Re: [PATCH] i2c: rust: fix I2cAdapter refcounts double increment

[Nicolás Antinori]

Hello Onur

On Mon, 25 May 2026 05:56:12 +0300
Onur Özkan <work@xxxxxxxxxxxxx> wrote:

> Hi Nicolás
> 
> On Sun, 24 May 2026 15:11:50 -0300
> Nicolás Antinori <nico.antinori.7@xxxxxxxxx> wrote:
> 
> > When `I2cAdapter::get` executes, it first calls
> > `bindings::i2c_get_adapter()` which increments the device and module
> > reference counts. It then takes a reference to the raw pointer and
> > converts it to an `ARef` via `.into()`.
> > 
> > The implementation of `From<&T> for ARef<T>` where `T:
> > AlwaysRefCounted` unconditionally calls `T::inc_ref()`. This leads
> > to a second call to `bindings::i2c_get_adapter()` inside
> > `I2cAdapter::inc_ref()`.
> > 
> > Since the returned `ARef` will only release a single reference when
> > dropped via `dec_ref()`, this leaks one device and module reference
> > count on every call.
> > 
> > This fix was suggested by sashiko.dev.
> 
> Including the link tag would be great.

I will include the missing Link tag in a v2. Apologies for the
oversight.

Separately, Sashiko reported a critical issue in reply to this patch:
https://sashiko.dev/#/patchset/20260524181151.24988-1-nico.antinori.7@xxxxxxxxx?part=1

Since the report indicates this is a pre-existing issue, I believe it 
is better to address it in a separate patch. I will send a v2 for this 
change shortly.

Thank you

> 
> > 
> > Signed-off-by: Nicolás Antinori <nico.antinori.7@xxxxxxxxx>
> > ---
> >  rust/kernel/i2c.rs | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
> > index 7b908f0c5a58..ca6476cf4399 100644
> > --- a/rust/kernel/i2c.rs
> > +++ b/rust/kernel/i2c.rs
> > @@ -405,7 +405,11 @@ pub fn get(index: i32) -> Result<ARef<Self>> {
> >  
> >          // SAFETY: `adapter` is non-null and points to a live
> > `i2c_adapter`. // `I2cAdapter` is #[repr(transparent)], so this
> > cast is valid.
> > -        Ok(unsafe {
> > (&*adapter.as_ptr().cast::<I2cAdapter<device::Normal>>()).into() })
> > +        Ok(unsafe {
> > +            ARef::from_raw(NonNull::new_unchecked(
> > +
> > adapter.as_ptr().cast::<I2cAdapter<device::Normal>>(),
> > +            ))
> > +        })
> >      }
> >  }
> >  
> > -- 
> > 2.53.0
> >