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, 116 insertions, 0 deletions

diff --git a/tools/perf/Documentation/Build.txt b/tools/perf/Documentation/Build.txt
new file mode 100644
index 000000000..57b226e7f
--- /dev/null
+++ b/tools/perf/Documentation/Build.txt
@@ -0,0 +1,116 @@
+
+1) perf build
+=============
+The perf build process consists of several separated building blocks,
+which are linked together to form the perf binary:
+  - libperf library (static)
+  - perf builtin commands
+  - traceevent library (static)
+  - GTK ui library
+
+Several makefiles govern the perf build:
+
+  - Makefile
+    top level Makefile working as a wrapper that calls the main
+    Makefile.perf with a -j option to do parallel builds.
+
+  - Makefile.perf
+    main makefile that triggers build of all perf objects including
+    installation and documentation processing.
+
+  - tools/build/Makefile.build
+    main makefile of the build framework
+
+  - tools/build/Build.include
+    build framework generic definitions
+
+  - Build makefiles
+    makefiles that defines build objects
+
+Please refer to tools/build/Documentation/Build.txt for more
+information about build framework.
+
+
+2) perf build
+=============
+The Makefile.perf triggers the build framework for build objects:
+   perf, libperf, gtk
+
+resulting in following objects:
+  $ ls  *-in.o
+  gtk-in.o  libperf-in.o  perf-in.o
+
+Those objects are then used in final linking:
+  libperf-gtk.so <- gtk-in.o  libperf-in.o
+  perf           <- perf-in.o libperf-in.o
+
+
+NOTE this description is omitting other libraries involved, only
+     focusing on build framework outcomes
+
+3) Build with ASan or UBSan
+==========================
+  $ cd tools/perf
+  $ make DESTDIR=/usr
+  $ make DESTDIR=/usr install
+
+AddressSanitizer (or ASan) is a GCC feature that detects memory corruption bugs
+such as buffer overflows and memory leaks.
+
+  $ cd tools/perf
+  $ make DEBUG=1 EXTRA_CFLAGS='-fno-omit-frame-pointer -fsanitize=address'
+  $ ASAN_OPTIONS=log_path=asan.log ./perf record -a
+
+ASan outputs all detected issues into a log file named 'asan.log.<pid>'.
+
+UndefinedBehaviorSanitizer (or UBSan) is a fast undefined behavior detector
+supported by GCC. UBSan detects undefined behaviors of programs at runtime.
+
+  $ cd tools/perf
+  $ make DEBUG=1 EXTRA_CFLAGS='-fno-omit-frame-pointer -fsanitize=undefined'
+  $ UBSAN_OPTIONS=print_stacktrace=1 ./perf record -a
+
+If UBSan detects any problem at runtime, it outputs a “runtime error:” message.
+
+4) Cross compilation
+====================
+As Multiarch is commonly supported in Linux distributions, we can install
+libraries for multiple architectures on the same system and then cross-compile
+Linux perf. For example, Aarch64 libraries and toolchains can be installed on
+an x86_64 machine, allowing us to compile perf for an Aarch64 target.
+
+Below is the command for building the perf with dynamic linking.
+
+  $ cd /path/to/Linux
+  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -C tools/perf
+
+For static linking, the option `LDFLAGS="-static"` is required.
+
+  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
+    LDFLAGS="-static" -C tools/perf
+
+In the embedded system world, a use case is to explicitly specify the package
+configuration paths for cross building:
+
+  $ PKG_CONFIG_SYSROOT_DIR="/path/to/cross/build/sysroot" \
+    PKG_CONFIG_LIBDIR="/usr/lib/:/usr/local/lib" \
+    make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -C tools/perf
+
+In this case, the variable PKG_CONFIG_SYSROOT_DIR can be used alongside the
+variable PKG_CONFIG_LIBDIR or PKG_CONFIG_PATH to prepend the sysroot path to
+the library paths for cross compilation.
+
+5) Build with Clang
+===================
+By default, the makefile uses GCC as compiler. With specifying environment
+variables HOSTCC, CC and CXX, it allows to build perf with Clang.
+
+Using Clang for a native build:
+
+  $ HOSTCC=clang CC=clang CXX=clang++ make -C tools/perf
+
+Specifying ARCH and CROSS_COMPILE for cross compilation:
+
+  $ HOSTCC=clang CC=clang CXX=clang++ \
+    ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
+    make -C tools/perf