Re: [PATCH] rust_binder: correctly handle FDA objects of length zero

[Carlos Llamas <cmllamas@google.com>]

On Mon, Dec 29, 2025 at 03:38:14PM +0000, Alice Ryhl wrote:
> Fix a bug where an empty FDA (fd array) object with 0 fds would cause an
> out-of-bounds error. The previous implementation used `skip == 0` to
> mean "this is a pointer fixup", but 0 is also the correct skip length
> for an empty FDA. If the FDA is at the end of the buffer, then this
> results in an attempt to write 8-bytes out of bounds. This is caught and
> results in an EINVAL error being returned to userspace.
> 
> The pattern of using `skip == 0` as a special value originates from the
> C-implementation of Binder. As part of fixing this bug, this pattern is
> replaced with a Rust enum.
> 
> I considered the alternate option of not pushing a fixup when the length
> is zero, but I think it's cleaner to just get rid of the zero-is-special
> stuff.
> 
> The root cause of this bug was diagnosed by Gemini CLI on first try. I
> used the following prompt:
> 
> > There appears to be a bug in @drivers/android/binder/thread.rs where
> > the Fixups oob bug is triggered with 316 304 316 324. This implies
> > that we somehow ended up with a fixup where buffer A has a pointer to
> > buffer B, but the pointer is located at an index in buffer A that is
> > out of bounds. Please investigate the code to find the bug. You may
> > compare with @drivers/android/binder.c that implements this correctly.
> 
> Cc: stable@vger.kernel.org
> Reported-by: DeepChirp <DeepChirp@outlook.com>
> Closes: https://github.com/waydroid/waydroid/issues/2157
> Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
> Tested-by: DeepChirp <DeepChirp@outlook.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---

Acked-by: Carlos Llamas <cmllamas@google.com>