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

[Alice Ryhl <aliceryhl@google.com>]

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. <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.
> 
> 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 <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);
> 
> 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