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, 21 insertions, 0 deletions

diff --git a/fs/bfs/Kconfig b/fs/bfs/Kconfig
new file mode 100644
index 000000000..8e7ef866b
--- /dev/null
+++ b/fs/bfs/Kconfig
@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: GPL-2.0-only
+config BFS_FS
+	tristate "BFS file system support"
+	depends on BLOCK
+	select BUFFER_HEAD
+	help
+	  Boot File System (BFS) is a file system used under SCO UnixWare to
+	  allow the bootloader access to the kernel image and other important
+	  files during the boot process.  It is usually mounted under /stand
+	  and corresponds to the slice marked as "STAND" in the UnixWare
+	  partition.  You should say Y if you want to read or write the files
+	  on your /stand slice from within Linux.  You then also need to say Y
+	  to "UnixWare slices support", below.  More information about the BFS
+	  file system is contained in the file
+	  <file:Documentation/filesystems/bfs.rst>.
+
+	  If you don't know what this is about, say N.
+
+	  To compile this as a module, choose M here: the module will be called
+	  bfs.  Note that the file system of your root partition (the one
+	  containing the directory /) cannot be compiled as a module.