kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 133 insertions, 0 deletions

diff --git a/samples/rust/rust_dma.rs b/samples/rust/rust_dma.rs
new file mode 100644
index 000000000..5046b4628
--- /dev/null
+++ b/samples/rust/rust_dma.rs
@@ -0,0 +1,133 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust DMA api test (based on QEMU's `pci-testdev`).
+//!
+//! To make this driver probe, QEMU must be run with `-device pci-testdev`.
+
+use kernel::{
+    device::Core,
+    dma::{
+        Coherent,
+        DataDirection,
+        Device,
+        DmaMask, //
+    },
+    page, pci,
+    prelude::*,
+    scatterlist::{Owned, SGTable},
+    sync::aref::ARef,
+};
+
+#[pin_data(PinnedDrop)]
+struct DmaSampleDriver {
+    pdev: ARef<pci::Device>,
+    ca: Coherent<[MyStruct]>,
+    #[pin]
+    sgt: SGTable<Owned<VVec<u8>>>,
+}
+
+const TEST_VALUES: [(u32, u32); 5] = [
+    (0xa, 0xb),
+    (0xc, 0xd),
+    (0xe, 0xf),
+    (0xab, 0xba),
+    (0xcd, 0xef),
+];
+
+struct MyStruct {
+    h: u32,
+    b: u32,
+}
+
+impl MyStruct {
+    fn new(h: u32, b: u32) -> Self {
+        Self { h, b }
+    }
+}
+// SAFETY: All bit patterns are acceptable values for `MyStruct`.
+unsafe impl kernel::transmute::AsBytes for MyStruct {}
+// SAFETY: Instances of `MyStruct` have no uninitialized portions.
+unsafe impl kernel::transmute::FromBytes for MyStruct {}
+
+kernel::pci_device_table!(
+    PCI_TABLE,
+    MODULE_PCI_TABLE,
+    <DmaSampleDriver as pci::Driver>::IdInfo,
+    [(pci::DeviceId::from_id(pci::Vendor::REDHAT, 0x5), ())]
+);
+
+impl pci::Driver for DmaSampleDriver {
+    type IdInfo = ();
+    type Data<'bound> = Self;
+    const ID_TABLE: pci::IdTable<Self::IdInfo> = &PCI_TABLE;
+
+    fn probe<'bound>(
+        pdev: &'bound pci::Device<Core<'_>>,
+        _info: &'bound Self::IdInfo,
+    ) -> impl PinInit<Self, Error> + 'bound {
+        pin_init::pin_init_scope(move || {
+            dev_info!(pdev, "Probe DMA test driver.\n");
+
+            let mask = DmaMask::new::<64>();
+
+            // SAFETY: There are no concurrent calls to DMA allocation and mapping primitives.
+            unsafe { pdev.dma_set_mask_and_coherent(mask)? };
+
+            let ca: Coherent<[MyStruct]> =
+                Coherent::zeroed_slice(pdev.as_ref(), TEST_VALUES.len(), GFP_KERNEL)?;
+
+            for (i, value) in TEST_VALUES.into_iter().enumerate() {
+                kernel::dma_write!(ca, [try: i], MyStruct::new(value.0, value.1));
+            }
+
+            let size = 4 * page::PAGE_SIZE;
+            let pages = VVec::with_capacity(size, GFP_KERNEL)?;
+
+            let sgt = SGTable::new(pdev.as_ref(), pages, DataDirection::ToDevice, GFP_KERNEL);
+
+            Ok(try_pin_init!(Self {
+                pdev: pdev.into(),
+                ca,
+                sgt <- sgt,
+            }))
+        })
+    }
+}
+
+impl DmaSampleDriver {
+    fn check_dma(&self) {
+        for (i, value) in TEST_VALUES.into_iter().enumerate() {
+            let val0 = kernel::dma_read!(self.ca, [panic: i].h);
+            let val1 = kernel::dma_read!(self.ca, [panic: i].b);
+
+            assert_eq!(val0, value.0);
+            assert_eq!(val1, value.1);
+        }
+    }
+}
+
+#[pinned_drop]
+impl PinnedDrop for DmaSampleDriver {
+    fn drop(self: Pin<&mut Self>) {
+        dev_info!(self.pdev, "Unload DMA test driver.\n");
+
+        self.check_dma();
+
+        for (i, entry) in self.sgt.iter().enumerate() {
+            dev_info!(
+                self.pdev,
+                "Entry[{}]: DMA address: {:#x}",
+                i,
+                entry.dma_address(),
+            );
+        }
+    }
+}
+
+kernel::module_pci_driver! {
+    type: DmaSampleDriver,
+    name: "rust_dma",
+    authors: ["Abdiel Janulgue"],
+    description: "Rust DMA test",
+    license: "GPL v2",
+}