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, 391 insertions, 0 deletions

diff --git a/tools/perf/util/expr.y b/tools/perf/util/expr.y
new file mode 100644
index 000000000..e20f64935
--- /dev/null
+++ b/tools/perf/util/expr.y
@@ -0,0 +1,391 @@
+/* Simple expression parser */
+%{
+#ifndef NDEBUG
+#define YYDEBUG 1
+#endif
+#include <assert.h>
+#include <math.h>
+#include <stdlib.h>
+#include "util/debug.h"
+#define IN_EXPR_Y 1
+#include "expr.h"
+#include "expr-bison.h"
+int expr_lex(YYSTYPE * yylval_param , void *yyscanner);
+%}
+
+%define api.pure full
+
+%parse-param { double *final_val }
+%parse-param { struct expr_parse_ctx *ctx }
+%parse-param { bool compute_ids }
+%parse-param {void *scanner}
+%lex-param {void* scanner}
+
+%union {
+	double	 num;
+	char	*str;
+	struct ids {
+		/*
+		 * When creating ids, holds the working set of event ids. NULL
+		 * implies the set is empty.
+		 */
+		struct hashmap *ids;
+		/*
+		 * The metric value. When not creating ids this is the value
+		 * read from a counter, a constant or some computed value. When
+		 * creating ids the value is either a constant or BOTTOM. NAN is
+		 * used as the special BOTTOM value, representing a "set of all
+		 * values" case.
+		 */
+		double val;
+	} ids;
+}
+
+%token ID NUMBER MIN MAX IF ELSE LITERAL D_RATIO SOURCE_COUNT AGGR_NR HAS_EVENT STRCMP_CPUID_STR EXPR_ERROR
+%left MIN MAX IF
+%left '|'
+%left '^'
+%left '&'
+%left '<' '>'
+%left '-' '+'
+%left '*' '/' '%'
+%left NEG NOT
+%type <num> NUMBER LITERAL
+%type <str> ID
+%destructor { free ($$); } <str>
+%type <ids> expr if_expr
+%destructor { ids__free($$.ids); } <ids>
+
+%{
+static void expr_error(double *final_val __maybe_unused,
+		       struct expr_parse_ctx *ctx __maybe_unused,
+		       bool compute_ids __maybe_unused,
+		       void *scanner __maybe_unused,
+		       const char *s)
+{
+	pr_debug("%s\n", s);
+}
+
+/*
+ * During compute ids, the special "bottom" value uses NAN to represent the set
+ * of all values. NAN is selected as it isn't a useful constant value.
+ */
+#define BOTTOM NAN
+
+/* During computing ids, does val represent a constant (non-BOTTOM) value? */
+static bool is_const(double val)
+{
+	return isfinite(val);
+}
+
+static struct ids union_expr(struct ids ids1, struct ids ids2)
+{
+	struct ids result = {
+		.val = BOTTOM,
+		.ids = ids__union(ids1.ids, ids2.ids),
+	};
+	return result;
+}
+
+enum expr_id_kind {
+	EXPR_ID_KIND__VALUE,
+	EXPR_ID_KIND__SOURCE_COUNT,
+	EXPR_ID_KIND__AGGR_NR,
+};
+
+static struct ids handle_id(struct expr_parse_ctx *ctx, char *id,
+			    bool compute_ids, enum expr_id_kind kind)
+{
+	struct ids result;
+
+	if (!compute_ids) {
+		/*
+		 * Compute the event's value from ID. If the ID isn't known then
+		 * it isn't used to compute the formula so set to NAN.
+		 */
+		struct expr_id_data *data;
+
+		result.val = NAN;
+		if (expr__resolve_id(ctx, id, &data) == 0) {
+			if (kind == EXPR_ID_KIND__SOURCE_COUNT)
+				result.val = expr_id_data__source_count(data);
+			else if (kind == EXPR_ID_KIND__AGGR_NR)
+				result.val = expr_id_data__aggr_nr(data);
+			else
+				result.val = expr_id_data__value(data);
+		}
+		result.ids = NULL;
+		free(id);
+	} else {
+		/*
+		 * Set the value to BOTTOM to show that any value is possible
+		 * when the event is computed. Create a set of just the ID.
+		 */
+		result.val = BOTTOM;
+		result.ids = ids__new();
+		if (!result.ids || ids__insert(result.ids, id)) {
+			pr_err("Error creating IDs for '%s'", id);
+			free(id);
+		}
+	}
+	return result;
+}
+
+/*
+ * If we're not computing ids or $1 and $3 are constants, compute the new
+ * constant value using OP. Its invariant that there are no ids.  If computing
+ * ids for non-constants union the set of IDs that must be computed.
+ */
+#define BINARY_OP(RESULT, OP, LHS, RHS)					\
+	if (!compute_ids || (is_const(LHS.val) && is_const(RHS.val))) { \
+		assert(LHS.ids == NULL);				\
+		assert(RHS.ids == NULL);				\
+		if (isnan(LHS.val) || isnan(RHS.val)) {			\
+			RESULT.val = NAN;				\
+		} else {						\
+			RESULT.val = LHS.val OP RHS.val;		\
+		}							\
+		RESULT.ids = NULL;					\
+	} else {							\
+	        RESULT = union_expr(LHS, RHS);				\
+	}
+
+%}
+%%
+
+start: if_expr
+{
+	if (compute_ids)
+		ctx->ids = ids__union($1.ids, ctx->ids);
+
+	if (final_val)
+		*final_val = $1.val;
+}
+;
+
+if_expr: expr IF expr ELSE if_expr
+{
+	if (fpclassify($3.val) == FP_ZERO) {
+		/*
+		 * The IF expression evaluated to 0 so treat as false, take the
+		 * ELSE and discard everything else.
+		 */
+		$$.val = $5.val;
+		$$.ids = $5.ids;
+		ids__free($1.ids);
+		ids__free($3.ids);
+	} else if (!compute_ids || is_const($3.val)) {
+		/*
+		 * If ids aren't computed then treat the expression as true. If
+		 * ids are being computed and the IF expr is a non-zero
+		 * constant, then also evaluate the true case.
+		 */
+		$$.val = $1.val;
+		$$.ids = $1.ids;
+		ids__free($3.ids);
+		ids__free($5.ids);
+	} else if ($1.val == $5.val) {
+		/*
+		 * LHS == RHS, so both are an identical constant. No need to
+		 * evaluate any events.
+		 */
+		$$.val = $1.val;
+		$$.ids = NULL;
+		ids__free($1.ids);
+		ids__free($3.ids);
+		ids__free($5.ids);
+	} else {
+		/*
+		 * Value is either the LHS or RHS and we need the IF expression
+		 * to compute it.
+		 */
+		$$ = union_expr($1, union_expr($3, $5));
+	}
+}
+| expr
+;
+
+expr: NUMBER
+{
+	$$.val = $1;
+	$$.ids = NULL;
+}
+| ID				{ $$ = handle_id(ctx, $1, compute_ids, EXPR_ID_KIND__VALUE); }
+| SOURCE_COUNT '(' ID ')'	{ $$ = handle_id(ctx, $3, compute_ids, EXPR_ID_KIND__SOURCE_COUNT); }
+| AGGR_NR '(' ID ')'		{ $$ = handle_id(ctx, $3, compute_ids, EXPR_ID_KIND__AGGR_NR); }
+| HAS_EVENT '(' ID ')'
+{
+	$$.val = expr__has_event(ctx, compute_ids, $3);
+	$$.ids = NULL;
+	free($3);
+}
+| STRCMP_CPUID_STR '(' ID ')'
+{
+	$$.val = expr__strcmp_cpuid_str(ctx, compute_ids, $3);
+	$$.ids = NULL;
+	free($3);
+}
+| expr '|' expr
+{
+	if (is_const($1.val) && is_const($3.val)) {
+		assert($1.ids == NULL);
+		assert($3.ids == NULL);
+		$$.ids = NULL;
+		$$.val = (fpclassify($1.val) == FP_ZERO && fpclassify($3.val) == FP_ZERO) ? 0 : 1;
+	} else if (is_const($1.val)) {
+		assert($1.ids == NULL);
+		if (fpclassify($1.val) == FP_ZERO) {
+			$$ = $3;
+		} else {
+			$$.val = 1;
+			$$.ids = NULL;
+			ids__free($3.ids);
+		}
+	} else if (is_const($3.val)) {
+		assert($3.ids == NULL);
+		if (fpclassify($3.val) == FP_ZERO) {
+			$$ = $1;
+		} else {
+			$$.val = 1;
+			$$.ids = NULL;
+			ids__free($1.ids);
+		}
+	} else {
+		$$ = union_expr($1, $3);
+	}
+}
+| expr '&' expr
+{
+	if (is_const($1.val) && is_const($3.val)) {
+		assert($1.ids == NULL);
+		assert($3.ids == NULL);
+		$$.val = (fpclassify($1.val) != FP_ZERO && fpclassify($3.val) != FP_ZERO) ? 1 : 0;
+		$$.ids = NULL;
+	} else if (is_const($1.val)) {
+		assert($1.ids == NULL);
+		if (fpclassify($1.val) != FP_ZERO) {
+			$$ = $3;
+		} else {
+			$$.val = 0;
+			$$.ids = NULL;
+			ids__free($3.ids);
+		}
+	} else if (is_const($3.val)) {
+		assert($3.ids == NULL);
+		if (fpclassify($3.val) != FP_ZERO) {
+			$$ = $1;
+		} else {
+			$$.val = 0;
+			$$.ids = NULL;
+			ids__free($1.ids);
+		}
+	} else {
+		$$ = union_expr($1, $3);
+	}
+}
+| expr '^' expr
+{
+	if (is_const($1.val) && is_const($3.val)) {
+		assert($1.ids == NULL);
+		assert($3.ids == NULL);
+		$$.val = (fpclassify($1.val) == FP_ZERO) != (fpclassify($3.val) == FP_ZERO) ? 1 : 0;
+		$$.ids = NULL;
+	} else {
+		$$ = union_expr($1, $3);
+	}
+}
+| expr '<' expr { BINARY_OP($$, <, $1, $3); }
+| expr '>' expr { BINARY_OP($$, >, $1, $3); }
+| expr '+' expr { BINARY_OP($$, +, $1, $3); }
+| expr '-' expr { BINARY_OP($$, -, $1, $3); }
+| expr '*' expr { BINARY_OP($$, *, $1, $3); }
+| expr '/' expr
+{
+	if (fpclassify($3.val) == FP_ZERO) {
+		pr_debug("division by zero\n");
+		assert($3.ids == NULL);
+		if (compute_ids)
+			ids__free($1.ids);
+		$$.val = NAN;
+		$$.ids = NULL;
+	} else if (!compute_ids || (is_const($1.val) && is_const($3.val))) {
+		assert($1.ids == NULL);
+		assert($3.ids == NULL);
+		$$.val = $1.val / $3.val;
+		$$.ids = NULL;
+	} else {
+		/* LHS and/or RHS need computing from event IDs so union. */
+		$$ = union_expr($1, $3);
+	}
+}
+| expr '%' expr
+{
+	if (fpclassify($3.val) == FP_ZERO) {
+		pr_debug("division by zero\n");
+		YYABORT;
+	} else if (!compute_ids || (is_const($1.val) && is_const($3.val))) {
+		assert($1.ids == NULL);
+		assert($3.ids == NULL);
+		$$.val = (long)$1.val % (long)$3.val;
+		$$.ids = NULL;
+	} else {
+		/* LHS and/or RHS need computing from event IDs so union. */
+		$$ = union_expr($1, $3);
+	}
+}
+| D_RATIO '(' expr ',' expr ')'
+{
+	if (fpclassify($5.val) == FP_ZERO) {
+		/*
+		 * Division by constant zero always yields zero and no events
+		 * are necessary.
+		 */
+		assert($5.ids == NULL);
+		$$.val = 0.0;
+		$$.ids = NULL;
+		ids__free($3.ids);
+	} else if (!compute_ids || (is_const($3.val) && is_const($5.val))) {
+		assert($3.ids == NULL);
+		assert($5.ids == NULL);
+		$$.val = $3.val / $5.val;
+		$$.ids = NULL;
+	} else {
+		/* LHS and/or RHS need computing from event IDs so union. */
+		$$ = union_expr($3, $5);
+	}
+}
+| '-' expr %prec NEG
+{
+	$$.val = -$2.val;
+	$$.ids = $2.ids;
+}
+| '(' if_expr ')'
+{
+	$$ = $2;
+}
+| MIN '(' expr ',' expr ')'
+{
+	if (!compute_ids) {
+		$$.val = $3.val < $5.val ? $3.val : $5.val;
+		$$.ids = NULL;
+	} else {
+		$$ = union_expr($3, $5);
+	}
+}
+| MAX '(' expr ',' expr ')'
+{
+	if (!compute_ids) {
+		$$.val = $3.val > $5.val ? $3.val : $5.val;
+		$$.ids = NULL;
+	} else {
+		$$ = union_expr($3, $5);
+	}
+}
+| LITERAL
+{
+	$$.val = $1;
+	$$.ids = NULL;
+}
+;
+
+%%