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, 34 insertions, 0 deletions

diff --git a/fs/jbd2/Kconfig b/fs/jbd2/Kconfig
new file mode 100644
index 000000000..9c19e1512
--- /dev/null
+++ b/fs/jbd2/Kconfig
@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: GPL-2.0-only
+config JBD2
+	tristate
+	select CRC32
+	help
+	  This is a generic journaling layer for block devices that support
+	  both 32-bit and 64-bit block numbers.  It is currently used by
+	  the ext4 and OCFS2 filesystems, but it could also be used to add
+	  journal support to other file systems or block devices such
+	  as RAID or LVM.
+
+	  If you are using ext4 or OCFS2, you need to say Y here.
+	  If you are not using ext4 or OCFS2 then you will
+	  probably want to say N.
+
+	  To compile this device as a module, choose M here. The module will be
+	  called jbd2.  If you are compiling ext4 or OCFS2 into the kernel,
+	  you cannot compile this code as a module.
+
+config JBD2_DEBUG
+	bool "JBD2 (ext4) debugging support"
+	depends on JBD2
+	help
+	  If you are using the ext4 journaled file system (or
+	  potentially any other filesystem/device using JBD2), this option
+	  allows you to enable debugging output while the system is running,
+	  in order to help track down any problems you are having.
+	  By default, the debugging output will be turned off.
+
+	  If you select Y here, then you will be able to turn on debugging
+	  with "echo N > /sys/module/jbd2/parameters/jbd2_debug", where N is a
+	  number between 1 and 5. The higher the number, the more debugging
+	  output is generated.  To turn debugging off again, do
+	  "echo 0 > /sys/module/jbd2/parameters/jbd2_debug".