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, 42 insertions, 0 deletions

diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
new file mode 100644
index 000000000..7b98f5d1e
--- /dev/null
+++ b/security/bpf/hooks.c
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (C) 2020 Google LLC.
+ */
+#include <linux/lsm_hooks.h>
+#include <linux/bpf_lsm.h>
+#include <uapi/linux/lsm.h>
+
+bool bpf_lsm_initialized __ro_after_init;
+
+static struct security_hook_list bpf_lsm_hooks[] __ro_after_init = {
+	#define LSM_HOOK(RET, DEFAULT, NAME, ...) \
+	LSM_HOOK_INIT(NAME, bpf_lsm_##NAME),
+	#include <linux/lsm_hook_defs.h>
+	#undef LSM_HOOK
+	LSM_HOOK_INIT(inode_free_security, bpf_inode_storage_free),
+};
+
+static const struct lsm_id bpf_lsmid = {
+	.name = "bpf",
+	.id = LSM_ID_BPF,
+};
+
+static int __init bpf_lsm_init(void)
+{
+	security_add_hooks(bpf_lsm_hooks, ARRAY_SIZE(bpf_lsm_hooks),
+			   &bpf_lsmid);
+	bpf_lsm_initialized = true;
+	pr_info("LSM support for eBPF active\n");
+	return 0;
+}
+
+struct lsm_blob_sizes bpf_lsm_blob_sizes __ro_after_init = {
+	.lbs_inode = sizeof(struct bpf_storage_blob),
+};
+
+DEFINE_LSM(bpf) = {
+	.id = &bpf_lsmid,
+	.init = bpf_lsm_init,
+	.blobs = &bpf_lsm_blob_sizes
+};