execve

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

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.

       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
       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 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
       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
       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
       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)