objtool: Add option to trace function validation

Add an option to trace and have information during the validation
of specified functions. Functions are specified with the --trace
option which can be a single function name (e.g. --trace foo to
trace the function with the name "foo"), or a shell wildcard
pattern (e.g. --trace foo* to trace all functions with a name
starting with "foo").

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://patch.msgid.link/20251121095340.464045-11-alexandre.chartre@oracle.com

tools/objtool/Build

@@ -9,6 +9,7 @@ objtool-y += elf.o
 objtool-y += objtool.o
 
 objtool-$(BUILD_DISAS) += disas.o
+objtool-$(BUILD_DISAS) += trace.o
 
 objtool-$(BUILD_ORC) += orc_gen.o orc_dump.o
 objtool-$(BUILD_KLP) += builtin-klp.o klp-diff.o klp-post-link.o

tools/objtool/builtin-check.c

@@ -103,6 +103,7 @@ static const struct option check_options[] = {
 	OPT_STRING('o',		 "output", &opts.output, "file", "output file name"),
 	OPT_BOOLEAN(0,		 "sec-address", &opts.sec_address, "print section addresses in warnings"),
 	OPT_BOOLEAN(0,		 "stats", &opts.stats, "print statistics"),
+	OPT_STRING(0,		 "trace", &opts.trace, "func", "trace function validation"),
 	OPT_BOOLEAN('v',	 "verbose", &opts.verbose, "verbose warnings"),
 	OPT_BOOLEAN(0,		 "werror", &opts.werror, "return error on warnings"),
 

tools/objtool/check.c

@@ -4,6 +4,7 @@
  */
 
 #define _GNU_SOURCE /* memmem() */
+#include <fnmatch.h>
 #include <string.h>
 #include <stdlib.h>
 #include <inttypes.h>
@@ -15,6 +16,7 @@
 #include <objtool/disas.h>
 #include <objtool/check.h>
 #include <objtool/special.h>
+#include <objtool/trace.h>
 #include <objtool/warn.h>
 #include <objtool/checksum.h>
 #include <objtool/util.h>
@@ -37,7 +39,9 @@ static struct cfi_state init_cfi;
 static struct cfi_state func_cfi;
 static struct cfi_state force_undefined_cfi;
 
-static size_t sym_name_max_len;
+struct disas_context *objtool_disas_ctx;
+
+size_t sym_name_max_len;
 
 struct instruction *find_insn(struct objtool_file *file,
 			      struct section *sec, unsigned long offset)
@@ -3556,8 +3560,10 @@ static bool skip_alt_group(struct instruction *insn)
 		return false;
 
 	/* ANNOTATE_IGNORE_ALTERNATIVE */
-	if (insn->alt_group->ignore)
+	if (insn->alt_group->ignore) {
+		TRACE_INSN(insn, "alt group ignored");
 		return true;
+	}
 
 	/*
 	 * For NOP patched with CLAC/STAC, only follow the latter to avoid
@@ -3663,6 +3669,8 @@ static void checksum_update_insn(struct objtool_file *file, struct symbol *func,
 
 static int validate_branch(struct objtool_file *file, struct symbol *func,
 			   struct instruction *insn, struct insn_state state);
+static int do_validate_branch(struct objtool_file *file, struct symbol *func,
+			      struct instruction *insn, struct insn_state state);
 
 static int validate_insn(struct objtool_file *file, struct symbol *func,
 			 struct instruction *insn, struct insn_state *statep,
@@ -3684,8 +3692,10 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		if (!insn->hint && !insn_cfi_match(insn, &statep->cfi))
 			return 1;
 
-		if (insn->visited & visited)
+		if (insn->visited & visited) {
+			TRACE_INSN(insn, "already visited");
 			return 0;
+		}
 	} else {
 		nr_insns_visited++;
 	}
@@ -3722,8 +3732,10 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 				 * It will be seen later via the
 				 * straight-line path.
 				 */
-				if (!prev_insn)
+				if (!prev_insn) {
+					TRACE_INSN(insn, "defer restore");
 					return 0;
+				}
 
 				WARN_INSN(insn, "objtool isn't smart enough to handle this CFI save/restore combo");
 				return 1;
@@ -3751,13 +3763,23 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		return 1;
 
 	if (insn->alts) {
+		int i, num_alts;
+
+		num_alts = 0;
+		for (alt = insn->alts; alt; alt = alt->next)
+			num_alts++;
+
+		i = 1;
 		for (alt = insn->alts; alt; alt = alt->next) {
+			TRACE_INSN(insn, "alternative %d/%d", i, num_alts);
 			ret = validate_branch(file, func, alt->insn, *statep);
 			if (ret) {
 				BT_INSN(insn, "(alt)");
 				return ret;
 			}
+			i++;
 		}
+		TRACE_INSN(insn, "alternative DEFAULT");
 	}
 
 	if (skip_alt_group(insn))
@@ -3769,10 +3791,16 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 	switch (insn->type) {
 
 	case INSN_RETURN:
+		TRACE_INSN(insn, "return");
 		return validate_return(func, insn, statep);
 
 	case INSN_CALL:
 	case INSN_CALL_DYNAMIC:
+		if (insn->type == INSN_CALL)
+			TRACE_INSN(insn, "call");
+		else
+			TRACE_INSN(insn, "indirect call");
+
 		ret = validate_call(file, insn, statep);
 		if (ret)
 			return ret;
@@ -3788,13 +3816,18 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 	case INSN_JUMP_CONDITIONAL:
 	case INSN_JUMP_UNCONDITIONAL:
 		if (is_sibling_call(insn)) {
+			TRACE_INSN(insn, "sibling call");
 			ret = validate_sibling_call(file, insn, statep);
 			if (ret)
 				return ret;
 
 		} else if (insn->jump_dest) {
-			ret = validate_branch(file, func,
-					      insn->jump_dest, *statep);
+			if (insn->type == INSN_JUMP_UNCONDITIONAL)
+				TRACE_INSN(insn, "unconditional jump");
+			else
+				TRACE_INSN(insn, "jump taken");
+
+			ret = validate_branch(file, func, insn->jump_dest, *statep);
 			if (ret) {
 				BT_INSN(insn, "(branch)");
 				return ret;
@@ -3804,10 +3837,12 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		if (insn->type == INSN_JUMP_UNCONDITIONAL)
 			return 0;
 
+		TRACE_INSN(insn, "jump not taken");
 		break;
 
 	case INSN_JUMP_DYNAMIC:
 	case INSN_JUMP_DYNAMIC_CONDITIONAL:
+		TRACE_INSN(insn, "indirect jump");
 		if (is_sibling_call(insn)) {
 			ret = validate_sibling_call(file, insn, statep);
 			if (ret)
@@ -3820,6 +3855,7 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		break;
 
 	case INSN_SYSCALL:
+		TRACE_INSN(insn, "syscall");
 		if (func && (!next_insn || !next_insn->hint)) {
 			WARN_INSN(insn, "unsupported instruction in callable function");
 			return 1;
@@ -3828,6 +3864,7 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		break;
 
 	case INSN_SYSRET:
+		TRACE_INSN(insn, "sysret");
 		if (func && (!next_insn || !next_insn->hint)) {
 			WARN_INSN(insn, "unsupported instruction in callable function");
 			return 1;
@@ -3836,6 +3873,7 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		return 0;
 
 	case INSN_STAC:
+		TRACE_INSN(insn, "stac");
 		if (!opts.uaccess)
 			break;
 
@@ -3848,6 +3886,7 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		break;
 
 	case INSN_CLAC:
+		TRACE_INSN(insn, "clac");
 		if (!opts.uaccess)
 			break;
 
@@ -3865,6 +3904,7 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		break;
 
 	case INSN_STD:
+		TRACE_INSN(insn, "std");
 		if (statep->df) {
 			WARN_INSN(insn, "recursive STD");
 			return 1;
@@ -3874,6 +3914,7 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		break;
 
 	case INSN_CLD:
+		TRACE_INSN(insn, "cld");
 		if (!statep->df && func) {
 			WARN_INSN(insn, "redundant CLD");
 			return 1;
@@ -3886,8 +3927,10 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 		break;
 	}
 
-	*dead_end = insn->dead_end;
+	if (insn->dead_end)
+		TRACE_INSN(insn, "dead end");
 
+	*dead_end = insn->dead_end;
 	return 0;
 }
 
@@ -3897,8 +3940,8 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
  * each instruction and validate all the rules described in
  * tools/objtool/Documentation/objtool.txt.
  */
-static int validate_branch(struct objtool_file *file, struct symbol *func,
-			   struct instruction *insn, struct insn_state state)
+static int do_validate_branch(struct objtool_file *file, struct symbol *func,
+			      struct instruction *insn, struct insn_state state)
 {
 	struct instruction *next_insn, *prev_insn = NULL;
 	bool dead_end;
@@ -3907,7 +3950,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 	if (func && func->ignore)
 		return 0;
 
-	while (1) {
+	do {
+		insn->trace = 0;
 		next_insn = next_insn_to_validate(file, insn);
 
 		if (opts.checksum && func && insn->sec)
@@ -3930,10 +3974,15 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		ret = validate_insn(file, func, insn, &state, prev_insn, next_insn,
 				    &dead_end);
-		if (dead_end)
-			break;
 
-		if (!next_insn) {
+		if (!insn->trace) {
+			if (ret)
+				TRACE_INSN(insn, "warning (%d)", ret);
+			else
+				TRACE_INSN(insn, NULL);
+		}
+
+		if (!dead_end && !next_insn) {
 			if (state.cfi.cfa.base == CFI_UNDEFINED)
 				return 0;
 			if (file->ignore_unreachables)
@@ -3947,7 +3996,20 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		prev_insn = insn;
 		insn = next_insn;
-	}
+
+	} while (!dead_end);
+
+	return ret;
+}
+
+static int validate_branch(struct objtool_file *file, struct symbol *func,
+			   struct instruction *insn, struct insn_state state)
+{
+	int ret;
+
+	trace_depth_inc();
+	ret = do_validate_branch(file, func, insn, state);
+	trace_depth_dec();
 
 	return ret;
 }
@@ -4408,10 +4470,18 @@ static int validate_symbol(struct objtool_file *file, struct section *sec,
 	if (opts.checksum)
 		checksum_init(func);
 
+	if (opts.trace && !fnmatch(opts.trace, sym->name, 0)) {
+		trace_enable();
+		TRACE("%s: validation begin\n", sym->name);
+	}
+
 	ret = validate_branch(file, func, insn, *state);
 	if (ret)
 		BT_INSN(insn, "<=== (sym)");
 
+	TRACE("%s: validation %s\n\n", sym->name, ret ? "failed" : "end");
+	trace_disable();
+
 	if (opts.checksum)
 		checksum_finish(func);
 
@@ -4823,8 +4893,6 @@ static void free_insns(struct objtool_file *file)
 		free(chunk->addr);
 }
 
-static struct disas_context *objtool_disas_ctx;
-
 const char *objtool_disas_insn(struct instruction *insn)
 {
 	struct disas_context *dctx = objtool_disas_ctx;
@@ -4846,8 +4914,10 @@ int check(struct objtool_file *file)
 	 * disassembly context to disassemble instruction or function
 	 * on warning or backtrace.
 	 */
-	if (opts.verbose || opts.backtrace) {
+	if (opts.verbose || opts.backtrace || opts.trace) {
 		disas_ctx = disas_context_create(file);
+		if (!disas_ctx)
+			opts.trace = false;
 		objtool_disas_ctx = disas_ctx;
 	}
 

tools/objtool/disas.c

@@ -308,6 +308,121 @@ char *disas_result(struct disas_context *dctx)
 	return dctx->result;
 }
 
+#define DISAS_INSN_OFFSET_SPACE		10
+#define DISAS_INSN_SPACE		60
+
+/*
+ * Print a message in the instruction flow. If insn is not NULL then
+ * the instruction address is printed in addition of the message,
+ * otherwise only the message is printed. In all cases, the instruction
+ * itself is not printed.
+ */
+static int disas_vprint(FILE *stream, struct section *sec, unsigned long offset,
+			int depth, const char *format, va_list ap)
+{
+	const char *addr_str;
+	int i, n;
+	int len;
+
+	len = sym_name_max_len + DISAS_INSN_OFFSET_SPACE;
+	if (depth < 0) {
+		len += depth;
+		depth = 0;
+	}
+
+	n = 0;
+
+	if (sec) {
+		addr_str = offstr(sec, offset);
+		n += fprintf(stream, "%6lx:  %-*s  ", offset, len, addr_str);
+		free((char *)addr_str);
+	} else {
+		len += DISAS_INSN_OFFSET_SPACE + 1;
+		n += fprintf(stream, "%-*s", len, "");
+	}
+
+	/* print vertical bars to show the code flow */
+	for (i = 0; i < depth; i++)
+		n += fprintf(stream, "| ");
+
+	if (format)
+		n += vfprintf(stream, format, ap);
+
+	return n;
+}
+
+/*
+ * Print a message in the instruction flow. If insn is not NULL then
+ * the instruction address is printed in addition of the message,
+ * otherwise only the message is printed. In all cases, the instruction
+ * itself is not printed.
+ */
+void disas_print_info(FILE *stream, struct instruction *insn, int depth,
+		      const char *format, ...)
+{
+	struct section *sec;
+	unsigned long off;
+	va_list args;
+
+	if (insn) {
+		sec = insn->sec;
+		off = insn->offset;
+	} else {
+		sec = NULL;
+		off = 0;
+	}
+
+	va_start(args, format);
+	disas_vprint(stream, sec, off, depth, format, args);
+	va_end(args);
+}
+
+/*
+ * Print an instruction address (offset and function), the instruction itself
+ * and an optional message.
+ */
+void disas_print_insn(FILE *stream, struct disas_context *dctx,
+		      struct instruction *insn, int depth,
+		      const char *format, ...)
+{
+	char fake_nop_insn[32];
+	const char *insn_str;
+	bool fake_nop;
+	va_list args;
+	int len;
+
+	/*
+	 * Alternative can insert a fake nop, sometimes with no
+	 * associated section so nothing to disassemble.
+	 */
+	fake_nop = (!insn->sec && insn->type == INSN_NOP);
+	if (fake_nop) {
+		snprintf(fake_nop_insn, 32, "<fake nop> (%d bytes)", insn->len);
+		insn_str = fake_nop_insn;
+	} else {
+		disas_insn(dctx, insn);
+		insn_str = disas_result(dctx);
+	}
+
+	/* print the instruction */
+	len = (depth + 1) * 2 < DISAS_INSN_SPACE ? DISAS_INSN_SPACE - (depth+1) * 2 : 1;
+	disas_print_info(stream, insn, depth, "%-*s", len, insn_str);
+
+	/* print message if any */
+	if (!format)
+		return;
+
+	if (strcmp(format, "\n") == 0) {
+		fprintf(stream, "\n");
+		return;
+	}
+
+	fprintf(stream, " - ");
+	va_start(args, format);
+	vfprintf(stream, format, args);
+	va_end(args);
+}
+
 /*
  * Disassemble a single instruction. Return the size of the instruction.
  */

tools/objtool/include/objtool/builtin.h

@@ -41,6 +41,7 @@ struct opts {
 	const char *output;
 	bool sec_address;
 	bool stats;
+	const char *trace;
 	bool verbose;
 	bool werror;
 };

tools/objtool/include/objtool/check.h

@@ -66,7 +66,8 @@ struct instruction {
 	    visited		: 4,
 	    no_reloc		: 1,
 	    hole		: 1,
-	    fake		: 1;
+	    fake		: 1,
+	    trace		: 1;
 		/* 9 bit hole */
 
 	struct alt_group *alt_group;
@@ -143,4 +144,7 @@ struct instruction *next_insn_same_sec(struct objtool_file *file, struct instruc
 
 const char *objtool_disas_insn(struct instruction *insn);
 
+extern size_t sym_name_max_len;
+extern struct disas_context *objtool_disas_ctx;
+
 #endif /* _CHECK_H */

tools/objtool/include/objtool/disas.h

@@ -19,6 +19,11 @@ int disas_info_init(struct disassemble_info *dinfo,
 		    const char *options);
 size_t disas_insn(struct disas_context *dctx, struct instruction *insn);
 char *disas_result(struct disas_context *dctx);
+void disas_print_info(FILE *stream, struct instruction *insn, int depth,
+		      const char *format, ...);
+void disas_print_insn(FILE *stream, struct disas_context *dctx,
+		      struct instruction *insn, int depth,
+		      const char *format, ...);
 
 #else /* DISAS */
 
@@ -51,6 +56,12 @@ static inline char *disas_result(struct disas_context *dctx)
 	return NULL;
 }
 
+static inline void disas_print_info(FILE *stream, struct instruction *insn,
+				    int depth, const char *format, ...) {}
+static inline void disas_print_insn(FILE *stream, struct disas_context *dctx,
+				    struct instruction *insn, int depth,
+				    const char *format, ...) {}
+
 #endif /* DISAS */
 
 #endif /* _DISAS_H */

tools/objtool/include/objtool/trace.h

@@ -0,0 +1,68 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates.
+ */
+
+#ifndef _TRACE_H
+#define _TRACE_H
+
+#include <objtool/check.h>
+#include <objtool/disas.h>
+
+#ifdef DISAS
+
+extern bool trace;
+extern int trace_depth;
+
+#define TRACE(fmt, ...)						\
+({	if (trace)						\
+		fprintf(stderr, fmt, ##__VA_ARGS__);		\
+})
+
+#define TRACE_INSN(insn, fmt, ...)				\
+({								\
+	if (trace) {						\
+		disas_print_insn(stderr, objtool_disas_ctx,	\
+				 insn, trace_depth - 1,	\
+				 fmt, ##__VA_ARGS__);		\
+		fprintf(stderr, "\n");				\
+		insn->trace = 1;				\
+	}							\
+})
+
+static inline void trace_enable(void)
+{
+	trace = true;
+	trace_depth = 0;
+}
+
+static inline void trace_disable(void)
+{
+	trace = false;
+}
+
+static inline void trace_depth_inc(void)
+{
+	if (trace)
+		trace_depth++;
+}
+
+static inline void trace_depth_dec(void)
+{
+	if (trace)
+		trace_depth--;
+}
+
+#else /* DISAS */
+
+#define TRACE(fmt, ...) ({})
+#define TRACE_INSN(insn, fmt, ...) ({})
+
+static inline void trace_enable(void) {}
+static inline void trace_disable(void) {}
+static inline void trace_depth_inc(void) {}
+static inline void trace_depth_dec(void) {}
+
+#endif
+
+#endif /* _TRACE_H */

tools/objtool/include/objtool/warn.h

@@ -97,6 +97,7 @@ static inline char *offstr(struct section *sec, unsigned long offset)
 		_len = (_len < 50) ? 50 - _len : 0;		\
 		WARN("  %s: " format "  %*s%s", _str, ##__VA_ARGS__, _len, "", _istr); \
 		free(_str);						\
+		__insn->trace = 1;				\
 	}							\
 })
 

tools/objtool/trace.c

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates.
+ */
+
+#include <objtool/trace.h>
+
+bool trace;
+int trace_depth;