On Fri, Nov 20, 2015 at 09:59:05AM +0000, David Howells wrote: > Al Viro <v...@zeniv.linux.org.uk> wrote: > > > 3) normally, readlink(2) fails for non-symlinks. Moreover, according to > > POSIX it should do so (with -EINVAL). There is a pathological case when > > it succeeds for a directory, though. Namely, one of the kinds of AFS > > "mountpoints". > > All AFS mountpoints are magic symlinks that are specially interpreted by the > client as far as I'm aware. I'm not sure why the designers didn't just select > a different file type for them, but they didn't.
All of them? I see two kinds there - one is magical symlink (recognized by contents in afs_iget()), another is this autocell thing, the latter having no ->readlink(). Both serve as automount points, don't they? > > stat(2) reports those as directories, stepping into them leads to > > automounting a directory there (why do we have ->open() for them, BTW?). > > I think I put that in to make sure the open() syscall returned EREMOTE rather > than another error if you tried to open it. It can probably be removed > because with the d_automount code you can't ever get there I think - unless > you can pass AT_NO_AUTOMOUNT to openat(). Just how would openat() get the AT_... flags? Only statat(2) accepts AT_NO_AUTOMOUNT, sorry. > > How the hell is userland supposed to guess to call readlink(2) on those > > suckers to get the information of what'll get automounted there if we step > > upon them? > > There's an AFS userspace command that could be used to query a mountpoint that > was going to use it. However, I suspect readlink() will now always trigger > the automount. This is one of the things OpenAFS uses pioctl() for - but > since I'm not allowed to add that to the kernel, I have to find some other way > of doing it. Well, pioctl() is a piec^H^Hle of shit interface; let's figure out what we'd actually want to implement and do that. One obvious thing is "here's a pathname, tell me what gets automounted here" (with interesting question of what to do if the automount is being triggered right now). Another thing is locating those guys; if we had a separate file type for them (i.e. could recognize them by st_mode _and_ d_type), we would be fine (the usual tree-walkers would be able to spot such places and query them for prospective automount targets), but without that... a syscall for everything in a tree just to list those suckers? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
