On Sat, Nov 09, 2019 at 08:55:38AM -0800, Linus Torvalds wrote: > On Fri, Nov 8, 2019 at 7:13 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > We have derived the parent from fhandle, we have a disconnected dentry for child, > > we go look for the name. We even find it. Now, we want to look it up. And > > some bastard goes and unlinks it, just as we are trying to lock the parent. > > We do a lookup, and get a negative dentry. Then we unlock the parent... and > > some other bastard does e.g. mkdir with the same name. OK, nresult->d_inode > > is not NULL (anymore). It has fuck-all to do with the original fhandle > > (different inumber, etc.) but we happily accept it. > > No arguments with your patch, although I doubt that this case has > actually ever happened in practice ;) Frankly, by this point I'm rather tempted to introduce new sparse annotation for dentries - "might not be positive". The thing is, there are four cases when ->d_inode is guaranteed to be stable: 1) nobody else has seen that dentry; that includes in-lookup ones - they can be found in in-lookup hash by d_alloc_parallel(), but it'll wait until they cease to be in-lookup. 2) ->d_lock is held 3) pinned positive 4) pinned, parent held at least shared Anything else can have ->d_inode changed by another thread. And class 3 is by far the most common. As the matter of fact, most of dentry pointers in the system (as opposed to (h)lists traversing dentries) are such. For obvious reasons we have shitloads of ->d_inode accesses; I'd been going through a tree-wide audit of those and doing that manually is bloody unpleasant. And we do have races in that area - the one above is the latest I'd caught; there had been more and I'm fairly sure that it's not the last. Redoing that kind of audit every once in a while is something I wouldn't wish on anyone; it would be nice to use sparse to narrow the field. Note that simply checking ->d_inode (or ->d_flags) is not enough unless we know them to be stable. E.g. callers of lookup_one_len_unlocked() tend to be racy exactly because of such tests. I'm going to add lookup_positive_unlocked() that would return ERR_PTR(-ENOENT) on negatives and convert the callers - with one exception they treat negatives the same way they would treat ERR_PTR(-ENOENT) and their tests are racy. I'd rather have sufficient barriers done in one common helper, instead of trying to add them in 11 places and hope that new users won't fuck it up... I'm still not sure what's the best way to do the annotations - propagation is not a big deal, but expressing the transitions and checking that maybe-negative ones are not misused is trickier. The last part can be actually left to manual audit - annotations would already reduce the search area big way. A not too ugly way to say that now this dentry is known to be non-negative, OTOH... I want to finish the audit and take a good look at the list of places where such transitions happen.
