execve(2) System Calls Manual execve(2)
NAME
execve - execute program
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int execve(const char *pathname, char *const _Nullable argv[],
char *const _Nullable envp[]);
DESCRIPTION
execve() executes the program referred to by pathname. This causes the
program that is currently being run by the calling process to be re-
placed with a new program, with newly initialized stack, heap, and
(initialized and uninitialized) data segments.
pathname must be either a binary executable, or a script starting with
a line of the form:
#!interpreter [optional-arg]
For details of the latter case, see "Interpreter scripts" below.
argv is an array of pointers to strings passed to the new program as
its command-line arguments. By convention, the first of these strings
(i.e., argv[0]) should contain the filename associated with the file
being executed. The argv array must be terminated by a NULL pointer.
(Thus, in the new program, argv[argc] will be NULL.)
envp is an array of pointers to strings, conventionally of the form
key=value, which are passed as the environment of the new program. The
envp array must be terminated by a NULL pointer.
This manual page describes the Linux system call in detail; for an
overview of the nomenclature and the many, often preferable, standard-
ised variants of this function provided by libc, including ones that
search the PATH environment variable, see exec(3).
The argument vector and environment can be accessed by the new pro-
gram's main function, when it is defined as:
int main(int argc, char *argv[], char *envp[])
Note, however, that the use of a third argument to the main function is
not specified in POSIX.1; according to POSIX.1, the environment should
be accessed via the external variable environ(7).
execve() does not return on success, and the text, initialized data,
uninitialized data (bss), and stack of the calling process are over-
written according to the contents of the newly loaded program.
If the current program is being ptraced, a SIGTRAP signal is sent to it
after a successful execve().
If the set-user-ID bit is set on the program file referred to by path-
name, then the effective user ID of the calling process is changed to
that of the owner of the program file. Similarly, if the set-group-ID
bit is set on the program file, then the effective group ID of the
calling process is set to the group of the program file.
The aforementioned transformations of the effective IDs are not per-
formed (i.e., the set-user-ID and set-group-ID bits are ignored) if any
of the following is true:
• the no_new_privs attribute is set for the calling thread (see
prctl(2));
• the underlying filesystem is mounted nosuid (the MS_NOSUID flag for
mount(2)); or
• the calling process is being ptraced.
The capabilities of the program file (see capabilities(7)) are also ig-
nored if any of the above are true.
The effective user ID of the process is copied to the saved set-user-
ID; similarly, the effective group ID is copied to the saved set-group-
ID. This copying takes place after any effective ID changes that occur
because of the set-user-ID and set-group-ID mode bits.
The process's real UID and real GID, as well as its supplementary group
IDs, are unchanged by a call to execve().
If the executable is an a.out dynamically linked binary executable con-
taining shared-library stubs, the Linux dynamic linker ld.so(8) is
called at the start of execution to bring needed shared objects into
memory and link the executable with them.
If the executable is a dynamically linked ELF executable, the inter-
preter named in the PT_INTERP segment is used to load the needed shared
objects. This interpreter is typically /lib/ld-linux.so.2 for binaries
linked with glibc (see ld-linux.so(8)).
Effect on process attributes
All process attributes are preserved during an execve(), except the
following:
• The dispositions of any signals that are being caught are reset to
the default (signal(7)).
• Any alternate signal stack is not preserved (sigaltstack(2)).
• Memory mappings are not preserved (mmap(2)).
• Attached System V shared memory segments are detached (shmat(2)).
• POSIX shared memory regions are unmapped (shm_open(3)).
• Open POSIX message queue descriptors are closed (mq_overview(7)).
• Any open POSIX named semaphores are closed (sem_overview(7)).
• POSIX timers are not preserved (timer_create(2)).
• Any open directory streams are closed (opendir(3)).
• Memory locks are not preserved (mlock(2), mlockall(2)).
• Exit handlers are not preserved (atexit(3), on_exit(3)).
• The floating-point environment is reset to the default (see
fenv(3)).
The process attributes in the preceding list are all specified in
POSIX.1. The following Linux-specific process attributes are also not
preserved during an execve():
• The process's "dumpable" attribute is set to the value 1, unless a
set-user-ID program, a set-group-ID program, or a program with capa-
bilities is being executed, in which case the dumpable flag may in-
stead be reset to the value in /proc/sys/fs/suid_dumpable, in the
circumstances described under PR_SET_DUMPABLE in prctl(2). Note
that changes to the "dumpable" attribute may cause ownership of
files in the process's /proc/pid directory to change to root:root,
as described in proc(5).
• The prctl(2) PR_SET_KEEPCAPS flag is cleared.
• (Since Linux 2.4.36 / 2.6.23) If a set-user-ID or set-group-ID pro-
gram is being executed, then the parent death signal set by prctl(2)
PR_SET_PDEATHSIG flag is cleared.
• The process name, as set by prctl(2) PR_SET_NAME (and displayed by
ps -o comm), is reset to the name of the new executable file.
• The SECBIT_KEEP_CAPS securebits flag is cleared. See capabili-
ties(7).
• The termination signal is reset to SIGCHLD (see clone(2)).
• The file descriptor table is unshared, undoing the effect of the
CLONE_FILES flag of clone(2).
Note the following further points:
• All threads other than the calling thread are destroyed during an
execve(). Mutexes, condition variables, and other pthreads objects
are not preserved.
• The equivalent of setlocale(LC_ALL, "C") is executed at program
start-up.
• POSIX.1 specifies that the dispositions of any signals that are ig-
nored or set to the default are left unchanged. POSIX.1 specifies
one exception: if SIGCHLD is being ignored, then an implementation
may leave the disposition unchanged or reset it to the default;
Linux does the former.
• Any outstanding asynchronous I/O operations are canceled
(aio_read(3), aio_write(3)).
• For the handling of capabilities during execve(), see capabili-
ties(7).
• By default, file descriptors remain open across an execve(). File
descriptors that are marked close-on-exec are closed; see the de-
scription of FD_CLOEXEC in fcntl(2). (If a file descriptor is
closed, this will cause the release of all record locks obtained on
the underlying file by this process. See fcntl(2) for details.)
POSIX.1 says that if file descriptors 0, 1, and 2 would otherwise be
closed after a successful execve(), and the process would gain priv-
ilege because the set-user-ID or set-group-ID mode bit was set on
the executed file, then the system may open an unspecified file for
each of these file descriptors. As a general principle, no portable
program, whether privileged or not, can assume that these three file
descriptors will remain closed across an execve().
Interpreter scripts
An interpreter script is a text file that has execute permission en-
abled and whose first line is of the form:
#!interpreter [optional-arg]
The interpreter must be a valid pathname for an executable file.
If the pathname argument of execve() specifies an interpreter script,
then interpreter will be invoked with the following arguments:
interpreter [optional-arg] pathname arg...
where pathname is the pathname of the file specified as the first argu-
ment of execve(), and arg... is the series of words pointed to by the
argv argument of execve(), starting at argv[1]. Note that there is no
way to get the argv[0] that was passed to the execve() call.
For portable use, optional-arg should either be absent, or be specified
as a single word (i.e., it should not contain white space); see NOTES
below.
Since Linux 2.6.28, the kernel permits the interpreter of a script to
itself be a script. This permission is recursive, up to a limit of
four recursions, so that the interpreter may be a script which is in-
terpreted by a script, and so on.
Limits on size of arguments and environment
Most UNIX implementations impose some limit on the total size of the
command-line argument (argv) and environment (envp) strings that may be
passed to a new program. POSIX.1 allows an implementation to advertise
this limit using the ARG_MAX constant (either defined in <limits.h> or
available at run time using the call sysconf(_SC_ARG_MAX)).
Before Linux 2.6.23, the memory used to store the environment and argu-
ment strings was limited to 32 pages (defined by the kernel constant
MAX_ARG_PAGES). On architectures with a 4-kB page size, this yields a
maximum size of 128 kB.
On Linux 2.6.23 and later, most architectures support a size limit de-
rived from the soft RLIMIT_STACK resource limit (see getrlimit(2)) that
is in force at the time of the execve() call. (Architectures with no
memory management unit are excepted: they maintain the limit that was
in effect before Linux 2.6.23.) This change allows programs to have a
much larger argument and/or environment list. For these architectures,
the total size is limited to 1/4 of the allowed stack size. (Imposing
the 1/4-limit ensures that the new program always has some stack
space.) Additionally, the total size is limited to 3/4 of the value of
the kernel constant _STK_LIM (8 MiB). Since Linux 2.6.25, the kernel
also places a floor of 32 pages on this size limit, so that, even when
RLIMIT_STACK is set very low, applications are guaranteed to have at
least as much argument and environment space as was provided by Linux
2.6.22 and earlier. (This guarantee was not provided in Linux 2.6.23
and 2.6.24.) Additionally, the limit per string is 32 pages (the ker-
nel constant MAX_ARG_STRLEN), and the maximum number of strings is
0x7FFFFFFF.
RETURN VALUE
On success, execve() does not return, on error -1 is returned, and er-
rno is set to indicate the error.
ERRORS
E2BIG The total number of bytes in the environment (envp) and argument
list (argv) is too large.
EACCES Search permission is denied on a component of the path prefix of
pathname or the name of a script interpreter. (See also
path_resolution(7).)
EACCES The file or a script interpreter is not a regular file.
EACCES Execute permission is denied for the file or a script or ELF in-
terpreter.
EACCES The filesystem is mounted noexec.
EAGAIN (since Linux 3.1)
Having changed its real UID using one of the set*uid() calls,
the caller was—and is now still—above its RLIMIT_NPROC resource
limit (see setrlimit(2)). For a more detailed explanation of
this error, see NOTES.
EFAULT pathname or one of the pointers in the vectors argv or envp
points outside your accessible address space.
EINVAL An ELF executable had more than one PT_INTERP segment (i.e.,
tried to name more than one interpreter).
EIO An I/O error occurred.
EISDIR An ELF interpreter was a directory.
ELIBBAD
An ELF interpreter was not in a recognized format.
ELOOP Too many symbolic links were encountered in resolving pathname
or the name of a script or ELF interpreter.
ELOOP The maximum recursion limit was reached during recursive script
interpretation (see "Interpreter scripts", above). Before Linux
3.8, the error produced for this case was ENOEXEC.
EMFILE The per-process limit on the number of open file descriptors has
been reached.
ENAMETOOLONG
pathname is too long.
ENFILE The system-wide limit on the total number of open files has been
reached.
ENOENT The file pathname or a script or ELF interpreter does not exist.
ENOEXEC
An executable is not in a recognized format, is for the wrong
architecture, or has some other format error that means it can-
not be executed.
ENOMEM Insufficient kernel memory was available.
ENOTDIR
A component of the path prefix of pathname or a script or ELF
interpreter is not a directory.
EPERM The filesystem is mounted nosuid, the user is not the superuser,
and the file has the set-user-ID or set-group-ID bit set.
EPERM The process is being traced, the user is not the superuser and
the file has the set-user-ID or set-group-ID bit set.
EPERM A "capability-dumb" applications would not obtain the full set
of permitted capabilities granted by the executable file. See
capabilities(7).
ETXTBSY
The specified executable was open for writing by one or more
processes.
STANDARDS
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. POSIX does not document the
#! behavior, but it exists (with some variations) on other UNIX sys-
tems.
NOTES
One sometimes sees execve() (and the related functions described in
exec(3)) described as "executing a new process" (or similar). This is
a highly misleading description: there is no new process; many at-
tributes of the calling process remain unchanged (in particular, its
PID). All that execve() does is arrange for an existing process (the
calling process) to execute a new program.
Set-user-ID and set-group-ID processes can not be ptrace(2)d.
The result of mounting a filesystem nosuid varies across Linux kernel
versions: some will refuse execution of set-user-ID and set-group-ID
executables when this would give the user powers they did not have al-
ready (and return EPERM), some will just ignore the set-user-ID and
set-group-ID bits and exec() successfully.
On Linux, argv and envp can be specified as NULL. In both cases, this
has the same effect as specifying the argument as a pointer to a list
containing a single null pointer. Do not take advantage of this non-
standard and nonportable misfeature! On many other UNIX systems, spec-
ifying argv as NULL will result in an error (EFAULT). Some other UNIX
systems treat the envp==NULL case the same as Linux.
POSIX.1 says that values returned by sysconf(3) should be invariant
over the lifetime of a process. However, since Linux 2.6.23, if the
RLIMIT_STACK resource limit changes, then the value reported by
_SC_ARG_MAX will also change, to reflect the fact that the limit on
space for holding command-line arguments and environment variables has
changed.
In most cases where execve() fails, control returns to the original ex-
ecutable image, and the caller of execve() can then handle the error.
However, in (rare) cases (typically caused by resource exhaustion),
failure may occur past the point of no return: the original executable
image has been torn down, but the new image could not be completely
built. In such cases, the kernel kills the process with a SIGSEGV
(SIGKILL until Linux 3.17) signal.
Interpreter scripts
The kernel imposes a maximum length on the text that follows the "#!"
characters at the start of a script; characters beyond the limit are
ignored. Before Linux 5.1, the limit is 127 characters. Since Linux
5.1, the limit is 255 characters.
The semantics of the optional-arg argument of an interpreter script
vary across implementations. On Linux, the entire string following the
interpreter name is passed as a single argument to the interpreter, and
this string can include white space. However, behavior differs on some
other systems. Some systems use the first white space to terminate op-
tional-arg. On some systems, an interpreter script can have multiple
arguments, and white spaces in optional-arg are used to delimit the ar-
guments.
Linux (like most other modern UNIX systems) ignores the set-user-ID and
set-group-ID bits on scripts.
execve() and EAGAIN
A more detailed explanation of the EAGAIN error that can occur (since
Linux 3.1) when calling execve() is as follows.
The EAGAIN error can occur when a preceding call to setuid(2), se-
treuid(2), or setresuid(2) caused the real user ID of the process to
change, and that change caused the process to exceed its RLIMIT_NPROC
resource limit (i.e., the number of processes belonging to the new real
UID exceeds the resource limit). From Linux 2.6.0 to Linux 3.0, this
caused the set*uid() call to fail. (Before Linux 2.6, the resource
limit was not imposed on processes that changed their user IDs.)
Since Linux 3.1, the scenario just described no longer causes the
set*uid() call to fail, because it too often led to security holes
where buggy applications didn't check the return status and assumed
that—if the caller had root privileges—the call would always succeed.
Instead, the set*uid() calls now successfully change the real UID, but
the kernel sets an internal flag, named PF_NPROC_EXCEEDED, to note that
the RLIMIT_NPROC resource limit has been exceeded. If the PF_NPROC_EX-
CEEDED flag is set and the resource limit is still exceeded at the time
of a subsequent execve() call, that call fails with the error EAGAIN.
This kernel logic ensures that the RLIMIT_NPROC resource limit is still
enforced for the common privileged daemon workflow—namely, fork(2) +
set*uid() + execve().
If the resource limit was not still exceeded at the time of the ex-
ecve() call (because other processes belonging to this real UID termi-
nated between the set*uid() call and the execve() call), then the ex-
ecve() call succeeds and the kernel clears the PF_NPROC_EXCEEDED
process flag. The flag is also cleared if a subsequent call to fork(2)
by this process succeeds.
Historical
With UNIX V6, the argument list of an exec() call was ended by 0, while
the argument list of main was ended by -1. Thus, this argument list
was not directly usable in a further exec() call. Since UNIX V7, both
are NULL.
EXAMPLES
The following program is designed to be execed by the second program
below. It just echoes its command-line arguments, one per line.
/* myecho.c */
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
for (size_t j = 0; j < argc; j++)
printf("argv[%zu]: %s\n", j, argv[j]);
exit(EXIT_SUCCESS);
}
This program can be used to exec the program named in its command-line
argument:
/* execve.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
static char *newargv[] = { NULL, "hello", "world", NULL };
static char *newenviron[] = { NULL };
if (argc != 2) {
fprintf(stderr, "Usage: %s <file-to-exec>\n", argv[0]);
exit(EXIT_FAILURE);
}
newargv[0] = argv[1];
execve(argv[1], newargv, newenviron);
perror("execve"); /* execve() returns only on error */
exit(EXIT_FAILURE);
}
We can use the second program to exec the first as follows:
$ cc myecho.c -o myecho
$ cc execve.c -o execve
$ ./execve ./myecho
argv[0]: ./myecho
argv[1]: hello
argv[2]: world
We can also use these programs to demonstrate the use of a script in-
terpreter. To do this we create a script whose "interpreter" is our
myecho program:
$ cat > script
#!./myecho script-arg
^D
$ chmod +x script
We can then use our program to exec the script:
$ ./execve ./script
argv[0]: ./myecho
argv[1]: script-arg
argv[2]: ./script
argv[3]: hello
argv[4]: world
SEE ALSO
chmod(2), execveat(2), fork(2), get_robust_list(2), ptrace(2), exec(3),
fexecve(3), getauxval(3), getopt(3), system(3), capabilities(7), cre-
dentials(7), environ(7), path_resolution(7), ld.so(8)
Linux man-pages 6.03 2023-02-05 execve(2)
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