holes
383 where buggy applications didn't check the return status and assumed
384 that—if the caller had root privileges—the call would always succeed.
385 Instead, the set*uid() calls now successfully change the real UID, but
386 the kernel sets an internal flag, named PF_NPROC_EXCEEDED, to note that
387 the RLIMIT_NPROC resource limit has been exceeded. If the
388 PF_NPROC_EXCEEDED flag is set and the resource limit is still exceeded
389 at the time of a subsequent execve() call, that call fails with the
390 error EAGAIN. This kernel logic ensures that the RLIMIT_NPROC resource
391 limit is still enforced for the common privileged daemon workflow—
392 namely, fork(2) + set*uid() + execve().
393 If the resource limit was not still exceeded at the time of the
395 execve() call (because other processes belonging to this real UID ter‐
396 minated between the set*uid() call and the execve() call), then the
397 execve() call succeeds and the kernel clears the PF_NPROC_EXCEEDED
398 process flag. The flag is also cleared if a subsequent call to fork(2)
399 by this process succeeds.
400 Historical
402 With UNIX V6, the argument list of an exec() call was ended by 0, while
403 the argument list of main was ended by -1. Thus, this argument list
404 was not directly usable in a further exec() call. Since UNIX V7, both
405 are NULL.
406
408 The following program is designed to be execed by the second program
409 below. It just echoes its command-line arguments, one per line.
410 /* myecho.c */
412 #include <stdio.h>
414 #include <stdlib.h>
415 int
417 main(int argc, char *argv[])
418 {
419 int j;
420 for (j = 0; j < argc; j++)
422 printf("argv[%d]: %s\n", j, argv[j]);
423 exit(EXIT_SUCCESS);
425 }
426 This program can be used to exec the program named in its command-line
428 argument:
429 /* execve.c */
431 #include <stdio.h>
433 #include <stdlib.h>
434 #include <unistd.h>
435 int
437 main(int argc, char *argv[])
438 {
439 char *newargv[] = { NULL, "hello", "world", NULL };
440 char *newenviron[] = { NULL };
441 if (argc != 2) {
443 fprintf(stderr, "Usage: %s <file-to-exec>\n", argv[0]);
444 exit(EXIT_FAILURE);
445 }
446 newargv[0] = argv[1];
448 execve(argv[1], newargv, newenviron);
450 perror("execve"); /* execve() returns only on error */
451 exit(EXIT_FAILURE);
452 }
453 We can use the second program to exec the first as follows:
455 $ cc myecho.c -o myecho
457 $ cc execve.c -o execve
458 $ ./execve ./myecho
459 argv[0]: ./myecho
460 argv[1]: hello
461 argv[2]: world
462 We can also use these programs to demonstrate the use of a script
464 interpreter. To do this we create a script whose "interpreter" is our
465 myecho program:
466 $ cat > script
468 #!./myecho script-arg
469 ^D
470 $ chmod +x script
471 We can then use our program to exec the script:
473 $ ./execve ./script
475 argv[0]: ./myecho
476 argv[1]: script-arg
477 argv[2]: ./script
478 argv[3]: hello
479 argv[4]: world
480
482 chmod(2), execveat(2), fork(2), get_robust_list(2), ptrace(2),
483 execl(3), fexecve(3), getopt(3), system(3), credentials(7), environ(7),
484 path_resolution(7), ld.so(8)
485
487 This page is part of release 4.16 of the Linux man-pages project. A
488 description of the project, information about reporting bugs, and the
489 latest version of this page, can be found at
490 https://www.kernel.org/doc/man-pages/.
491Linux 2018-04-30 EXECVE(2)