execve

execve - execute program

#include <unistd.h> I int execve(const char * filename , char *const argv [],
I char *const envp []);

R execve () executes the program pointed to by filename. filename 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 argument strings passed to the new program. envp is an array of strings, conventionally of the form key=value, which are passed as environment to the new program. Both argv and envp must be terminated by a null pointer. The argument vector and environment can be accessed by the called program's main function, when it is defined as:

int main(int argc, char *argv[], char *envp[]).

R execve () does not return on success, and the text, data, bss, and stack of the calling process are overwritten by that of the program loaded. If the current program is being ptraced, a SIGTRAP is sent to it after a successful R execve (). If the set-user-ID bit is set on the program file pointed to by filename, and the underlying file system is not mounted R nosuid (the MS_NOSUID flag for mount(2)), and the calling process is not being ptraced, then the effective user ID of the calling process is changed to that of the owner of the program file. Similarly, when the set-group-ID bit of the program file is set the effective group ID of the calling process is set to the group of the program file. 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 permission bits. 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 libraries 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 libraries. This interpreter is typically /lib/ld-linux.so.1 for binaries linked with the Linux libc version 5, or /lib/ld-linux.so.2 for binaries linked with the GNU libc version 2. All process attributes are preserved during an R execve (), except the following:

* The set of pending signals is cleared (sigpending(2)).

* The dispositions of any signals that are being caught are reset to being ignored.

* 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(3)).

* 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 process attributes in the preceding list are all specified in POSIX.1-2001. The following Linux-specific process attributes are also not preserved during an R execve ():

* The prctl(2) PR_SET_DUMPABLE flag is set, unless a set-user-ID or set-group ID program is being executed, in which case it is cleared.

* The prctl(2) PR_SET_KEEPCAPS flag is cleared.

* The process name, as set by prctl(2) R PR_SET_NAME (and displayed by R ps -o comm ), is reset to the name of the new executable file.

* The termination signal is reset to SIGCHLD (see clone(2)).

Note the following further points:

* All threads other than the calling thread are destroyed during an R 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.1-2001 specifies that the dispositions of any signals that are ignored or set to the default are left unchanged. POSIX.1-2001 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(2), see capabilities(7).

* By default, file descriptors remain open across an R execve (). File descriptors that are marked close-on-exec are closed ; see the description of R 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.1-2001 says that if file descriptors 0, 1, and 2 would otherwise be closed after a successful R execve (), and the process would gain privilege because the set-user_ID or set-group_ID permission 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 R 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 which is not itself a script. If the filename argument of R execve () specifies an interpreter script, then interpreter will be invoked with the following arguments:

interpreter [optional-arg] filename arg...

where arg... is the series of words pointed to by the argv argument of R execve (). 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.

On success, R execve () does not return, on error -1 is returned, and errno is set appropriately.

E2BIG

EACCES

EACCES

EACCES

EACCES

EFAULT

EINVAL

EIO

EISDIR

ELIBBAD

ELOOP

EMFILE

ENAMETOOLONG

ENFILE

ENOENT

ENOEXEC

ENOMEM

ENOTDIR

EPERM

EPERM

ETXTBSY

SVr4, 4.3BSD, POSIX.1-2001. POSIX.1-2001 does not document the #! behavior but is otherwise compatible.

Set-user-ID and set-group-ID processes can not be ptrace(2)d. Linux ignores the set-user-ID and set-group-ID bits on scripts. 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 she did not have already (and return R EPERM ), some will just ignore the set-user-ID and set-group-ID bits and R exec () successfully. A maximum line length of 127 characters is allowed for the first line in a #! executable shell script. 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 R optional-arg . On some systems, an interpreter script can have multiple arguments, and white spaces in optional-arg are used to delimit the arguments. On Linux, argv and envp can be specified as NULL, which has the same effect as specifying these arguments as pointers to lists containing a single NULL pointer. R Do not take advantage of this misfeature! It is non-standard and non-portable: on most other Unix systems doing this will result in an error.

Historical

With Unix V6 the argument list of an R 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 R exec () call. Since Unix V7 both are NULL.

The following program is designed to execed by the second program below. It just echoes its command-line one per line.

/* myecho.c */

#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char *argv[])
{
    int j;

    for (j = 0; j < argc; j++)
        printf("argv[%d]: %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>
#include <assert.h>

int
main(int argc, char *argv[])
{
    char *newargv[] = { NULL, "hello", "world", NULL };
    char *newenviron[] = { NULL };

    assert(argc == 2);  /* argv[1] identifies
                           program to exec */
    newargv[0] = argv[1];

    execve(argv[1], newargv, newenviron);
    perror("execve");   /* execve() only returns 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.sh
#! ./myecho script-arg
^D
$ chmod +x script.sh

We can then use our program to exec the script:

$ ./execve ./script.sh
argv[0]: ./myecho
argv[1]: script-arg
argv[2]: ./script.sh
argv[3]: hello
argv[4]: world

chmod(2), fork(2), ptrace(2), execl(3), fexecve(3), environ(7), credentials(7), path_resolution(7), ld.so(8)