fexecve

EXECVE(2)                    BSD System Calls Manual                   EXECVE(2)

NAME
     execve, fexecve — execute a file

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

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

     int
     fexecve(int fd, char *const argv[], char *const envp[]);

DESCRIPTION
     The execve() system call transforms the calling process into a new process.
     The new process is constructed from an ordinary file, whose name is pointed
     to by path, called the new process file.  The fexecve() system call is
     equivalent to execve() except that the file to be executed is determined by
     the file descriptor fd instead of a path.  This file is either an
     executable object file, or a file of data for an interpreter.  An
     executable object file consists of an identifying header, followed by pages
     of data representing the initial program (text) and initialized data pages.
     Additional pages may be specified by the header to be initialized with zero
     data; see elf(5) and a.out(5).

     An interpreter file begins with a line of the form:

           #! interpreter [arg]

     When an interpreter file is execve'd, the system actually execve's the
     specified interpreter.  If the optional arg is specified, it becomes the
     first argument to the interpreter, and the name of the originally execve'd
     file becomes the second argument; otherwise, the name of the originally
     execve'd file becomes the first argument.  The original arguments are
     shifted over to become the subsequent arguments.  The zeroth argument is
     set to the specified interpreter.

     The argument argv is a pointer to a null-terminated array of character
     pointers to null-terminated character strings.  These strings construct the
     argument list to be made available to the new process.  At least one
     argument must be present in the array; by custom, the first element should
     be the name of the executed program (for example, the last component of
     path).

     The argument envp is also a pointer to a null-terminated array of character
     pointers to null-terminated strings.  A pointer to this array is normally
     stored in the global variable environ.  These strings pass information to
     the new process that is not directly an argument to the command (see
     environ(7)).

     File descriptors open in the calling process image remain open in the new
     process image, except for those for which the close-on-exec flag is set
     (see close(2) and fcntl(2)).  Descriptors that remain open are unaffected
     by execve().  If any of the standard descriptors (0, 1, and/or 2) are
     closed at the time execve() is called, and the process will gain privilege
     as a result of set-id semantics, those descriptors will be re-opened
     automatically.  No programs, whether privileged or not, should assume that
     these descriptors will remain closed across a call to execve().

     Signals set to be ignored in the calling process are set to be ignored in
     the new process.  Signals which are set to be caught in the calling process
     image are set to default action in the new process image.  Blocked signals
     remain blocked regardless of changes to the signal action.  The signal
     stack is reset to be undefined (see sigaction(2) for more information).

     If the set-user-ID mode bit of the new process image file is set (see
     chmod(2)), the effective user ID of the new process image is set to the
     owner ID of the new process image file.  If the set-group-ID mode bit of
     the new process image file is set, the effective group ID of the new
     process image is set to the group ID of the new process image file.  (The
     effective group ID is the first element of the group list.)  The real user
     ID, real group ID and other group IDs of the new process image remain the
     same as the calling process image.  After any set-user-ID and set-group-ID
     processing, the effective user ID is recorded as the saved set-user-ID, and
     the effective group ID is recorded as the saved set-group-ID.  These values
     may be used in changing the effective IDs later (see setuid(2)).

     The set-ID bits are not honored if the respective file system has the
     nosuid option enabled or if the new process file is an interpreter file.
     Syscall tracing is disabled if effective IDs are changed.

     The new process also inherits the following attributes from the calling
     process:

           process ID           see getpid(2)
           parent process ID    see getppid(2)
           process group ID     see getpgrp(2)
           access groups        see getgroups(2)
           working directory    see chdir(2)
           root directory       see chroot(2)
           control terminal     see termios(4)
           resource usages      see getrusage(2)
           interval timers      see getitimer(2)
           resource limits      see getrlimit(2)
           file mode mask       see umask(2)
           signal mask          see sigaction(2), sigprocmask(2)

     When a program is executed as a result of an execve() system call, it is
     entered as follows:

           main(argc, argv, envp)
           int argc;
           char **argv, **envp;

     where argc is the number of elements in argv (the ``arg count'') and argv
     points to the array of character pointers to the arguments themselves.

     The fexecve() ignores the file offset of fd.  Since execute permission is
     checked by fexecve(), the file descriptor fd need not have been opened with
     the O_EXEC flag.  However, if the file to be executed denies read
     permission for the process preparing to do the exec, the only way to
     provide the fd to fexecve() is to use the O_EXEC flag when opening fd.
     Note that the file to be executed can not be open for writing.

RETURN VALUES
     As the execve() system call overlays the current process image with a new
     process image the successful call has no process to return to.  If execve()
     does return to the calling process an error has occurred; the return value
     will be -1 and the global variable errno is set to indicate the error.

ERRORS
     The execve() system call will fail and return to the calling process if:

     [ENOTDIR]          A component of the path prefix is not a directory.

     [ENAMETOOLONG]     A component of a pathname exceeded 255 characters, or an
                        entire path name exceeded 1023 characters.

     [ENOEXEC]          When invoking an interpreted script, the length of the
                        first line, inclusive of the #! prefix and terminating
                        newline, exceeds MAXSHELLCMDLEN characters.

     [ENOENT]           The new process file does not exist.

     [ELOOP]            Too many symbolic links were encountered in translating
                        the pathname.

     [EACCES]           Search permission is denied for a component of the path
                        prefix.

     [EACCES]           The new process file is not an ordinary file.

     [EACCES]           The new process file mode denies execute permission.

     [ENOEXEC]          The new process file has the appropriate access
                        permission, but has an invalid magic number in its
                        header.

     [ETXTBSY]          The new process file is a pure procedure (shared text)
                        file that is currently open for writing by some process.

     [ENOMEM]           The new process requires more virtual memory than is
                        allowed by the imposed maximum (getrlimit(2)).

     [E2BIG]            The number of bytes in the new process' argument list is
                        larger than the system-imposed limit.  This limit is
                        specified by the sysctl(3) MIB variable KERN_ARGMAX.

     [EFAULT]           The new process file is not as long as indicated by the
                        size values in its header.

     [EFAULT]           The path, argv, or envp arguments point to an illegal
                        address.

     [EIO]              An I/O error occurred while reading from the file
                        system.

     [EINTEGRITY]       Corrupted data was detected while reading from the file
                        system.

     In addition, the fexecve() will fail and return to the calling process if:

     [EBADF]            The fd argument is not a valid file descriptor open for
                        executing.

SEE ALSO
     ktrace(1), _exit(2), fork(2), open(2), execl(3), exit(3), sysctl(3),
     a.out(5), elf(5), fdescfs(5), environ(7), mount(8)

STANDARDS
     The execve() system call conforms to IEEE Std 1003.1-2001 (“POSIX.1”), with
     the exception of reopening descriptors 0, 1, and/or 2 in certain
     circumstances.  A future update of the Standard is expected to require this
     behavior, and it may become the default for non-privileged processes as
     well.  The support for executing interpreted programs is an extension.  The
     fexecve() system call conforms to The Open Group Extended API Set 2
     specification.

HISTORY
     The execve() system call appeared in Version 7 AT&T UNIX.  The fexecve()
     system call appeared in FreeBSD 8.0.

CAVEATS
     If a program is setuid to a non-super-user, but is executed when the real
     uid is ``root'', then the program has some of the powers of a super-user as
     well.

     When executing an interpreted program through fexecve(), kernel supplies
     /dev/fd/n as a second argument to the interpreter, where n is the file
     descriptor passed in the fd argument to fexecve().  For this construction
     to work correctly, the fdescfs(5) filesystem shall be mounted on /dev/fd.

BSD                              March 30, 2020                              BSD