[PATCH v2] rust: auxiliary: validate DeviceId name length

[Georgios Androutsopoulos <georgeandrout13@gmail.com>]

`DeviceId::new()` copies `modname` and `name` into the fixed 40-byte
`auxiliary_device_id::name` array without checking that they fit. An
oversized name is caught by the array bounds check, but the error
reports an out-of-bounds index in the copy loop rather than the
constraint the caller violated.

Check the invariant explicitly instead, so the failure states the length
limit rather than an array index.

This should only be reached when constructing a device ID table, so the
failure is a compile time error. Document that intent.

Signed-off-by: Georgios Androutsopoulos <georgeandrout13@gmail.com>
---
Changes in v2:
- Drop Fixes: following feedback from Danilo Krummrich and Alexandre
  Courbot.
- Replace the `# Panics` section with a note that this is for device ID
  table construction, following feedback from Danilo Krummrich and Gary
  Guo.
- Reword the commit message so it does not suggest runtime evaluation,
  following feedback from Alexandre Courbot.
- Link to v1: https://lore.kernel.org/rust-for-linux/20260909033246.2779303-1-georgeandrout13@gmail.com/
---
 rust/kernel/auxiliary.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
index 60dfbec8f330..2ace0428e45e 100644
--- a/rust/kernel/auxiliary.rs
+++ b/rust/kernel/auxiliary.rs
@@ -137,10 +137,18 @@ macro_rules! module_auxiliary_driver {
 
 impl DeviceId {
     /// Create a new [`DeviceId`] from name.
+    ///
+    /// This is only intended to be called in const context, when constructing a
+    /// device ID table, where exceeding `AUXILIARY_NAME_SIZE` is a compile time error.
     pub const fn new(modname: &'static CStr, name: &'static CStr) -> Self {
         let name = name.to_bytes_with_nul();
         let modname = modname.to_bytes_with_nul();
 
+        assert!(
+            modname.len().saturating_add(name.len()) <= bindings::AUXILIARY_NAME_SIZE as usize,
+            "auxiliary device ID is too long"
+        );
+
         let mut id: bindings::auxiliary_device_id = pin_init::zeroed();
         let mut i = 0;
         while i < modname.len() {

base-commit: 28924df2a08f440c73991b83028032c901de2ae4
-- 
2.47.3