On Sat, Jun 20, 2020 at 11:07:43AM -0700, Bart Van Assche wrote: > On 2020-06-19 13:47, Luis Chamberlain wrote: > > We were only creating the request_queue debugfs_dir only > > for make_request block drivers (multiqueue), but never for > > request-based block drivers. We did this as we were only > > creating non-blktrace additional debugfs files on that directory > > for make_request drivers. However, since blktrace *always* creates > > that directory anyway, we special-case the use of that directory > > on blktrace. Other than this being an eye-sore, this exposes > > request-based block drivers to the same debugfs fragile > > race that used to exist with make_request block drivers > > where if we start adding files onto that directory we can later > > run a race with a double removal of dentries on the directory > > if we don't deal with this carefully on blktrace. > > > > Instead, just simplify things by always creating the request_queue > > debugfs_dir on request_queue registration. Rename the mutex also to > > reflect the fact that this is used outside of the blktrace context. > > There are two changes in this patch: a bug fix and a rename of a mutex. > I don't like it to see two changes in a single patch. I thought about doing the split first, and I did it at first, but then I could hear Christoph yelling at me for it. So I merged the two together. Although it makes it more difficult for review, the changes do go together. Kind of late to split this as its already merged now. > Additionally, is the new mutex name really better than the old name? The > proper way to use mutexes is to use mutexes to protect data instead of > code. Where is the documentation that mentions which member variable(s) > of which data structures are protected by the mutex formerly called > blk_trace_mutex? It does not exist, and that is the point. The debugfs_dir use after free showed us *when* that UAF can happen, and so care must be taken if we are to use the mutex to protect the debugfs_dir but also re-use the same directory for other block core shenanigans. > Since the new name makes it even less clear which data > is protected by this mutex, is the new name really better than the old name? I thought the new name makes it crystal clear what is being protected. I can however add a comment to explain that the q->debugfs_mutex protects the q->debugfs_dir if it is created, otherwise it protects the ephemeral debugfs_dir directory which would otherwise be created in lieue of q->debugfs_dir, however the patch still lies under <debugfs_root>/block/. Let me know if you think that will help. Luis
