[Bug 294780] fexecve(2) only works for scripts if /dev/fd is mounted with the non-default nodup option.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=294780

Kyle Evans <kevans@freebsd.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kevans@freebsd.org,
                   |                            |kib@FreeBSD.org

--- Comment #1 from Kyle Evans <kevans@freebsd.org> ---
I only spent a minute looking at the fexecve(2) implementation, so CC kib@, but
I think the official response is probably: just don't do that.  I note the
following comment:

```
                /*                                                              
                 * If the descriptors was not opened with O_PATH, then          
                 * we require that it was opened with O_EXEC or                 
                 * O_RDONLY.  In either case, exec_check_permissions()          
                 * below checks _current_ file access mode regardless           
                 * of the permissions additionally checked at the               
                 * open(2).                                                     
                 */            
```

You're not benefiting yourself by opening it O_EXEC because it will be
re-checked, you might as well just open it O_RDONLY and use faccessat(2) with
AT_EMPTY_PATH to see if you should even try fexecve(2) since your userland
checks are racey anyways, given the above.  I think that solves your problem,
and I don't know if we want to try and do anything else here.

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