On Thu, Oct 3, 2013 at 5:50 PM, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote: > On 10/03/2013 07:14 PM, Miklos Szeredi wrote: >> >> On Thu, Oct 03, 2013 at 05:28:30PM +0400, Maxim Patlasov wrote: >> >>> 1. There is an in-flight primary request with a chain of secondary ones. >>> 2. User calls ftruncate(2) to extend file; fuse_set_nowrite() makes >>> fi->writectr negative and starts waiting for completion of that >>> in-flight request >>> 3. Userspace fuse daemon ACKs the request and fuse_writepage_end() >>> is called; it calls __fuse_flush_writepages(), but the latter does >>> nothing because fi->writectr < 0 >>> 4. fuse_do_setattr() proceeds extending i_size and calling >>> __fuse_release_nowrite(). But now new (increased) i_size will be >>> used as 'crop' arg of __fuse_flush_writepages() >>> >>> stale data can leak to the server. >> >> So, lets do this then: skip fuse_flush_writepages() and call >> fuse_send_writepage() directly. It will ignore the NOWRITE logic, but >> that's >> okay, this happens rarely and cannot happen more than once in a row. >> >> Does this look good? > > > Yes, but let's at least add a comment explaining why it's safe. There are > three different cases and what you write above explains only one of them: > > 1st case (trivial): there are no concurrent activities using > fuse_set/release_nowrite. Then we're on safe side because > fuse_flush_writepages() would call fuse_send_writepage() anyway. > 2nd case: someone called fuse_set_nowrite and it is waiting now for > completion of all in-flight requests. Here what you wrote about "happening > rarely and no more than once" is applicable. > 3rd case: someone (e.g. fuse_do_setattr()) is in the middle of > fuse_set_nowrite..fuse_release_nowrite section. The fact that > fuse_set_nowrite returned implies that all in-flight requests were completed > along with all its secondary requests (because we increment writectr for a > secondry before decrementing it for the primary -- that's how > fuse_writepage_end is implemeted). Further requests are blocked by negative > writectr. Hence there cannot be any in-flight requests and no invocations of > fuse_writepage_end while we're in fuse_set_nowrite..fuse_release_nowrite > section. > > It looks obvious now, but I'm not sure we'll able to recollect it later. Added your analysis as a comment and all patches pushed to writepages.v2. >> Can you actually trigger this path with your testing? > > > No. Hmm, did you do any testing with the wait-for-page-writeback disabled in fuse_mkwrite()? Thanks, Miklos -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
