fexecve

FEXECVE(3)                 Linux Programmer's Manual                FEXECVE(3)

NAME
       fexecve - execute program specified via file descriptor

SYNOPSIS
       #include <unistd.h>

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fexecve():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _GNU_SOURCE

DESCRIPTION
       fexecve() performs the same task as execve(2), with the difference that
       the file to be executed is specified via a file descriptor, fd,  rather
       than  via  a pathname.  The file descriptor fd must be opened read-only
       (O_RDONLY) or with the O_PATH flag and the caller must have  permission
       to execute the file that it refers to.

RETURN VALUE
       A  successful  call to fexecve() never returns.  On error, the function
       does return, with a result value of -1, and errno is set appropriately.

ERRORS
       Errors are as for execve(2), with the following additions:

       EINVAL fd is not a valid file descriptor, or argv is NULL, or  envp  is
              NULL.

       ENOENT The  close-on-exec flag is set on fd, and fd refers to a script.
              See BUGS.

       ENOSYS The kernel does not provide the execveat(2) system call, and the
              /proc filesystem could not be accessed.

VERSIONS
       fexecve() is implemented since glibc 2.3.2.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +----------+---------------+---------+
       |Interface | Attribute     | Value   |
       +----------+---------------+---------+
       |fexecve() | Thread safety | MT-Safe |
       +----------+---------------+---------+

CONFORMING TO
       POSIX.1-2008.  This function is not specified in POSIX.1-2001,  and  is
       not   widely   available   on   other  systems.   It  is  specified  in
       POSIX.1-2008.

NOTES
       On Linux with glibc versions 2.26 and earlier, fexecve() is implemented
       using  the  proc(5) filesystem, so /proc needs to be mounted and avail-
       able at the time of the call.  Since glibc 2.27, if the underlying ker-
       nel supports the execveat(2) system call, then fexecve() is implemented
       using that system call, with the benefit that /proc does not need to be
       mounted.

       The  idea  behind fexecve() is to allow the caller to verify (checksum)
       the contents of an executable before executing it.  Simply opening  the
       file,  checksumming the contents, and then doing an execve(2) would not
       suffice, since, between the two steps, the  filename,  or  a  directory
       prefix  of  the  pathname,  could have been exchanged (by, for example,
       modifying the target of a symbolic link).  fexecve() does not  mitigate
       the  problem  that  the contents of a file could be changed between the
       checksumming and the call to fexecve(); for that, the  solution  is  to
       ensure  that the permissions on the file prevent it from being modified
       by malicious users.

       The natural idiom when using fexecve() is to set the close-on-exec flag
       on fd, so that the file descriptor does not leak through to the program
       that is executed.  This approach is natural for two reasons.  First, it
       prevents  file descriptors being consumed unnecessarily.  (The executed
       program normally has no need of a file descriptor that  refers  to  the
       program  itself.)   Second, if fexecve() is used recursively, employing
       the close-on-exec flag prevents the  file  descriptor  exhaustion  that
       would  result from the fact that each step in the recursion would cause
       one more file descriptor to be passed to the  new  program.   (But  see
       BUGS.)

BUGS
       If  fd  refers  to  a  script (i.e., it is an executable text file that
       names a script interpreter with a first line that begins with the char-
       acters  #!)   and the close-on-exec flag has been set for fd, then fex-
       ecve() fails with the error ENOENT.  This error occurs because, by  the
       time the script interpreter is executed, fd has already been closed be-
       cause of the close-on-exec flag.  Thus, the close-on-exec flag can't be
       set  on  fd if it refers to a script, leading to the problems described
       in NOTES.

SEE ALSO
       execve(2), execveat(2)

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                        FEXECVE(3)