objtool: Improve call destination function detection

A recent clang change, combined with a binutils bug, can trigger a
situation where a ".Lprintk$local" STT_NOTYPE symbol gets created at the
same offset as the "printk" STT_FUNC symbol.  This confuses objtool:

  kernel/printk/printk.o: warning: objtool: ignore_loglevel_setup()+0x10: can't find call dest symbol at .text+0xc67

Improve the call destination detection by looking specifically for an
STT_FUNC symbol.
Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/872
Link: https://sourceware.org/bugzilla/show_bug.cgi?id=25551
Link: https://lkml.kernel.org/r/0a7ee320bc0ea4469bd3dc450a7b4725669e0ea9.1581997059.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -420,8 +420,8 @@ static void add_ignores(struct objtool_file *file)
 			break;
 
 		case STT_SECTION:
-			func = find_symbol_by_offset(rela->sym->sec, rela->addend);
-			if (!func || func->type != STT_FUNC)
+			func = find_func_by_offset(rela->sym->sec, rela->addend);
+			if (!func)
 				continue;
 			break;
 
@@ -665,10 +665,14 @@ static int add_call_destinations(struct objtool_file *file)
 					       insn->len);
 		if (!rela) {
 			dest_off = insn->offset + insn->len + insn->immediate;
-			insn->call_dest = find_symbol_by_offset(insn->sec,
-								dest_off);
+			insn->call_dest = find_func_by_offset(insn->sec, dest_off);
+			if (!insn->call_dest)
+				insn->call_dest = find_symbol_by_offset(insn->sec, dest_off);
 
-			if (!insn->call_dest && !insn->ignore) {
+			if (insn->ignore)
+				continue;
+
+			if (!insn->call_dest) {
 				WARN_FUNC("unsupported intra-function call",
 					  insn->sec, insn->offset);
 				if (retpoline)
@@ -676,11 +680,16 @@ static int add_call_destinations(struct objtool_file *file)
 				return -1;
 			}
 
+			if (insn->func && insn->call_dest->type != STT_FUNC) {
+				WARN_FUNC("unsupported call to non-function",
+					  insn->sec, insn->offset);
+				return -1;
+			}
+
 		} else if (rela->sym->type == STT_SECTION) {
-			insn->call_dest = find_symbol_by_offset(rela->sym->sec,
-								rela->addend+4);
-			if (!insn->call_dest ||
-			    insn->call_dest->type != STT_FUNC) {
+			insn->call_dest = find_func_by_offset(rela->sym->sec,
+							      rela->addend+4);
+			if (!insn->call_dest) {
 				WARN_FUNC("can't find call dest symbol at %s+0x%x",
 					  insn->sec, insn->offset,
 					  rela->sym->sec->name,

tools/objtool/elf.c

@@ -62,8 +62,18 @@ struct symbol *find_symbol_by_offset(struct section *sec, unsigned long offset)
 	struct symbol *sym;
 
 	list_for_each_entry(sym, &sec->symbol_list, list)
-		if (sym->type != STT_SECTION &&
-		    sym->offset == offset)
+		if (sym->type != STT_SECTION && sym->offset == offset)
+			return sym;
+
+	return NULL;
+}
+
+struct symbol *find_func_by_offset(struct section *sec, unsigned long offset)
+{
+	struct symbol *sym;
+
+	list_for_each_entry(sym, &sec->symbol_list, list)
+		if (sym->type == STT_FUNC && sym->offset == offset)
 			return sym;
 
 	return NULL;

tools/objtool/elf.h

@@ -77,6 +77,7 @@ struct elf {
 
 struct elf *elf_read(const char *name, int flags);
 struct section *find_section_by_name(struct elf *elf, const char *name);
+struct symbol *find_func_by_offset(struct section *sec, unsigned long offset);
 struct symbol *find_symbol_by_offset(struct section *sec, unsigned long offset);
 struct symbol *find_symbol_by_name(struct elf *elf, const char *name);
 struct symbol *find_symbol_containing(struct section *sec, unsigned long offset);