objtool: Explicitly avoid self modifying code in .altinstr_replacement

Assume ALTERNATIVE()s know what they're doing and do not change, or
cause to change, instructions in .altinstr_replacement sections.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120309.722511775@infradead.org

tools/objtool/check.c

@@ -993,18 +993,27 @@ static void remove_insn_ops(struct instruction *insn)
 	}
 }
 
-static void add_call_dest(struct objtool_file *file, struct instruction *insn,
-			  struct symbol *dest, bool sibling)
+static void annotate_call_site(struct objtool_file *file,
+			       struct instruction *insn, bool sibling)
 {
 	struct reloc *reloc = insn_reloc(file, insn);
+	struct symbol *sym = insn->call_dest;
 
-	insn->call_dest = dest;
-	if (!dest)
+	if (!sym)
+		sym = reloc->sym;
+
+	/*
+	 * Alternative replacement code is just template code which is
+	 * sometimes copied to the original instruction. For now, don't
+	 * annotate it. (In the future we might consider annotating the
+	 * original instruction if/when it ever makes sense to do so.)
+	 */
+	if (!strcmp(insn->sec->name, ".altinstr_replacement"))
 		return;
 
-	if (insn->call_dest->static_call_tramp) {
-		list_add_tail(&insn->call_node,
-			      &file->static_call_list);
+	if (sym->static_call_tramp) {
+		list_add_tail(&insn->call_node, &file->static_call_list);
+		return;
 	}
 
 	/*
@@ -1012,7 +1021,7 @@ static void add_call_dest(struct objtool_file *file, struct instruction *insn,
 	 * so they need a little help, NOP out any KCOV calls from noinstr
 	 * text.
 	 */
-	if (insn->sec->noinstr && insn->call_dest->kcov) {
+	if (insn->sec->noinstr && sym->kcov) {
 		if (reloc) {
 			reloc->type = R_NONE;
 			elf_write_reloc(file->elf, reloc);
@@ -1024,9 +1033,10 @@ static void add_call_dest(struct objtool_file *file, struct instruction *insn,
 			               : arch_nop_insn(insn->len));
 
 		insn->type = sibling ? INSN_RETURN : INSN_NOP;
+		return;
 	}
 
-	if (mcount && insn->call_dest->fentry) {
+	if (mcount && sym->fentry) {
 		if (sibling)
 			WARN_FUNC("Tail call to __fentry__ !?!?", insn->sec, insn->offset);
 
@@ -1041,9 +1051,17 @@ static void add_call_dest(struct objtool_file *file, struct instruction *insn,
 
 		insn->type = INSN_NOP;
 
-		list_add_tail(&insn->mcount_loc_node,
-			      &file->mcount_loc_list);
+		list_add_tail(&insn->mcount_loc_node, &file->mcount_loc_list);
+		return;
 	}
+}
+
+static void add_call_dest(struct objtool_file *file, struct instruction *insn,
+			  struct symbol *dest, bool sibling)
+{
+	insn->call_dest = dest;
+	if (!dest)
+		return;
 
 	/*
 	 * Whatever stack impact regular CALLs have, should be undone
@@ -1053,6 +1071,8 @@ static void add_call_dest(struct objtool_file *file, struct instruction *insn,
 	 * are converted to JUMP, see read_intra_function_calls().
 	 */
 	remove_insn_ops(insn);
+
+	annotate_call_site(file, insn, sibling);
 }
 
 /*