On Sat, Jan 11, 2020 at 08:07:19AM +1100, Aleksa Sarai wrote: > If I'm understanding this proposal correctly, this would be a problem > for the libpathrs use-case -- if this is done then there's no way to > avoid a TOCTOU with someone mounting and the userspace program checking > whether something is a mountpoint (unless you have Linux >5.6 and > RESOLVE_NO_XDEV). Today, you can (in theory) do it with MNT_EXPIRE: > > 1. Open the candidate directory. > 2. umount2(MNT_EXPIRE) the fd. > * -EINVAL means it wasn't a mountpoint when we got the fd, and the > fd is a stable handle to the underlying directory. > * -EAGAIN or -EBUSY means that it was a mountpoint or became a > mountpoint after the fd was opened (we don't care about that, but > fail-safe is better here). > 3. Use the fd from (1) for all operations. ... except that foo/../bar *WILL* cross into the covering mount, on any kernel that supports ...at(2) at all, so I would be very cautious about any kind "hardening" claims in that case. I'm not sure about Linus' proposal - it looks rather convoluted and we get a hard to describe twist of semantics in an area (procfs symlinks vs. mount traversal) on top of everything else in there... Anyway, a couple of questions: 1) do you see any problems on your testcases with the current #fixes? That's commit 7a955b7363b8 as branch tip. 2) do you have any updates you would like to fold into stuff in #work.openat2? Right now I have a local variant of #work.namei (with fairly cosmetical change compared to vfs.git one) that merges clean with #work.openat2; I would like to do any updates/fold-ins/etc. of #work.openat2 *before* doing a merge and continuing to work on top of the merge results...
