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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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execve(2) System Calls Manual execve(2)
execve - execute program
Standard C library (libc, -lc)
#include <unistd.h>
int execve(const char *pathname, char *const _Nullable argv[],
char *const _Nullable envp[]);
execve() executes the program referred to by pathname. This
causes the program that is currently being run by the calling
process to be replaced 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
a null pointer.)
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,
standardised 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
program'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 overwritten 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
pathname, 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
performed (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 ignored 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 containing 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
interpreter 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 capabilities is being executed, in which case the
dumpable flag may instead 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
program 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
capabilities(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 ignored 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
capabilities(7).
• By default, file descriptors remain open across an execve().
File descriptors that are marked close-on-exec are closed; see
the description 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 privilege
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
enabled 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
argument 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 interpreted 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
argument 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 derived 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 kernel
constant MAX_ARG_STRLEN), and the maximum number of strings is
0x7FFFFFFF.
On success, execve() does not return, on error -1 is returned,
and errno is set to indicate the error.
E2BIG The total number of bytes in the environment (envp) and
argument list (argv) is too large, an argument or
environment string is too long, or the full pathname of
the executable is too long. The terminating null byte is
counted as part of the string length.
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 interpreter.
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 cannot 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.
POSIX does not document the #! behavior, but it exists (with some
variations) on other UNIX systems.
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 nonstandard and nonportable misfeature! On
many other UNIX systems, specifying 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.
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 optional-arg. On some
systems, an interpreter script can have multiple arguments, and
white spaces in optional-arg are used to delimit the arguments.
Linux (like most other modern UNIX systems) ignores the set-user-
ID and set-group-ID bits on scripts.
POSIX.1-2008.
POSIX.1-2001, SVr4, 4.3BSD.
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.
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 attributes 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 already (and return EPERM), some will just
ignore the set-user-ID and set-group-ID bits and exec()
successfully.
In most cases where execve() fails, control returns to the
original executable 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.
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),
setreuid(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_EXCEEDED 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
execve() call (because other processes belonging to this real UID
terminated between the set*uid() call and the execve() call),
then the execve() 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.
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
interpreter. 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
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exec(3), fexecve(3), getauxval(3), getopt(3), system(3),
capabilities(7), credentials(7), environ(7), path_resolution(7),
ld.so(8)
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Linux man-pages 6.9.1 2024-06-15 execve(2)
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