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, 80 insertions, 0 deletions

diff --git a/lib/crypto/mpi/mpi-sub-ui.c b/lib/crypto/mpi/mpi-sub-ui.c
new file mode 100644
index 000000000..0edcdbd24
--- /dev/null
+++ b/lib/crypto/mpi/mpi-sub-ui.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* mpi-sub-ui.c - Subtract an unsigned integer from an MPI.
+ *
+ * Copyright 1991, 1993, 1994, 1996, 1999-2002, 2004, 2012, 2013, 2015
+ * Free Software Foundation, Inc.
+ *
+ * This file was based on the GNU MP Library source file:
+ * https://gmplib.org/repo/gmp-6.2/file/510b83519d1c/mpz/aors_ui.h
+ *
+ * The GNU MP Library is free software; you can redistribute it and/or modify
+ * it under the terms of either:
+ *
+ *   * the GNU Lesser General Public License as published by the Free
+ *     Software Foundation; either version 3 of the License, or (at your
+ *     option) any later version.
+ *
+ * or
+ *
+ *   * the GNU General Public License as published by the Free Software
+ *     Foundation; either version 2 of the License, or (at your option) any
+ *     later version.
+ *
+ * or both in parallel, as here.
+ *
+ * The GNU MP Library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * for more details.
+ *
+ * You should have received copies of the GNU General Public License and the
+ * GNU Lesser General Public License along with the GNU MP Library.  If not,
+ * see https://www.gnu.org/licenses/.
+ */
+
+#include <linux/export.h>
+
+#include "mpi-internal.h"
+
+int mpi_sub_ui(MPI w, MPI u, unsigned long vval)
+{
+	if (u->nlimbs == 0) {
+		if (mpi_resize(w, 1) < 0)
+			return -ENOMEM;
+		w->d[0] = vval;
+		w->nlimbs = (vval != 0);
+		w->sign = (vval != 0);
+		return 0;
+	}
+
+	/* If not space for W (and possible carry), increase space. */
+	if (mpi_resize(w, u->nlimbs + 1))
+		return -ENOMEM;
+
+	if (u->sign) {
+		mpi_limb_t cy;
+
+		cy = mpihelp_add_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
+		w->d[u->nlimbs] = cy;
+		w->nlimbs = u->nlimbs + cy;
+		w->sign = 1;
+	} else {
+		/* The signs are different.  Need exact comparison to determine
+		 * which operand to subtract from which.
+		 */
+		if (u->nlimbs == 1 && u->d[0] < vval) {
+			w->d[0] = vval - u->d[0];
+			w->nlimbs = 1;
+			w->sign = 1;
+		} else {
+			mpihelp_sub_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
+			/* Size can decrease with at most one limb. */
+			w->nlimbs = (u->nlimbs - (w->d[u->nlimbs - 1] == 0));
+			w->sign = 0;
+		}
+	}
+
+	mpi_normalize(w);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mpi_sub_ui);