objtool: Parse options from OBJTOOL_ARGS

Teach objtool to parse options from the OBJTOOL_ARGS environment
variable.

This enables things like:

  $ OBJTOOL_ARGS="--backup" make O=defconfig-build/ kernel/ponies.o

to obtain both defconfig-build/kernel/ponies.o{,.orig} and easily
inspect what objtool actually did.
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20210226110004.252553847@infradead.org

tools/objtool/builtin-check.c

@@ -15,6 +15,7 @@
 
 #include <subcmd/parse-options.h>
 #include <string.h>
+#include <stdlib.h>
 #include <objtool/builtin.h>
 #include <objtool/objtool.h>
 
@@ -26,6 +27,11 @@ static const char * const check_usage[] = {
 	NULL,
 };
 
+static const char * const env_usage[] = {
+	"OBJTOOL_ARGS=\"<options>\"",
+	NULL,
+};
+
 const struct option check_options[] = {
 	OPT_BOOLEAN('f', "no-fp", &no_fp, "Skip frame pointer validation"),
 	OPT_BOOLEAN('u', "no-unreachable", &no_unreachable, "Skip 'unreachable instruction' warnings"),
@@ -44,6 +50,25 @@ const struct option check_options[] = {
 
 int cmd_parse_options(int argc, const char **argv, const char * const usage[])
 {
+	const char *envv[16] = { };
+	char *env;
+	int envc;
+
+	env = getenv("OBJTOOL_ARGS");
+	if (env) {
+		envv[0] = "OBJTOOL_ARGS";
+		for (envc = 1; envc < ARRAY_SIZE(envv); ) {
+			envv[envc++] = env;
+			env = strchr(env, ' ');
+			if (!env)
+				break;
+			*env = '\0';
+			env++;
+		}
+
+		parse_options(envc, envv, check_options, env_usage, 0);
+	}
+
 	argc = parse_options(argc, argv, check_options, usage, 0);
 	if (argc != 1)
 		usage_with_options(usage, check_options);