objtool: Teach get_alt_entry() about more relocation types

Occasionally objtool encounters symbol (as opposed to section)
relocations in .altinstructions. Typically they are the alternatives
written by elf_add_alternative() as encountered on a noinstr
validation run on vmlinux after having already ran objtool on the
individual .o files.

Basically this is the counterpart of commit 44f6a7c0755d ("objtool:
Fix seg fault with Clang non-section symbols"), because when these new
assemblers (binutils now also does this) strip the section symbols,
elf_add_reloc_to_insn() is forced to emit symbol based relocations.

As such, teach get_alt_entry() about different relocation types.

Fixes: 9bc0bb50727c ("objtool/x86: Rewrite retpoline thunk calls")
Reported-by: Stephen Rothwell <[e-mail]>
Reported-by: Borislav Petkov <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Acked-by: Josh Poimboeuf <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/objtool/special.c

···
 {
 }
 
+static bool reloc2sec_off(struct reloc *reloc, struct section **sec, unsigned long *off)
+{
+	switch (reloc->sym->type) {
+	case STT_FUNC:
+		*sec = reloc->sym->sec;
+		*off = reloc->sym->offset + reloc->addend;
+		return true;
+
+	case STT_SECTION:
+		*sec = reloc->sym->sec;
+		*off = reloc->addend;
+		return true;
+
+	default:
+		return false;
+	}
+}
+
 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
 			 struct section *sec, int idx,
 			 struct special_alt *alt)
···
 		WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
 		return -1;
 	}
-	if (orig_reloc->sym->type != STT_SECTION) {
-		WARN_FUNC("don't know how to handle non-section reloc symbol %s",
+	if (!reloc2sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off)) {
+		WARN_FUNC("don't know how to handle reloc symbol type: %s",
 			   sec, offset + entry->orig, orig_reloc->sym->name);
 		return -1;
 	}
-
-	alt->orig_sec = orig_reloc->sym->sec;
-	alt->orig_off = orig_reloc->addend;
 
 	if (!entry->group || alt->new_len) {
 		new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
···
 		if (arch_is_retpoline(new_reloc->sym))
 			return 1;
 
-		alt->new_sec = new_reloc->sym->sec;
-		alt->new_off = (unsigned int)new_reloc->addend;
+		if (!reloc2sec_off(new_reloc, &alt->new_sec, &alt->new_off)) {
+			WARN_FUNC("don't know how to handle reloc symbol type: %s",
+				  sec, offset + entry->new, new_reloc->sym->name);
+			return -1;
+		}
 
 		/* _ASM_EXTABLE_EX hack */
 		if (alt->new_off >= 0x7ffffff0)