execve(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

EXECVE(2)               Linux Programmer's Manual              EXECVE(2)

NAME         top

       execve - execute program

SYNOPSIS         top

       #include <unistd.h>

       int execve(const char *pathname, char *const argv[],
                  char *const envp[]);

DESCRIPTION         top

       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 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 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 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.

   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)).

       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.

RETURN VALUE         top

       On success, execve() does not return, on error -1 is returned,
       and errno is set to indicate the error.

ERRORS         top

       E2BIG  The total number of bytes in the environment (envp) and
              argument list (argv) is too large.

       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.

CONFORMING TO         top

       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.

NOTES         top

       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.

   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.

   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 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.

   Historical
       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.

EXAMPLES         top

       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

SEE ALSO         top

       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)

COLOPHON         top

       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/.

Linux                          2021-03-22                      EXECVE(2)

Pages that refer to this page: pmcd(1)setpriv(1)strace(1)access(2)alarm(2)arch_prctl(2)brk(2)chdir(2)chmod(2)chroot(2)clone(2)close(2)eventfd(2)execveat(2)_exit(2)fanotify_mark(2)fcntl(2)flock(2)fork(2)getgroups(2)getitimer(2)getpriority(2)getrlimit(2)get_robust_list(2)getrusage(2)ioctl(2)ioctl_console(2)ioperm(2)iopl(2)keyctl(2)madvise(2)memfd_create(2)mlock(2)mount(2)open(2)perf_event_open(2)prctl(2)ptrace(2)sched_setaffinity(2)seccomp(2)semop(2)set_mempolicy(2)setpgid(2)setresuid(2)setreuid(2)setsid(2)setuid(2)shmop(2)sigaction(2)sigaltstack(2)signalfd(2)sigpending(2)sigprocmask(2)syscalls(2)timer_create(2)timerfd_create(2)umask(2)vfork(2)cap_get_file(3)cap_iab(3)cap_launch(3)catopen(3)exec(3)exit(3)fexecve(3)getexeccon(3)getfscreatecon(3)getkeycreatecon(3)getsockcreatecon(3)libexpect(3)mq_close(3)posix_spawn(3)pthread_atfork(3)pthread_kill_other_threads_np(3)pthread_mutexattr_setrobust(3)sd_bus_creds_get_pid(3)sem_close(3)sigvec(3)system(3)core(5)elf(5)proc(5)systemd.exec(5)systemd-system.conf(5)capabilities(7)cgroups(7)credentials(7)environ(7)inode(7)inotify(7)persistent-keyring(7)process-keyring(7)pthreads(7)sched(7)session-keyring(7)signal(7)signal-safety(7)thread-keyring(7)user-keyring(7)user_namespaces(7)user-session-keyring(7)vdso(7)pam_selinux(8)