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

[Gary Guo]

On Mon, 29 Dec 2025 15:38:14 +0000
Alice Ryhl <aliceryhl@xxxxxxxxxx> 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 was curious and checked the C binder implementation. Apparently the C
binder implementation returns early when translating a FD array with
length 0.

Would it still make sense to do something similar in the Rust binder? The
enum change is still good to make, though.

Best,
Gary

> 
> 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@xxxxxxxxxxxxxxx
> Reported-by: DeepChirp <DeepChirp@xxxxxxxxxxx>
> Closes: https://github.com/waydroid/waydroid/issues/2157
> Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
> Tested-by: DeepChirp <DeepChirp@xxxxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>