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[73.241.217.19]) by smtp.gmail.com with ESMTPSA id q22sm11398036pgn.91.2020.06.22.20.18.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 22 Jun 2020 20:18:28 -0700 (PDT) Subject: Re: [PATCH v7 8/8] block: create the request_queue debugfs_dir on registration To: Luis Chamberlain Cc: axboe@kernel.dk, viro@zeniv.linux.org.uk, gregkh@linuxfoundation.org, rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com, martin.petersen@oracle.com, jejb@linux.ibm.com, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20200619204730.26124-1-mcgrof@kernel.org> <20200619204730.26124-9-mcgrof@kernel.org> <02112994-4cd7-c749-6bd7-66a772593c90@acm.org> <20200622124208.GW11244@42.do-not-panic.com> From: Bart Van Assche Autocrypt: addr=bvanassche@acm.org; prefer-encrypt=mutual; keydata= mQENBFSOu4oBCADcRWxVUvkkvRmmwTwIjIJvZOu6wNm+dz5AF4z0FHW2KNZL3oheO3P8UZWr LQOrCfRcK8e/sIs2Y2D3Lg/SL7qqbMehGEYcJptu6mKkywBfoYbtBkVoJ/jQsi2H0vBiiCOy fmxMHIPcYxaJdXxrOG2UO4B60Y/BzE6OrPDT44w4cZA9DH5xialliWU447Bts8TJNa3lZKS1 AvW1ZklbvJfAJJAwzDih35LxU2fcWbmhPa7EO2DCv/LM1B10GBB/oQB5kvlq4aA2PSIWkqz4 3SI5kCPSsygD6wKnbRsvNn2mIACva6VHdm62A7xel5dJRfpQjXj2snd1F/YNoNc66UUTABEB AAG0JEJhcnQgVmFuIEFzc2NoZSA8YnZhbmFzc2NoZUBhY20ub3JnPokBOQQTAQIAIwUCVI67 igIbAwcLCQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJEHFcPTXFzhAJ8QkH/1AdXblKL65M Y1Zk1bYKnkAb4a98LxCPm/pJBilvci6boefwlBDZ2NZuuYWYgyrehMB5H+q+Kq4P0IBbTqTa jTPAANn62A6jwJ0FnCn6YaM9TZQjM1F7LoDX3v+oAkaoXuq0dQ4hnxQNu792bi6QyVdZUvKc macVFVgfK9n04mL7RzjO3f+X4midKt/s+G+IPr4DGlrq+WH27eDbpUR3aYRk8EgbgGKvQFdD CEBFJi+5ZKOArmJVBSk21RHDpqyz6Vit3rjep7c1SN8s7NhVi9cjkKmMDM7KYhXkWc10lKx2 RTkFI30rkDm4U+JpdAd2+tP3tjGf9AyGGinpzE2XY1K5AQ0EVI67igEIAKiSyd0nECrgz+H5 PcFDGYQpGDMTl8MOPCKw/F3diXPuj2eql4xSbAdbUCJzk2ETif5s3twT2ER8cUTEVOaCEUY3 eOiaFgQ+nGLx4BXqqGewikPJCe+UBjFnH1m2/IFn4T9jPZkV8xlkKmDUqMK5EV9n3eQLkn5g lco+FepTtmbkSCCjd91EfThVbNYpVQ5ZjdBCXN66CKyJDMJ85HVr5rmXG/nqriTh6cv1l1Js T7AFvvPjUPknS6d+BETMhTkbGzoyS+sywEsQAgA+BMCxBH4LvUmHYhpS+W6CiZ3ZMxjO8Hgc ++w1mLeRUvda3i4/U8wDT3SWuHcB3DWlcppECLkAEQEAAYkBHwQYAQIACQUCVI67igIbDAAK CRBxXD01xc4QCZ4dB/0QrnEasxjM0PGeXK5hcZMT9Eo998alUfn5XU0RQDYdwp6/kMEXMdmT oH0F0xB3SQ8WVSXA9rrc4EBvZruWQ+5/zjVrhhfUAx12CzL4oQ9Ro2k45daYaonKTANYG22y //x8dLe2Fv1By4SKGhmzwH87uXxbTJAUxiWIi1np0z3/RDnoVyfmfbbL1DY7zf2hYXLLzsJR mSsED/1nlJ9Oq5fALdNEPgDyPUerqHxcmIub+pF0AzJoYHK5punqpqfGmqPbjxrJLPJfHVKy goMj5DlBMoYqEgpbwdUYkH6QdizJJCur4icy8GUNbisFYABeoJ91pnD4IGei3MTdvINSZI5e Message-ID: <4d25dbd1-a001-9869-58d5-630696440abc@acm.org> Date: Mon, 22 Jun 2020 20:18:27 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200622124208.GW11244@42.do-not-panic.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 625EC28A4E9 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2020-06-22 05:42, Luis Chamberlain wrote: > 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. During the past weeks I have been more busy than usual. I will try to make sure that in the future I have the time to read all comments on the previous versions of a patch series before replying to the latest version of a patch series. >> 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 /block/. > > Let me know if you think that will help. My concern is that q->debugfs_mutex would evolve the same way as q->sysfs_lock: at the time of introduction the role of a mutex is very clear but over time the number of use cases grows to a point where it is no longer possible to recognize the original purpose. I think there are two possible approaches: either a comment is added now that explains the role of q->debugfs_mutex or someone who has followed this conversation yells when someone tries to use q->debugfs_mutex for another purpose than what it was intended for. Thanks, Bart.