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, 40 insertions, 0 deletions

diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
new file mode 100644
index 000000000..cd934e31d
--- /dev/null
+++ b/fs/crypto/Kconfig
@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: GPL-2.0-only
+config FS_ENCRYPTION
+	bool "FS Encryption (Per-file encryption)"
+	select BLK_INLINE_ENCRYPTION if BLOCK
+	select BLK_INLINE_ENCRYPTION_FALLBACK if BLOCK
+	select CRYPTO
+	select CRYPTO_SKCIPHER
+	select CRYPTO_LIB_AES
+	select CRYPTO_LIB_SHA256
+	select CRYPTO_LIB_SHA512
+	select KEYS
+	help
+	  Enable encryption of files and directories.  This
+	  feature is similar to ecryptfs, but it is more memory
+	  efficient since it avoids caching the encrypted and
+	  decrypted pages in the page cache.  Currently Ext4,
+	  F2FS, UBIFS, and CephFS make use of this feature.
+
+# Filesystems supporting encryption must select this if FS_ENCRYPTION.  This
+# allows the algorithms to be built as modules when all the filesystems are,
+# whereas selecting them from FS_ENCRYPTION would force them to be built-in.
+#
+# Note: this option only pulls in the algorithms that filesystem encryption
+# needs "by default".  If userspace will use "non-default" encryption modes such
+# as Adiantum encryption, then those other modes need to be explicitly enabled
+# in the crypto API; see Documentation/filesystems/fscrypt.rst for details.
+#
+# Also note that this option only pulls in the generic implementations of the
+# algorithms, not any per-architecture optimized implementations.  It is
+# strongly recommended to enable optimized implementations too.
+config FS_ENCRYPTION_ALGS
+	tristate
+	select CRYPTO_AES
+	select CRYPTO_CBC
+	select CRYPTO_CTS
+	select CRYPTO_XTS
+
+config FS_ENCRYPTION_INLINE_CRYPT
+	bool
+	default y if FS_ENCRYPTION && BLOCK