Re: [PATCH 1/5] export file_close_fd and task_work_add

[Christian Brauner <brauner@kernel.org>]

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. <binder returns from ioctl>
> 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.
> 
> Link: https://lore.kernel.org/all/20180730203633.GC12962@bombadil.infradead.org/ [1]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  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);
>  
>  void do_close_on_exec(struct files_struct *files)
>  {
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 0f7519f8e7c93f9a4536c26a341255799c320432..08eb29abaea6b98cc443d1087ddb1d0f1a38c9ae 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -102,6 +102,7 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>  
>  	return 0;
>  }
> +EXPORT_SYMBOL(task_work_add);

Uhm, no. We're not going to export task_work_add() to let random drivers
queue up work for a task when it returns to userspace. That just screams
bugs and deadlocks at full capacity. Sorry, no.