objtool: Add verbose option for disassembling affected functions

When a warning is associated with a function, add an option to
disassemble that function.

This makes it easier for reporters to submit the information needed to
diagnose objtool warnings.

Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lore.kernel.org/r/dd0fe13428ede186f09c74059a8001f4adcea5fc.1681853186.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

tools/objtool/Documentation/objtool.txt

@@ -244,6 +244,11 @@ To achieve the validation, objtool enforces the following rules:
 Objtool warnings
 ----------------
 
+NOTE: When requesting help with an objtool warning, please recreate with
+OBJTOOL_VERBOSE=1 (e.g., "make OBJTOOL_VERBOSE=1") and send the full
+output, including any disassembly below the warning, to the objtool
+maintainers.
+
 For asm files, if you're getting an error which doesn't make sense,
 first make sure that the affected code follows the above rules.
 

tools/objtool/builtin-check.c

@@ -93,6 +93,7 @@ static const struct option check_options[] = {
 	OPT_BOOLEAN(0, "no-unreachable", &opts.no_unreachable, "skip 'unreachable instruction' warnings"),
 	OPT_BOOLEAN(0, "sec-address", &opts.sec_address, "print section addresses in warnings"),
 	OPT_BOOLEAN(0, "stats", &opts.stats, "print statistics"),
+	OPT_BOOLEAN('v', "verbose", &opts.verbose, "verbose warnings"),
 
 	OPT_END(),
 };
@@ -118,6 +119,10 @@ int cmd_parse_options(int argc, const char **argv, const char * const usage[])
 		parse_options(envc, envv, check_options, env_usage, 0);
 	}
 
+	env = getenv("OBJTOOL_VERBOSE");
+	if (env && !strcmp(env, "1"))
+		opts.verbose = true;
+
 	argc = parse_options(argc, argv, check_options, usage, 0);
 	if (argc != 1)
 		usage_with_options(usage, check_options);

tools/objtool/check.c

@@ -4530,6 +4530,81 @@ static int validate_reachable_instructions(struct objtool_file *file)
 	return warnings;
 }
 
+/* 'funcs' is a space-separated list of function names */
+static int disas_funcs(const char *funcs)
+{
+	const char *objdump_str, *cross_compile;
+	int size, ret;
+	char *cmd;
+
+	cross_compile = getenv("CROSS_COMPILE");
+
+	objdump_str = "%sobjdump -wdr %s | gawk -M -v _funcs='%s' '"
+			"BEGIN { split(_funcs, funcs); }"
+			"/^$/ { func_match = 0; }"
+			"/<.*>:/ { "
+				"f = gensub(/.*<(.*)>:/, \"\\\\1\", 1);"
+				"for (i in funcs) {"
+					"if (funcs[i] == f) {"
+						"func_match = 1;"
+						"base = strtonum(\"0x\" $1);"
+						"break;"
+					"}"
+				"}"
+			"}"
+			"{"
+				"if (func_match) {"
+					"addr = strtonum(\"0x\" $1);"
+					"printf(\"%%04x \", addr - base);"
+					"print;"
+				"}"
+			"}' 1>&2";
+
+	/* fake snprintf() to calculate the size */
+	size = snprintf(NULL, 0, objdump_str, cross_compile, objname, funcs) + 1;
+	if (size <= 0) {
+		WARN("objdump string size calculation failed");
+		return -1;
+	}
+
+	cmd = malloc(size);
+
+	/* real snprintf() */
+	snprintf(cmd, size, objdump_str, cross_compile, objname, funcs);
+	ret = system(cmd);
+	if (ret) {
+		WARN("disassembly failed: %d", ret);
+		return -1;
+	}
+
+	return 0;
+}
+
+static int disas_warned_funcs(struct objtool_file *file)
+{
+	struct symbol *sym;
+	char *funcs = NULL, *tmp;
+
+	for_each_sym(file, sym) {
+		if (sym->warned) {
+			if (!funcs) {
+				funcs = malloc(strlen(sym->name) + 1);
+				strcpy(funcs, sym->name);
+			} else {
+				tmp = malloc(strlen(funcs) + strlen(sym->name) + 2);
+				sprintf(tmp, "%s %s", funcs, sym->name);
+				free(funcs);
+				funcs = tmp;
+			}
+		}
+	}
+
+	if (funcs)
+		disas_funcs(funcs);
+
+	return 0;
+}
+
 int check(struct objtool_file *file)
 {
 	int ret, warnings = 0;
@@ -4674,6 +4749,8 @@ int check(struct objtool_file *file)
 		warnings += ret;
 	}
 
+	if (opts.verbose)
+		disas_warned_funcs(file);
 
 	if (opts.stats) {
 		printf("nr_insns_visited: %ld\n", nr_insns_visited);

tools/objtool/include/objtool/builtin.h

@@ -37,6 +37,7 @@ struct opts {
 	bool no_unreachable;
 	bool sec_address;
 	bool stats;
+	bool verbose;
 };
 
 extern struct opts opts;