objtool: Support Clang non-section symbols in ORC generation

When compiling the kernel with AS=clang, objtool produces a lot of
warnings:

  warning: objtool: missing symbol for section .text
  warning: objtool: missing symbol for section .init.text
  warning: objtool: missing symbol for section .ref.text

It then fails to generate the ORC table.

The problem is that objtool assumes text section symbols always exist.
But the Clang assembler is aggressive about removing them.

When generating relocations for the ORC table, objtool always tries to
reference instructions by their section symbol offset.  If the section
symbol doesn't exist, it bails.

Do a fallback: when a section symbol isn't available, reference a
function symbol instead.

Link: ClangBuiltLinux#669
Cc: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: Dmitry Golovin <dima@golovin.in>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

tools/objtool/orc_gen.c

@@ -88,11 +88,6 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
 	struct orc_entry *orc;
 	struct rela *rela;
 
-	if (!insn_sec->sym) {
-		WARN("missing symbol for section %s", insn_sec->name);
-		return -1;
-	}
-
 	/* populate ORC data */
 	orc = (struct orc_entry *)u_sec->data->d_buf + idx;
 	memcpy(orc, o, sizeof(*orc));
@@ -105,8 +100,32 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
 	}
 	memset(rela, 0, sizeof(*rela));
 
-	rela->sym = insn_sec->sym;
-	rela->addend = insn_off;
+	if (insn_sec->sym) {
+		rela->sym = insn_sec->sym;
+		rela->addend = insn_off;
+	} else {
+		/*
+		 * The Clang assembler doesn't produce section symbols, so we
+		 * have to reference the function symbol instead:
+		 */
+		rela->sym = find_symbol_containing(insn_sec, insn_off);
+		if (!rela->sym) {
+			/*
+			 * Hack alert.  This happens when we need to reference
+			 * the NOP pad insn immediately after the function.
+			 */
+			rela->sym = find_symbol_containing(insn_sec,
+							   insn_off - 1);
+		}
+		if (!rela->sym) {
+			WARN("missing symbol for insn at offset 0x%lx\n",
+			     insn_off);
+			return -1;
+		}
+
+		rela->addend = insn_off - rela->sym->offset;
+	}
+
 	rela->type = R_X86_64_PC32;
 	rela->offset = idx * sizeof(int);
 	rela->sec = ip_relasec;