include/uapi: Define AT_FDNODIR constant as future guarantee

From: Steven Stewart-Gallus <[email protected]

This constant means that in the far future it might be possible to define other AT_FD* constants.

---

This is my first kernel patch but this is really trivial so I hope I'm doing this right.

index 074b886..92223f0 100644
@@ -38,9 +38,15 @@
 #define DN_ATTRIB	0x00000020	/* File changed attibutes */
 #define DN_MULTISHOT	0x80000000	/* Don't remove notifier */
 
+#define AT_FDNODDIR		-1      /* Special value used to indicate
+					   openat should not use any directory.
+					   Currently, other values work for this
+					   but in the future that might
+					   change. */
 #define AT_FDCWD		-100    /* Special value used to indicate
-                                           openat should use the current
-                                           working directory. */
+					   openat should use the current
+					   working directory. */
+
 #define AT_SYMLINK_NOFOLLOW	0x100   /* Do not follow symbolic links.  */
 #define AT_REMOVEDIR		0x200   /* Remove directory instead of
                                            unlinking file.  */

Almost. For one you really should explain what the flag does, especially if it's not actually use.

It took me a while that you're trying to document the way Linux currently behaves for the cases where the dfd argument to the various *at is neither a valid file descriptor nor the magic AT_FDCWD constant.

Posix says that openat with an absolute path is equivalent to open, so the only strictly allowed other flags would those that allow some form of relative path, but treat it differently than the currently possible options. I can't really think of a good way to do that, although we recently had a case where a different AT_* value would be useful, even if not fully conforming to that Posix wording.

Your explanation of what this constant does should also go into the man page, so in addition to the kernel patch you should also propose a patch to the man page documenting it.

Also I don't think AT_FDNODIR is a good choice of name, AT_ABSOLUTE would be a lot more descriptive.

Thank you for the criticisms Christopher Hellwig.

I will edit the commit message to be better and comments in the code to be clearer. I will also document the constant in the manpages. I think I will call the constant AT_FDABSOLUTE though because I want it to be prepended with FD the same way AT_FDCWD is.

Thank you,
Steven Stewart-Gallus