objtool: Improve tracing of alternative instructions

When tracing function validation, improve the reporting of
alternative instruction by more clearly showing the different
alternatives beginning and end.

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/20251121095340.464045-16-alexandre.chartre@oracle.com

Author: achartre

tools/objtool/check.c

@@ -3564,7 +3564,7 @@ static bool skip_alt_group(struct instruction *insn)
 
 	/* ANNOTATE_IGNORE_ALTERNATIVE */
 	if (insn->alt_group->ignore) {
-		TRACE_INSN(insn, "alt group ignored");
+		TRACE_ALT(insn, "alt group ignored");
 		return true;
 	}
 
@@ -3680,8 +3680,9 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 			 struct instruction *prev_insn, struct instruction *next_insn,
 			 bool *dead_end)
 {
-	/* prev_state is not used if there is no disassembly support */
+	/* prev_state and alt_name are not used if there is no disassembly support */
 	struct insn_state prev_state __maybe_unused;
+	char *alt_name __maybe_unused = NULL;
 	struct alternative *alt;
 	u8 visited;
 	int ret;
@@ -3768,23 +3769,16 @@ 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);
+			TRACE_ALT_BEGIN(insn, alt, alt_name);
 			ret = validate_branch(file, func, alt->insn, *statep);
+			TRACE_ALT_END(insn, alt, alt_name);
 			if (ret) {
 				BT_INSN(insn, "(alt)");
 				return ret;
 			}
-			i++;
 		}
-		TRACE_INSN(insn, "alternative DEFAULT");
+		TRACE_ALT_INFO_NOADDR(insn, "/ ", "DEFAULT");
 	}
 
 	if (skip_alt_group(insn))

tools/objtool/include/objtool/trace.h

@@ -19,11 +19,26 @@ extern int trace_depth;
 		fprintf(stderr, fmt, ##__VA_ARGS__);		\
 })
 
+/*
+ * Print the instruction address and a message. The instruction
+ * itself is not printed.
+ */
+#define TRACE_ADDR(insn, fmt, ...)				\
+({								\
+	if (trace) {						\
+		disas_print_info(stderr, insn, trace_depth - 1, \
+				 fmt "\n", ##__VA_ARGS__);	\
+	}							\
+})
+
+/*
+ * Print the instruction address, the instruction and a message.
+ */
 #define TRACE_INSN(insn, fmt, ...)				\
 ({								\
 	if (trace) {						\
 		disas_print_insn(stderr, objtool_disas_ctx,	\
-				 insn, trace_depth - 1,	\
+				 insn, trace_depth - 1,		\
 				 fmt, ##__VA_ARGS__);		\
 		fprintf(stderr, "\n");				\
 		insn->trace = 1;				\
@@ -36,6 +51,37 @@ extern int trace_depth;
 		trace_insn_state(insn, sprev, snext);		\
 })
 
+#define TRACE_ALT_FMT(pfx, fmt) pfx "<%s.%lx> " fmt
+#define TRACE_ALT_ARG(insn) disas_alt_type_name(insn), (insn)->offset
+
+#define TRACE_ALT(insn, fmt, ...)				\
+	TRACE_INSN(insn, TRACE_ALT_FMT("", fmt),		\
+		   TRACE_ALT_ARG(insn), ##__VA_ARGS__)
+
+#define TRACE_ALT_INFO(insn, pfx, fmt, ...)			\
+	TRACE_ADDR(insn, TRACE_ALT_FMT(pfx, fmt),		\
+		   TRACE_ALT_ARG(insn), ##__VA_ARGS__)
+
+#define TRACE_ALT_INFO_NOADDR(insn, pfx, fmt, ...)		\
+	TRACE_ADDR(NULL, TRACE_ALT_FMT(pfx, fmt),		\
+		   TRACE_ALT_ARG(insn), ##__VA_ARGS__)
+
+#define TRACE_ALT_BEGIN(insn, alt, alt_name)			\
+({								\
+	if (trace) {						\
+		alt_name = disas_alt_name(alt);			\
+		trace_alt_begin(insn, alt, alt_name);		\
+	}							\
+})
+
+#define TRACE_ALT_END(insn, alt, alt_name)			\
+({								\
+	if (trace) {						\
+		trace_alt_end(insn, alt, alt_name);		\
+		free(alt_name);					\
+	}							\
+})
+
 static inline void trace_enable(void)
 {
 	trace = true;
@@ -61,17 +107,34 @@ static inline void trace_depth_dec(void)
 
 void trace_insn_state(struct instruction *insn, struct insn_state *sprev,
 		      struct insn_state *snext);
+void trace_alt_begin(struct instruction *orig_insn, struct alternative *alt,
+		     char *alt_name);
+void trace_alt_end(struct instruction *orig_insn, struct alternative *alt,
+		   char *alt_name);
 
 #else /* DISAS */
 
 #define TRACE(fmt, ...) ({})
+#define TRACE_ADDR(insn, fmt, ...) ({})
 #define TRACE_INSN(insn, fmt, ...) ({})
 #define TRACE_INSN_STATE(insn, sprev, snext) ({})
+#define TRACE_ALT(insn, fmt, ...) ({})
+#define TRACE_ALT_INFO(insn, fmt, ...) ({})
+#define TRACE_ALT_INFO_NOADDR(insn, fmt, ...) ({})
+#define TRACE_ALT_BEGIN(insn, alt, alt_name) ({})
+#define TRACE_ALT_END(insn, alt, alt_name) ({})
+
 
 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) {}
+static inline void trace_alt_begin(struct instruction *orig_insn,
+				   struct alternative *alt,
+				   char *alt_name) {};
+static inline void trace_alt_end(struct instruction *orig_insn,
+				 struct alternative *alt,
+				 char *alt_name) {};
 
 #endif
 

tools/objtool/trace.c

@@ -146,3 +146,58 @@ void trace_insn_state(struct instruction *insn, struct insn_state *sprev,
 
 	insn->trace = 1;
 }
+
+void trace_alt_begin(struct instruction *orig_insn, struct alternative *alt,
+		     char *alt_name)
+{
+	struct instruction *alt_insn;
+	char suffix[2];
+
+	alt_insn = alt->insn;
+
+	if (alt->type == ALT_TYPE_EX_TABLE) {
+		/*
+		 * When there is an exception table then the instruction
+		 * at the original location is executed but it can cause
+		 * an exception. In that case, the execution will be
+		 * redirected to the alternative instruction.
+		 *
+		 * The instruction at the original location can have
+		 * instruction alternatives, so we just print the location
+		 * of the instruction that can cause the exception and
+		 * not the instruction itself.
+		 */
+		TRACE_ALT_INFO_NOADDR(orig_insn, "/ ", "%s for instruction at 0x%lx <%s+0x%lx>",
+				      alt_name,
+				      orig_insn->offset, orig_insn->sym->name,
+				      orig_insn->offset - orig_insn->sym->offset);
+	} else {
+		TRACE_ALT_INFO_NOADDR(orig_insn, "/ ", "%s", alt_name);
+	}
+
+	if (alt->type == ALT_TYPE_JUMP_TABLE) {
+		/*
+		 * For a jump alternative, if the default instruction is
+		 * a NOP then it is replaced with the jmp instruction,
+		 * otherwise it is replaced with a NOP instruction.
+		 */
+		trace_depth++;
+		if (orig_insn->type == INSN_NOP) {
+			suffix[0] = (orig_insn->len == 5) ? 'q' : '\0';
+			TRACE_ADDR(orig_insn, "jmp%-3s %lx <%s+0x%lx>", suffix,
+				   alt_insn->offset, alt_insn->sym->name,
+				   alt_insn->offset - alt_insn->sym->offset);
+		} else {
+			TRACE_ADDR(orig_insn, "nop%d", orig_insn->len);
+			trace_depth--;
+		}
+	}
+}
+
+void trace_alt_end(struct instruction *orig_insn, struct alternative *alt,
+		   char *alt_name)
+{
+	if (alt->type == ALT_TYPE_JUMP_TABLE && orig_insn->type == INSN_NOP)
+		trace_depth--;
+	TRACE_ALT_INFO_NOADDR(orig_insn, "\\ ", "%s", alt_name);
+}