On Mon, 29 Dec 2025 15:38:14 +0000 Alice Ryhl aliceryhl@google.com 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@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