Re: [git pull] vfs.git part 2

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Jul 12, 2013 at 12:42:15PM -0700, Linus Torvalds wrote:

> As to the mode argument: we should encourage people to have it, since
> the inode *may* become visible afterwards. See above (can you do
> linkat() to just turn an fd into a name? I didn't really check, but I
> think you can do it as a "link(/proc/sef/fd/..)" thing regardless).

link() won't do it, linkat() with the right arguments will (link(2)
doesn't follow symlinks on the oldname).  It's a bit more complicated,
though:
	* vfs_link() will refuse to link with link count that went down to 0
(always had, and that's a damn good idea for security reasons)
	* with O_TMPFILE, vfs_link() will act as usual *if* you give open()
O_EXCL as well - files created that way cannot be linked in, ever.  They
stay with link count 0 and they die as soon as they are closed.  Again,
that's the behaviour we'd have with O_CREAT open + unlink.  tmpfile(3) ought
to call it that way.
	* O_TMPFILE _without_ O_EXCL == you may link it in once you've
decided that you've set it up enough.  In that case you are supposed to
open it, write whatever you want to write, fchmod/fchown/fsetxattr/etc.
and use linkat() to put it in place.  Or use vfs_link() if you are doing
that kernel-side.