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.  This causes the
       program that is currently being run by the calling process  to  be  re-
       placed  with  a  new  program,  with newly initialized stack, heap, and
       (initialized and uninitialized) data segments.

       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 (i.e., argv[0])  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.  The argv and envp arrays must each in-
       clude a null pointer at the end of the array.

       The argument vector and environment can be accessed by the called  pro-
       gram'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  over-
       written 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 pointed to  by  file-
       name,  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 aforementioned transformations of the effective IDs  are  not  per-
       formed (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 ig-
       nored 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 con-
       taining 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  inter-
       preter 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 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)).

       *  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 ig-
          nored 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  capabili-
          ties(7).

       *  By  default,  file descriptors remain open across an execve().  File
          descriptors that are marked close-on-exec are closed;  see  the  de-
          scription  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 priv-
          ilege 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  en-
       abled 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] 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.

       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  in-
       terpreted 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 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 de-
       rived 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, ap-
       plications are guaranteed to have at least as much argument  and  envi-
       ronment space as was provided by Linux 2.6.23 and earlier.  (This guar-
       antee 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  er-
       rno 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 in-
              terpreter.

       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  re-
              source  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.

       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
              filename is too long.

       ENFILE The system-wide 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 the 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.

       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
       POSIX.1-2001,  POSIX.1-2008, SVr4, 4.3BSD.  POSIX does not document the
       #! behavior, but it exists (with some variations) on  other  UNIX  sys-
       tems.

NOTES
       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 at-
       tributes of the calling process remain unchanged  (in  particular,  its
       PID).   All that execve(2) 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 she did not  have  al-
       ready  (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  non-
       standard and nonportable misfeature!  On many other UNIX systems, spec-
       ifying 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 ex-
       ecutable  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 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 op-
       tional-arg.  On some systems, an interpreter script can  have  multiple
       arguments, and white spaces in optional-arg are used to delimit the ar-
       guments.

       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), se-
       treuid(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_EX-
       CEEDED 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  ex-
       ecve()  call (because other processes belonging to this real UID termi-
       nated between the set*uid() call and the execve() call), then  the  ex-
       ecve()  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() 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  in-
       terpreter.   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
       chmod(2),  execveat(2),  fork(2),  get_robust_list(2),  ptrace(2),  ex-
       ecl(3),  fexecve(3),  getopt(3), system(3), credentials(7), environ(7),
       path_resolution(7), ld.so(8)

COLOPHON
       This page is part of release 4.16 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                             2018-04-30                         EXECVE(2)

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