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AJvYcCXnaigKRk+ophvQF6EM/mInUWDhtB5MC7WrAZAvjsCbWUj7D6E6CqMdeXMdBHxZA2lYpGjNrcyhQg==@kvack.org X-Gm-Message-State: AOJu0Yw5k9pKYB4aTOnAPpTHXdh1fIZJgLJq3yGvekL0lFknCWN2hWPD 4vfYMXXc0FwlPksztg7/Y6Ljwms1do+NKfHqZPIuan7Woz8cy8op8u63OZzUaMfoPEEIiaYYDo2 B7ZevaR3cJZWzBNStSQ== X-Received: from wmbz6.prod.google.com ([2002:a05:600c:c086:b0:480:4a03:7b6c]) (user=aliceryhl job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:530f:b0:47a:935f:618e with SMTP id 5b1f17b1804b1-4830e9552a2mr89288445e9.15.1770291769579; Thu, 05 Feb 2026 03:42:49 -0800 (PST) Date: Thu, 5 Feb 2026 11:42:46 +0000 In-Reply-To: <9d0d6edd-eab4-4f31-9691-78ed48e7ad5b@lucifer.local> Mime-Version: 1.0 References: <20260205-binder-tristate-v1-0-dfc947c35d35@google.com> <20260205-binder-tristate-v1-1-dfc947c35d35@google.com> <9d0d6edd-eab4-4f31-9691-78ed48e7ad5b@lucifer.local> Message-ID: Subject: Re: [PATCH 1/5] export file_close_fd and task_work_add From: Alice Ryhl To: Lorenzo Stoakes Cc: Greg Kroah-Hartman , Carlos Llamas , Alexander Viro , Christian Brauner , Jan Kara , Paul Moore , James Morris , "Serge E. Hallyn" , Andrew Morton , Dave Chinner , Qi Zheng , Roman Gushchin , Muchun Song , David Hildenbrand , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Miguel Ojeda , Boqun Feng , Gary Guo , "=?utf-8?B?QmrDtnJu?= Roy Baron" , Benno Lossin , Andreas Hindborg , Trevor Gross , Danilo Krummrich , kernel-team@android.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, linux-mm@kvack.org, rust-for-linux@vger.kernel.org Content-Type: text/plain; charset="utf-8" X-Rspamd-Server: rspam11 X-Stat-Signature: hghb3a661wiz3m319kwh3gbo9nw5x4ap X-Rspam-User: X-Rspamd-Queue-Id: 1D0C6100003 X-HE-Tag: 1770291770-380950 X-HE-Meta: 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 AXjY3Iwo 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 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: List-Subscribe: List-Unsubscribe: On Thu, Feb 05, 2026 at 11:20:33AM +0000, Lorenzo Stoakes wrote: > On Thu, Feb 05, 2026 at 10:51:26AM +0000, Alice Ryhl wrote: > > This exports the functionality needed by Binder to close file > > descriptors. > > > > When you send a fd over Binder, what happens is this: > > > > 1. The sending process turns the fd into a struct file and stores it in > > the transaction object. > > 2. When the receiving process gets the message, the fd is installed as a > > fd into the current process. > > 3. When the receiving process is done handling the message, it tells > > Binder to clean up the transaction. As part of this, fds embedded in > > the transaction are closed. > > > > Note that it was not always implemented like this. Previously the > > sending process would install the fd directly into the receiving proc in > > step 1, but as discussed previously [1] this is not ideal and has since > > been changed so that fd install happens during receive. > > > > The functions being exported here are for closing the fd in step 3. They > > are required because closing a fd from an ioctl is in general not safe. > > This is to meet the requirements for using fdget(), which is used by the > > ioctl framework code before calling into the driver's implementation of > > the ioctl. Binder works around this with this sequence of operations: > > > > 1. file_close_fd() > > 2. get_file() > > 3. filp_close() > > 4. task_work_add(current, TWA_RESUME) > > 5. > > 6. fput() > > > > This ensures that when fput() is called in the task work, the fdget() > > that the ioctl framework code uses has already been fdput(), so if the > > fd being closed happens to be the same fd, then the fd is not closed > > in violation of the fdget() rules. > > I'm not really familiar with this mechanism but you're already talking about > this being a workaround so strikes me the correct thing to do is to find a way > to do this in the kernel sensibly rather than exporting internal implementation > details and doing it in binder. I did previously submit a patch that implemented this logic outside of Binder, but I was advised to move it into Binder. But I'm happy to submit a patch to extract this logic into some sort of close_fd_safe() method that can be called even if said fd is currently held using fdget(). > > Link: https://lore.kernel.org/all/20180730203633.GC12962@bombadil.infradead.org/ [1] > > Signed-off-by: Alice Ryhl > > --- > > fs/file.c | 1 + > > kernel/task_work.c | 1 + > > 2 files changed, 2 insertions(+) > > > > diff --git a/fs/file.c b/fs/file.c > > index 0a4f3bdb2dec6284a0c7b9687213137f2eecb250..0046d0034bf16270cdea7e30a86866ebea3a5a81 100644 > > --- a/fs/file.c > > +++ b/fs/file.c > > @@ -881,6 +881,7 @@ struct file *file_close_fd(unsigned int fd) > > > > return file; > > } > > +EXPORT_SYMBOL(file_close_fd); > > As a matter of policy we generally don't like to export without GPL like this > unless there's a _really_ good reason. > > Christian or Al may have a different viewpoint but generally this should be an > EXPORT_SYMBOL_GPL() and also - there has to be a _really_ good reason to export > it. Sorry I should just have done _GPL from the beginning. My mistake. Alice