man_2_execve.txt

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

NAME
       execve - execute program

SYNOPSIS
       #include <unistd.h>

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

DESCRIPTION
       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 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 ar‐
       rays must each include a null pointer at the end of the array.

       The argument vector and environment can be accessed by the called program's main  func‐
       tion, 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 suc‐
       cessful execve().

       If the set-user-ID bit is set on the program file referred to by pathname, then the ef‐
       fective  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 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 typi‐
       cally /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 (sig‐
          nal(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 follow‐
       ing 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 pro‐
          gram 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 program is being exe‐
          cuted,  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().   Mu‐
          texes, 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  be‐
          ing  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 ob‐
          tained 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 success‐
          ful 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 unspeci‐
          fied 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 ex‐
       ecve(), 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 argu‐
       ment  (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 ar‐
       chitectures 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  main‐
       tain  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 en‐
       sures 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 Mibibytes).  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 2.6.23 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 appro‐
       priately.

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 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, or a shared li‐
              brary 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 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  ca‐
              pabilities 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 systems.

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() 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 ef‐
       fect 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  (typi‐
       cally  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  com‐
       pletely built.  In such cases, the kernel kills the process with a SIGKILL 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 imple‐
       mentations.  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 ter‐
       minate  optional-arg.   On  some systems, an interpreter script can have multiple argu‐
       ments, 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  setre‐
       suid(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  be‐
       longing 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 al‐
       ways 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.

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

SEE ALSO
       chmod(2), execveat(2), fork(2),  get_robust_list(2),  ptrace(2),  exec(3),  fexecve(3),
       getopt(3), system(3), credentials(7), environ(7), path_resolution(7), ld.so(8)

COLOPHON
       This page is part of release 5.05 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                                     2019-10-10                                 EXECVE(2)