fexecve(3)                                            Library Functions Manual                                           fexecve(3)

       fexecve - execute program specified via file descriptor

       Standard C library (libc, -lc)

       #include <unistd.h>

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

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

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

       fexecve()  performs the same task as execve(2), with the difference that the file to be executed is specified via a file de‐
       scriptor, 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.

       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 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.

       fexecve() is implemented since glibc 2.3.2.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                                                                        │ Attribute     │ Value   │
       │fexecve()                                                                                        │ Thread safety │ MT-Safe │

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

       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 be‐
       ing consumed unnecessarily.  (The executed program normally has no need of a file descriptor that refers to the program  it‐
       self.)   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.)

       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-ex‐
       ec 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

       execve(2), execveat(2)

Linux man-pages 6.03                                         2022-12-15                                                  fexecve(3)