execve — execute program
#include <unistd.h>
int
execve( |
const char * | filename, |
char *const | argv, | |
char *const | envp) ; |
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[])
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
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 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
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
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) PR_SET_NAME
(and displayed by
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 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
cancelled (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
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.1-2001 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 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 execve
().
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 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 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, execve
() does
not return, on error −1 is returned, and errno
is set appropriately.
The total number of bytes in the environment
(envp
) and
argument list (argv
) is too large.
Search permission is denied on a component of the
path prefix of filename
or the name of a
script interpreter. (See also path_resolution(7).)
The file or a script interpreter is not a regular file.
Execute permission is denied for the file or a script or ELF interpreter.
The file system is mounted noexec
.
filename
points outside your accessible address space.
An ELF executable had more than one PT_INTERP segment (i.e., tried to name more than one interpreter).
An I/O error occurred.
An ELF interpreter was a directory.
An ELF interpreter was not in a recognised format.
Too many symbolic links were encountered in
resolving filename
or the name of a
script or ELF interpreter.
The process has the maximum number of files open.
filename
is
too long.
The system limit on the total number of open files has been reached.
The file filename
or a script or
ELF interpreter does not exist, or a shared library
needed for file or interpreter cannot be found.
An executable is not in a recognised format, is for the wrong architecture, or has some other format error that means it cannot be executed.
Insufficient kernel memory was available.
A component of the path prefix of filename
or a script or
ELF interpreter is not a directory.
The file system is mounted nosuid
, the user is not
the superuser, and the file has the set-user-ID or
set-group-ID bit set.
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.
Executable was open for writing by one or more processes.
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 EPERM), some will
just ignore the set-user-ID and set-group-ID bits and
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
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. 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.
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 $ ./exceve ./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)
|