Let's say I need to get the file descriptor of a file (or a directory) which has only execution (or search) permission.
The X/Open norm defines a O_SEARCH flag for the open() function. See http://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html
However, Linux doesn't. There are only 3 available flags (O_RDONLY, O_WRONLY and O_RDWR). See http://man7.org/linux/man-pages/man2/open.2.html
Why? And how can I get the fd of a directory with only search permission?
Thanks
On Linux, you can obtain an fd for a directory that only has search permission using O_PATH. On other POSIX systems, you can fork a process and chdir to the directory; whenever you want to do openat relative to that directory, you can have the process perform open for you and send the fd to your main process (AF_UNIX and SCM_RIGHTS are a portable way of doing this).
Neither is strictly equivalent to O_SEARCH in the case where search permissions are revoked from the directory before access. POSIX guarantees that there will be no further permissions checks for an fd opened with O_SEARCH, but both O_PATH and chdir will check on every access.
