objtool: Skip magical retpoline .altinstr_replacement

When the .altinstr_replacement is a retpoline, skip the alternative.
We already special case retpolines anyway.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lkml.kernel.org/r/20210326151300.259429287@infradead.org

Author: Peter Zijlstra

tools/objtool/special.c

@@ -106,6 +106,14 @@ static int get_alt_entry(struct elf *elf, struct special_entry *entry,
 			return -1;
 		}
 
+		/*
+		 * Skip retpoline .altinstr_replacement... we already rewrite the
+		 * instructions for retpolines anyway, see arch_is_retpoline()
+		 * usage in add_{call,jump}_destinations().
+		 */
+		if (arch_is_retpoline(new_reloc->sym))
+			return 1;
+
 		alt->new_sec = new_reloc->sym->sec;
 		alt->new_off = (unsigned int)new_reloc->addend;
 
@@ -154,7 +162,9 @@ int special_get_alts(struct elf *elf, struct list_head *alts)
 			memset(alt, 0, sizeof(*alt));
 
 			ret = get_alt_entry(elf, entry, sec, idx, alt);
-			if (ret)
+			if (ret > 0)
+				continue;
+			if (ret < 0)
 				return ret;
 
 			list_add_tail(&alt->list, alts);