execve — execute program
#include <unistd.h>
int
execve( |
const char *pathname, |
| char *const argv[], | |
char*const 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
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.
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 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 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)).
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().
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]pathnamearg...
where pathname
is the absolute 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.
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.) 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.
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 pathname 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.
pathname 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 pathname 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.
pathname is
too long.
The system-wide limit on the total number of open files has been reached.
The file pathname or a script or
ELF interpreter does not exist.
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 pathname 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.
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.
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 SIGSEGV (SIGKILL until Linux 3.17) signal.
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.
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[]) { for (int 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), get_robust_list(2), ptrace(2), exec(3), fexecve(3), getopt(3), system(3), capabilities(7), credentials(7), environ(7), path_resolution(7), ld.so(8)
This page is part of release 5.11 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 |