objtool: Add --dry-run

Add a --dry-run argument to skip writing the modifications. This is
convenient for debugging.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154317.282720146@infradead.org

tools/objtool/builtin-check.c

@@ -20,7 +20,7 @@
 #include <objtool/objtool.h>
 
 bool no_fp, no_unreachable, retpoline, module, backtrace, uaccess, stats,
-     validate_dup, vmlinux, mcount, noinstr, backup, sls;
+     validate_dup, vmlinux, mcount, noinstr, backup, sls, dryrun;
 
 static const char * const check_usage[] = {
 	"objtool check [<options>] file.o",
@@ -46,6 +46,7 @@ const struct option check_options[] = {
 	OPT_BOOLEAN('M', "mcount", &mcount, "generate __mcount_loc"),
 	OPT_BOOLEAN('B', "backup", &backup, "create .orig files before modification"),
 	OPT_BOOLEAN('S', "sls", &sls, "validate straight-line-speculation"),
+	OPT_BOOLEAN(0, "dry-run", &dryrun, "don't write the modifications"),
 	OPT_END(),
 };
 

tools/objtool/elf.c

@@ -1019,6 +1019,9 @@ int elf_write(struct elf *elf)
 	struct section *sec;
 	Elf_Scn *s;
 
+	if (dryrun)
+		return 0;
+
 	/* Update changed relocation sections and section headers: */
 	list_for_each_entry(sec, &elf->sections, list) {
 		if (sec->changed) {

tools/objtool/include/objtool/builtin.h

@@ -9,7 +9,7 @@
 
 extern const struct option check_options[];
 extern bool no_fp, no_unreachable, retpoline, module, backtrace, uaccess, stats,
-            validate_dup, vmlinux, mcount, noinstr, backup, sls;
+            validate_dup, vmlinux, mcount, noinstr, backup, sls, dryrun;
 
 extern int cmd_parse_options(int argc, const char **argv, const char * const usage[]);