objtool: Fix clang switch table edge case

Clang has the ability to create a switch table which is not a jump
table, but is rather a table of string pointers.  This confuses objtool,
because it sees the relocations for the string pointers and assumes
they're part of a jump table:

  drivers/ata/sata_dwc_460ex.o: warning: objtool: sata_dwc_bmdma_start_by_tag()+0x3a2: can't find switch jump table
  net/ceph/messenger.o: warning: objtool: ceph_con_workfn()+0x47c: can't find switch jump table

Make objtool's find_jump_table() smart enough to distinguish between a
switch jump table (which has relocations to text addresses in the same
function as the original instruction) and other anonymous rodata (which
may have relocations to elsewhere).

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/485
Link: https://lkml.kernel.org/r/263f6aae46d33da0b86d7030ced878cb5cab1788.1581997059.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -1025,7 +1025,7 @@ static struct rela *find_jump_table(struct objtool_file *file,
 				      struct instruction *insn)
 {
 	struct rela *text_rela, *table_rela;
-	struct instruction *orig_insn = insn;
+	struct instruction *dest_insn, *orig_insn = insn;
 	struct section *table_sec;
 	unsigned long table_offset;
 
@@ -1077,10 +1077,17 @@ static struct rela *find_jump_table(struct objtool_file *file,
 		    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
 			continue;
 
-		/* Each table entry has a rela associated with it. */
+		/*
+		 * Each table entry has a rela associated with it.  The rela
+		 * should reference text in the same function as the original
+		 * instruction.
+		 */
 		table_rela = find_rela_by_dest(table_sec, table_offset);
 		if (!table_rela)
 			continue;
+		dest_insn = find_insn(file, table_rela->sym->sec, table_rela->addend);
+		if (!dest_insn || !dest_insn->func || dest_insn->func->pfunc != func)
+			continue;
 
 		/*
 		 * Use of RIP-relative switch jumps is quite rare, and