NAME

       fexecve - execute program specified via file descriptor

LIBRARY

       Standard C library (libc, -lc)

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 to indicate the error.

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 attributes(7).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │fexecve()                                                      │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       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 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 characters #!)  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)