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, 121 insertions, 0 deletions

diff --git a/samples/bpf/README.rst b/samples/bpf/README.rst
new file mode 100644
index 000000000..cabe2d216
--- /dev/null
+++ b/samples/bpf/README.rst
@@ -0,0 +1,121 @@
+eBPF sample programs
+====================
+
+This directory contains a test stubs, verifier test-suite and examples
+for using eBPF. The examples use libbpf from tools/lib/bpf.
+
+Note that the XDP-specific samples have been removed from this directory and
+moved to the xdp-tools repository: https://github.com/xdp-project/xdp-tools
+See the commit messages removing each tool from this directory for how to
+convert specific command invocations between the old samples and the utilities
+in xdp-tools.
+
+Build dependencies
+==================
+
+Compiling requires having installed:
+ * clang
+ * llvm
+ * pahole
+
+Consult :ref:`Documentation/process/changes.rst <changes>` for the minimum
+version numbers required and how to update them. Note that LLVM's tool
+'llc' must support target 'bpf', list version and supported targets with
+command: ``llc --version``
+
+Clean and configuration
+-----------------------
+
+It can be needed to clean tools, samples or kernel before trying new arch or
+after some changes (on demand)::
+
+ make -C tools clean
+ make -C samples/bpf clean
+ make clean
+
+Configure kernel, defconfig for instance
+(see "tools/testing/selftests/bpf/config" for a reference config)::
+
+ make defconfig
+
+Kernel headers
+--------------
+
+There are usually dependencies to header files of the current kernel.
+To avoid installing devel kernel headers system wide, as a normal
+user, simply call::
+
+ make headers_install
+
+This will create a local "usr/include" directory in the git/build top
+level directory, that the make system will automatically pick up first.
+
+Compiling
+=========
+
+For building the BPF samples, issue the below command from the kernel
+top level directory::
+
+ make M=samples/bpf
+
+It is also possible to call make from this directory.  This will just
+hide the invocation of make as above.
+
+Manually compiling LLVM with 'bpf' support
+------------------------------------------
+
+Since version 3.7.0, LLVM adds a proper LLVM backend target for the
+BPF bytecode architecture.
+
+By default llvm will build all non-experimental backends including bpf.
+To generate a smaller llc binary one can use::
+
+ -DLLVM_TARGETS_TO_BUILD="BPF"
+
+We recommend that developers who want the fastest incremental builds
+use the Ninja build system, you can find it in your system's package
+manager, usually the package is ninja or ninja-build.
+
+Quick sniplet for manually compiling LLVM and clang
+(build dependencies are ninja, cmake and gcc-c++)::
+
+ $ git clone https://github.com/llvm/llvm-project.git
+ $ mkdir -p llvm-project/llvm/build
+ $ cd llvm-project/llvm/build
+ $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
+            -DLLVM_ENABLE_PROJECTS="clang"    \
+            -DCMAKE_BUILD_TYPE=Release        \
+            -DLLVM_BUILD_RUNTIME=OFF
+ $ ninja
+
+It is also possible to point make to the newly compiled 'llc' or
+'clang' command via redefining LLC or CLANG on the make command line::
+
+ make M=samples/bpf LLC=~/git/llvm-project/llvm/build/bin/llc CLANG=~/git/llvm-project/llvm/build/bin/clang
+
+Cross compiling samples
+-----------------------
+In order to cross-compile, say for arm64 targets, export CROSS_COMPILE and ARCH
+environment variables before calling make. But do this before clean,
+configuration and header install steps described above. This will direct make to
+build samples for the cross target::
+
+ export ARCH=arm64
+ export CROSS_COMPILE="aarch64-linux-gnu-"
+
+Headers can be also installed on RFS of target board if need to keep them in
+sync (not necessarily and it creates a local "usr/include" directory also)::
+
+ make INSTALL_HDR_PATH=~/some_sysroot/usr headers_install
+
+Pointing LLC and CLANG is not necessarily if it's installed on HOST and have
+in its targets appropriate arm64 arch (usually it has several arches).
+Build samples::
+
+ make M=samples/bpf
+
+Or build samples with SYSROOT if some header or library is absent in toolchain,
+say libelf, providing address to file system containing headers and libs,
+can be RFS of target board::
+
+ make M=samples/bpf SYSROOT=~/some_sysroot