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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
     replaced  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
     include 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
     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 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

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 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 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
     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] 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
     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  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.

     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
     attributes  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
     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 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
     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 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
     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() 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