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, 216 insertions, 0 deletions

diff --git a/tools/perf/util/libdw.c b/tools/perf/util/libdw.c
new file mode 100644
index 000000000..d5d295890
--- /dev/null
+++ b/tools/perf/util/libdw.c
@@ -0,0 +1,216 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "dso.h"
+#include "libdw.h"
+#include "srcline.h"
+#include "symbol.h"
+#include "dwarf-aux.h"
+#include "callchain.h"
+#include <fcntl.h>
+#include <unistd.h>
+#include <elfutils/libdwfl.h>
+
+static const Dwfl_Callbacks offline_callbacks = {
+	.find_debuginfo = dwfl_standard_find_debuginfo,
+	.section_address = dwfl_offline_section_address,
+	.find_elf = dwfl_build_id_find_elf,
+};
+
+void dso__free_libdw(struct dso *dso)
+{
+	Dwfl *dwfl = dso__libdw(dso);
+
+	if (dwfl) {
+		dwfl_end(dwfl);
+		dso__set_libdw(dso, NULL);
+	}
+}
+
+struct Dwfl *dso__libdw_dwfl(struct dso *dso)
+{
+	Dwfl *dwfl = dso__libdw(dso);
+	const char *dso_name;
+	Dwfl_Module *mod;
+	int fd;
+
+	if (dwfl)
+		return dwfl;
+
+	dso_name = dso__long_name(dso);
+	/*
+	 * Initialize Dwfl session.
+	 * We need to open the DSO file to report it to libdw.
+	 */
+	fd = open(dso_name, O_RDONLY);
+	if (fd < 0)
+		return NULL;
+
+	dwfl = dwfl_begin(&offline_callbacks);
+	if (!dwfl) {
+		close(fd);
+		return NULL;
+	}
+
+	/*
+	 * If the report is successful, the file descriptor fd is consumed
+	 * and closed by the Dwfl. If not, it is not closed.
+	 */
+	mod = dwfl_report_offline(dwfl, dso_name, dso_name, fd);
+	if (!mod) {
+		dwfl_end(dwfl);
+		close(fd);
+		return NULL;
+	}
+
+	if (dwfl_report_end(dwfl, /*removed=*/NULL, /*arg=*/NULL) != 0) {
+		dwfl_end(dwfl);
+		return NULL;
+	}
+	dso__set_libdw(dso, dwfl);
+
+	return dwfl;
+}
+
+struct libdw_a2l_cb_args {
+	struct dso *dso;
+	struct symbol *sym;
+	struct inline_node *node;
+	char *leaf_srcline;
+	bool leaf_srcline_used;
+	int err;
+};
+
+static int libdw_a2l_cb(Dwarf_Die *die, void *_args)
+{
+	struct libdw_a2l_cb_args *args  = _args;
+	struct symbol *inline_sym = new_inline_sym(args->dso, args->sym, die_name(die));
+	const char *call_fname = die_get_call_file(die);
+	int call_lineno = die_get_call_lineno(die);
+	char *call_srcline = srcline__unknown;
+
+	if (!inline_sym)
+		goto abort_enomem;
+
+	/* Assign caller information to the parent. */
+	if (call_fname)
+		call_srcline = srcline_from_fileline(call_fname, call_lineno >= 0 ? call_lineno : 0);
+
+	if (!list_empty(&args->node->val)) {
+		struct inline_list *parent;
+
+		if (callchain_param.order == ORDER_CALLEE)
+			parent = list_first_entry(&args->node->val, struct inline_list, list);
+		else
+			parent = list_last_entry(&args->node->val, struct inline_list, list);
+
+		if (args->leaf_srcline == parent->srcline)
+			args->leaf_srcline_used = false;
+		else if (parent->srcline != srcline__unknown)
+			free(parent->srcline);
+		parent->srcline = call_srcline;
+		call_srcline = NULL;
+	}
+	if (call_srcline && call_srcline != srcline__unknown)
+		free(call_srcline);
+
+	/* Add this symbol to the chain as the leaf. */
+	if (!args->leaf_srcline_used) {
+		if (inline_list__append_tail(inline_sym, args->leaf_srcline, args->node) != 0)
+			goto abort_delete_sym;
+		args->leaf_srcline_used = true;
+	} else {
+		char *srcline = strdup(args->leaf_srcline);
+
+		if (!srcline)
+			goto abort_delete_sym;
+		if (inline_list__append_tail(inline_sym, srcline, args->node) != 0) {
+			free(srcline);
+			goto abort_delete_sym;
+		}
+	}
+	return 0;
+
+abort_delete_sym:
+	if (symbol__inlined(inline_sym))
+		symbol__delete(inline_sym);
+abort_enomem:
+	args->err = -ENOMEM;
+	return DWARF_CB_ABORT;
+}
+
+int libdw__addr2line(u64 addr, char **file, unsigned int *line_nr,
+		     struct dso *dso, bool unwind_inlines,
+		     struct inline_node *node, struct symbol *sym)
+{
+	Dwfl *dwfl = dso__libdw_dwfl(dso);
+	Dwfl_Module *mod;
+	Dwfl_Line *dwline;
+	Dwarf_Addr bias;
+	const char *src;
+	int lineno = 0;
+
+	if (!dwfl)
+		return 0;
+
+	mod = dwfl_addrmodule(dwfl, addr);
+	if (!mod)
+		return 0;
+
+	/*
+	 * Get/ignore the dwarf information. Determine the bias, difference
+	 * between the regular ELF addr2line addresses and those to use with
+	 * libdw.
+	 */
+	if (!dwfl_module_getdwarf(mod, &bias))
+		return 0;
+
+	/* Find source line information for the address. */
+	dwline = dwfl_module_getsrc(mod, addr + bias);
+	if (!dwline)
+		return 0;
+
+	/* Get line information. */
+	src = dwfl_lineinfo(dwline, /*addr=*/NULL, &lineno, /*col=*/NULL, /*mtime=*/NULL,
+			    /*length=*/NULL);
+
+	if (file)
+		*file = src ? strdup(src) : NULL;
+	if (line_nr)
+		*line_nr = lineno;
+
+	/* Optionally unwind inline function call chain. */
+	if (unwind_inlines && node) {
+		Dwarf_Addr unused_bias;
+		Dwarf_Die *cudie = dwfl_module_addrdie(mod, addr + bias, &unused_bias);
+		struct libdw_a2l_cb_args args = {
+			.dso = dso,
+			.sym = sym,
+			.node = node,
+			.leaf_srcline = srcline_from_fileline(src ?: "<unknown>", lineno),
+		};
+
+		if (!args.leaf_srcline) {
+			if (file && *file) {
+				free(*file);
+				*file = NULL;
+			}
+			return 0;
+		}
+
+		/* Walk from the parent down to the leaf. */
+		if (cudie)
+			cu_walk_functions_at(cudie, addr, libdw_a2l_cb, &args);
+
+		if (!args.leaf_srcline_used)
+			free(args.leaf_srcline);
+
+		if (args.err) {
+			if (file && *file) {
+				free(*file);
+				*file = NULL;
+			}
+			inline_node__clear_frames(node);
+			return 0;
+		}
+	}
+	return 1;
+}