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 appropri-
       ately.

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.

       ENOSYS 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 attributes(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
       available at the time of the call.  Since glibc 2.27, if the underlying kernel 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 direc-
       tory prefix of the pathname, could have been exchanged (by, for example, modifying the target of a symbolic link).  fexecve() does not mit-
       igate 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 recur-
       sively, 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 fexecve() fails with the error ENOENT.  This error occurs because, by  the
       time  the script interpreter is executed, fd has already been closed because 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 4.15 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                                                               2017-09-15                                                          FEXECVE(3)