objtool: Print headers for alternatives

When using the --disas option, objtool doesn't currently disassemble
any alternative. Print an header for each alternative. This identifies
places where alternatives are present but alternative code is still
not disassembled at the moment.

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/20251121095340.464045-20-alexandre.chartre@oracle.com

Author: achartre

tools/objtool/disas.c

@@ -29,6 +29,43 @@ struct disas_context {
 	struct disassemble_info info;
 };
 
+/*
+ * Maximum number of alternatives
+ */
+#define DISAS_ALT_MAX		5
+
+/*
+ * Maximum number of instructions per alternative
+ */
+#define DISAS_ALT_INSN_MAX	50
+
+/*
+ * Information to disassemble an alternative
+ */
+struct disas_alt {
+	struct instruction *orig_insn;		/* original instruction */
+	struct alternative *alt;		/* alternative or NULL if default code */
+	char *name;				/* name for this alternative */
+	int width;				/* formatting width */
+};
+
+/*
+ * Wrapper around asprintf() to allocate and format a string.
+ * Return the allocated string or NULL on error.
+ */
+static char *strfmt(const char *fmt, ...)
+{
+	va_list ap;
+	char *str;
+	int rv;
+
+	va_start(ap, fmt);
+	rv = vasprintf(&str, fmt, ap);
+	va_end(ap);
+
+	return rv == -1 ? NULL : str;
+}
+
 static int sprint_name(char *str, const char *name, unsigned long offset)
 {
 	int len;
@@ -314,6 +351,9 @@ char *disas_result(struct disas_context *dctx)
 #define DISAS_INSN_OFFSET_SPACE		10
 #define DISAS_INSN_SPACE		60
 
+#define DISAS_PRINSN(dctx, insn, depth)			\
+	disas_print_insn(stdout, dctx, insn, depth, "\n")
+
 /*
  * Print a message in the instruction flow. If insn is not NULL then
  * the instruction address is printed in addition of the message,
@@ -354,6 +394,19 @@ static int disas_vprint(FILE *stream, struct section *sec, unsigned long offset,
 	return n;
 }
 
+static int disas_print(FILE *stream, struct section *sec, unsigned long offset,
+			int depth, const char *format, ...)
+{
+	va_list args;
+	int len;
+
+	va_start(args, format);
+	len = disas_vprint(stream, sec, offset, depth, format, args);
+	va_end(args);
+
+	return len;
+}
+
 /*
  * Print a message in the instruction flow. If insn is not NULL then
  * the instruction address is printed in addition of the message,
@@ -523,21 +576,144 @@ char *disas_alt_name(struct alternative *alt)
 	return str;
 }
 
+/*
+ * Initialize an alternative. The default alternative should be initialized
+ * with alt=NULL.
+ */
+static int disas_alt_init(struct disas_alt *dalt,
+			  struct instruction *orig_insn,
+			  struct alternative *alt)
+{
+	dalt->orig_insn = orig_insn;
+	dalt->alt = alt;
+	dalt->name = alt ? disas_alt_name(alt) : strdup("DEFAULT");
+	if (!dalt->name)
+		return -1;
+	dalt->width = strlen(dalt->name);
+
+	return 0;
+}
+
+/*
+ * Print all alternatives one above the other.
+ */
+static void disas_alt_print_compact(char *alt_name, struct disas_alt *dalts,
+				    int alt_count)
+{
+	struct instruction *orig_insn;
+	int len;
+	int i;
+
+	orig_insn = dalts[0].orig_insn;
+
+	len = disas_print(stdout, orig_insn->sec, orig_insn->offset, 0, NULL);
+	printf("%s\n", alt_name);
+
+	for (i = 0; i < alt_count; i++)
+		printf("%*s= %s\n", len, "", dalts[i].name);
+}
+
+/*
+ * Disassemble an alternative.
+ *
+ * Return the last instruction in the default alternative so that
+ * disassembly can continue with the next instruction. Return NULL
+ * on error.
+ */
+static void *disas_alt(struct disas_context *dctx,
+		       struct instruction *orig_insn)
+{
+	struct disas_alt dalts[DISAS_ALT_MAX] = { 0 };
+	struct alternative *alt;
+	int alt_count = 0;
+	char *alt_name;
+	int err;
+	int i;
+
+	alt_name = strfmt("<%s.%lx>", disas_alt_type_name(orig_insn),
+			  orig_insn->offset);
+	if (!alt_name) {
+		WARN("Failed to define name for alternative at instruction 0x%lx",
+		     orig_insn->offset);
+		goto done;
+	}
+
+	/*
+	 * Initialize the default alternative.
+	 */
+	err = disas_alt_init(&dalts[0], orig_insn, NULL);
+	if (err) {
+		WARN("%s: failed to initialize default alternative", alt_name);
+		goto done;
+	}
+
+	/*
+	 * Initialize all other alternatives.
+	 */
+	i = 1;
+	for (alt = orig_insn->alts; alt; alt = alt->next) {
+		if (i >= DISAS_ALT_MAX) {
+			WARN("%s has more alternatives than supported", alt_name);
+			break;
+		}
+		err = disas_alt_init(&dalts[i], orig_insn, alt);
+		if (err) {
+			WARN("%s: failed to disassemble alternative", alt_name);
+			goto done;
+		}
+
+		i++;
+	}
+	alt_count = i;
+
+	/*
+	 * Print default and non-default alternatives.
+	 *
+	 * At the moment, this just prints an header for each alternative.
+	 */
+	disas_alt_print_compact(alt_name, dalts, alt_count);
+
+done:
+	for (i = 0; i < alt_count; i++)
+		free(dalts[i].name);
+
+	free(alt_name);
+
+	/*
+	 * Currently we are not disassembling any alternative but just
+	 * printing alternative names. Return NULL to have disas_func()
+	 * resume the disassembly with the default alternative.
+	 */
+	return NULL;
+}
+
 /*
  * Disassemble a function.
  */
 static void disas_func(struct disas_context *dctx, struct symbol *func)
 {
+	struct instruction *insn_start;
 	struct instruction *insn;
-	size_t addr;
 
 	printf("%s:\n", func->name);
 	sym_for_each_insn(dctx->file, func, insn) {
-		addr = insn->offset;
-		disas_insn(dctx, insn);
-		printf(" %6lx:  %s+0x%-6lx      %s\n",
-		       addr, func->name, addr - func->offset,
-		       disas_result(dctx));
+		if (insn->alts) {
+			insn_start = insn;
+			insn = disas_alt(dctx, insn);
+			if (insn)
+				continue;
+			/*
+			 * There was an error with disassembling
+			 * the alternative. Resume disassembling
+			 * at the current instruction, this will
+			 * disassemble the default alternative
+			 * only and continue with the code after
+			 * the alternative.
+			 */
+			insn = insn_start;
+		}
+
+		DISAS_PRINSN(dctx, insn, 0);
 	}
 	printf("\n");
 }