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. By convention,
the first of these strings should contain the filename
associated with the file being executed. 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 filesystem 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 mode 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 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)).
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 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.
(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().
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 filename argument of
execve() specifies an
interpreter script, then interpreter will be invoked
with the following arguments:
interpreter[optional-arg]filenamearg...
where arg...
is the series of words pointed to by the argv argument of execve(), starting at argv[1].
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.
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)).
On Linux prior to kernel 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 kernel 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 kernel 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.) Since Linux 2.6.25, the kernel 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
2.6.23 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 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 filesystem is mounted noexec.
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.
filename or
one of the pointers in the vectors argv or envp 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 recognized format.
Too many symbolic links were encountered in
resolving filename or the name of a
script or ELF interpreter.
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.
The per-process limit on the number of open file descriptors has been reached.
filename is
too long.
The system-wide 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 the file or interpreter cannot be found.
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.
Insufficient kernel memory was available.
A component of the path prefix of filename or a script or
ELF interpreter is not a directory.
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.
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.
A "capability-dumb" applications would not obtain the full set of permitted capabilities granted by the executable file. See capabilities(7).
The specified executable was open for writing by one or more processes.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. POSIX does not document the #! behavior, but it exists (with some variations) on other UNIX systems.
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 she did not have already (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 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.
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 SIGKILL signal.
A maximum line length of 127 characters is allowed for the first line in an interpreter scripts.
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 ignores the set-user-ID and set-group-ID bits on scripts.
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 3.0, this caused
the set*uid() call to fail.
(Prior to 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[]) { 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> int main(int argc, char *argv[]) { char *newargv[] = { NULL, "hello", "world", NULL }; 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
chmod(2), execveat(2), fork(2), ptrace(2), execl(3), fexecve(3), getopt(3), system(3), credentials(7), environ(7), path_resolution(7), ld.so(8)
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
Copyright (c) 1992 Drew Eckhardt (drewcs.colorado.edu), March 28, 1992 and Copyright (c) 2006 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified by Michael Haardt <michaelmoria.de> Modified 1993-07-21 by Rik Faith <faithcs.unc.edu> Modified 1994-08-21 by Michael Chastain <mecshell.portal.com>: Modified 1997-01-31 by Eric S. Raymond <esrthyrsus.com> Modified 1999-11-12 by Urs Thuermann <ursisnogud.escape.de> Modified 2004-06-23 by Michael Kerrisk <mtk.manpagesgmail.com> 2006-09-04 Michael Kerrisk <mtk.manpagesgmail.com> Added list of process attributes that are not preserved on exec(). 2007-09-14 Ollie Wild <aawgoogle.com>, mtk Add text describing limits on command-line arguments + environment |