Why is there no O_SEARCH flag under Linux for open function?

Question

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

Answer 1

It turns out Linux doesn't support, yet, this flag, as stated in W. Richard Stevens Stephen A. Rago's book "Advanced Programming in the UNIX Environment" which you can have a look at here Link to the book on google books

Actually, the flag is defined in POSIX, implemented in standard C library (which is in this case glibc, that's why you find it under man 3 open) but is not implemented in Linux kernel (thus not found under man 2 open).

EDIT 1 : Since we use GNU under Linux, it includes specific headers for Linux to be able to make appropriate system calls that are feasable by Linux (in this case, it includes fcntl-linux.h in addition to fcntl.h).

EDIT 2 : Bug ticket https://sourceware.org/bugzilla/show_bug.cgi?id=18228

Please, correct me if I'm wrong!

Answer 2

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.