EXECVE(2)



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

NAME
       execve - execute program

SYNOPSIS
       #include <unistd.h>

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

DESCRIPTION
       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 file-
       name,  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 permission bits.

       If the executable is an a.out dynamically linked binary executable con-
       taining  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 inter-
       preter named in the PT_INTERP segment is used to load the needed shared
       libraries.   This interpreter is typically /lib/ld-linux.so.2 for bina-
       ries linked with glibc.

       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.1-2001.  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.

       *  (Since Linux 2.4.36 / 2.6.23) If a set-user-ID or set-group-ID  pro-
          gram 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 capabili-
          ties(7).

       *  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 imple-
          mentation 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  capabili-
          ties(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  other-
          wise  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().

   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 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(), 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.

   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  environ-
       ment  and argument strings was limited to 32 pages (defined by the ker-
       nel 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  archi-
       tectures,  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.

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

ERRORS
       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
              filename 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 explana-
              tion of this error, see NOTES.

       EFAULT filename  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 filename
              or the name of a script or ELF interpreter.

       EMFILE The process has the maximum number of files open.

       ENAMETOOLONG
              filename is too long.

       ENFILE The system limit on the total number  of  open  files  has  been
              reached.

       ENOENT The file filename or a script or ELF interpreter does not exist,
              or a shared library needed for file  or  interpreter  cannot  be
              found.

       ENOEXEC
              An  executable  is  not in a recognized format, is for the wrong
              architecture, or has some other format error that means it  can-
              not be executed.

       ENOMEM Insufficient kernel memory was available.

       ENOTDIR
              A  component  of  the path prefix of filename 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.

       ETXTBSY
              Executable was open for writing by one or more processes.

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

NOTES
       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 spec-
       ifying  the  argument  as  a pointer to a list containing a single null
       pointer.  Do not take advantage of this misfeature!  It is  nonstandard
       and  nonportable:  on most other UNIX systems doing this will result in
       an error (EFAULT).

       POSIX.1-2001 says that values returned by sysconf(3) should be  invari-
       ant  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 sig-
       nal.

   Interpreter scripts
       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.

   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  ter-
       minated  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.

EXAMPLE
       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() 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

SEE ALSO
       chmod(2),  fork(2), ptrace(2), execl(3), fexecve(3), getopt(3), creden-
       tials(7), environ(7), path_resolution(7), ld.so(8)

COLOPHON
       This page is part of release 3.71 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-08-19                         EXECVE(2)

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