[PATCH AUTOSEL 6.19] rust_binder: Fix build failure if !CONFIG_COMPAT

[Sasha Levin]

From: Xi Ruoyao <xry111@xxxxxxxxxxx>

[ Upstream commit 174e2a339bf731e080ced67c215ad609a677560b ]

The bindgen utility cannot handle "#define compat_ptr_ioctl NULL" in the
C header, so we need to handle this case on our own.

Simply skip this field in the initializer when !CONFIG_COMPAT as the
SAFETY comment above this initializer implies this is allowed.

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>
Closes: https://lore.kernel.org/all/CANiq72mrVzqXnAV=Hy2XBOonLHA6YQgH-ckZoc_h0VBvTGK8rA@xxxxxxxxxxxxxx/
Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Link: https://patch.msgid.link/20251209125029.1117897-1-xry111@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---

LLM Generated explanations, may be completely bogus:

Confirmed: v6.19.2 has the `compat_ptr_ioctl` usage but does NOT yet
have the `#[cfg(CONFIG_COMPAT)]` fix. This means **v6.19.x stable is
affected by the build failure when CONFIG_COMPAT is not set**.

## Classification

This is a **build fix** - one of the explicitly allowed categories for
stable backporting. Without this fix, the kernel cannot be compiled with
the Rust Binder driver enabled on configurations where `CONFIG_COMPAT`
is disabled. This is a real build failure affecting real configurations.

## Scope and Risk

- **Change size**: 1 line added (`#[cfg(CONFIG_COMPAT)]`)
- **Risk**: Extremely low. When CONFIG_COMPAT is enabled, behavior is
  identical. When disabled, the field defaults to zero (NULL), which is
  the correct behavior matching what the C equivalent would produce.
- **The SAFETY comment** in the code explicitly states that zeroed
  fields are safe for `file_operations`.

## Verification

- **git log** showed `drivers/android/binder/rust_binder_main.rs` was
  introduced by commit `eafedbc7c050c` during v6.18-rc cycle (v6.17-rc3
  + 103 commits)
- **git describe** confirmed `d4b83ba11cf22` (the compat_ptr_ioctl
  change that introduced the bug) was at v6.18-rc3 + 277 commits
- **git log v6.19.2 --grep** confirmed `d4b83ba11cf22` IS in v6.19.2
  stable
- **git log v6.18.12 --grep** confirmed `d4b83ba11cf22` is NOT in
  v6.18.12 stable (so 6.18.x is unaffected)
- **git show v6.19.2:...** confirmed v6.19.2 has the buggy
  `compat_ioctl: Some(bindings::compat_ptr_ioctl)` line without the
  `#[cfg(CONFIG_COMPAT)]` guard
- **git show v6.18.12:...** confirmed v6.18.12 uses a different approach
  (`rust_binder_compat_ioctl`) not affected by this bug
- The commit has `Reviewed-by: Alice Ryhl` (the Rust Binder maintainer)
  and was merged by Greg KH
- The lore.kernel.org link in the commit confirms the build failure was
  reported by Miguel Ojeda

## Summary

This is a minimal, single-line build fix for the Rust Binder driver in
v6.19.x stable. It fixes a real build failure when `CONFIG_COMPAT` is
not enabled, caused by bindgen's inability to handle `#define
compat_ptr_ioctl NULL`. The fix is obviously correct (zeroed default is
safe and matches intended C behavior), reviewed by the subsystem
maintainer, and has zero risk of runtime regression. Build fixes are
explicitly called out as stable-appropriate in the stable kernel rules.

The only applicable stable tree is v6.19.x (the bug-introducing commit
`d4b83ba11cf22` is not in v6.18.x). The fix applies cleanly.

**YES**

 drivers/android/binder/rust_binder_main.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/android/binder/rust_binder_main.rs b/drivers/android/binder/rust_binder_main.rs
index c79a9e7422401..9a527268f5b45 100644
--- a/drivers/android/binder/rust_binder_main.rs
+++ b/drivers/android/binder/rust_binder_main.rs
@@ -314,6 +314,7 @@ unsafe impl<T> Sync for AssertSync<T> {}
         owner: THIS_MODULE.as_ptr(),
         poll: Some(rust_binder_poll),
         unlocked_ioctl: Some(rust_binder_ioctl),
+        #[cfg(CONFIG_COMPAT)]
         compat_ioctl: Some(bindings::compat_ptr_ioctl),
         mmap: Some(rust_binder_mmap),
         open: Some(rust_binder_open),
-- 
2.51.0