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, 68 insertions, 0 deletions

diff --git a/tools/perf/util/fncache.c b/tools/perf/util/fncache.c
new file mode 100644
index 000000000..bf9559c55
--- /dev/null
+++ b/tools/perf/util/fncache.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Manage a cache of file names' existence */
+#include <pthread.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <linux/compiler.h>
+#include "fncache.h"
+#include "hashmap.h"
+
+static struct hashmap *fncache;
+
+static size_t fncache__hash(long key, void *ctx __maybe_unused)
+{
+	return str_hash((const char *)key);
+}
+
+static bool fncache__equal(long key1, long key2, void *ctx __maybe_unused)
+{
+	return strcmp((const char *)key1, (const char *)key2) == 0;
+}
+
+static void fncache__init(void)
+{
+	fncache = hashmap__new(fncache__hash, fncache__equal, /*ctx=*/NULL);
+}
+
+static struct hashmap *fncache__get(void)
+{
+	static pthread_once_t fncache_once = PTHREAD_ONCE_INIT;
+
+	pthread_once(&fncache_once, fncache__init);
+
+	return fncache;
+}
+
+static bool lookup_fncache(const char *name, bool *res)
+{
+	long val;
+
+	if (!hashmap__find(fncache__get(), name, &val))
+		return false;
+
+	*res = (val != 0);
+	return true;
+}
+
+static void update_fncache(const char *name, bool res)
+{
+	char *old_key = NULL, *key = strdup(name);
+
+	if (key) {
+		hashmap__set(fncache__get(), key, res, &old_key, /*old_value*/NULL);
+		free(old_key);
+	}
+}
+
+/* No LRU, only use when bounded in some other way. */
+bool file_available(const char *name)
+{
+	bool res;
+
+	if (lookup_fncache(name, &res))
+		return res;
+	res = access(name, R_OK) == 0;
+	update_fncache(name, res);
+	return res;
+}