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man: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.
     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  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
     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 4.16 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)

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