objtool: Fix CONFIG_UBSAN_TRAP unreachable warnings

CONFIG_UBSAN_TRAP causes GCC to emit a UD2 whenever it encounters an
unreachable code path.  This includes __builtin_unreachable().  Because
the BUG() macro uses __builtin_unreachable() after it emits its own UD2,
this results in a double UD2.  In this case objtool rightfully detects
that the second UD2 is unreachable:

  init/main.o: warning: objtool: repair_env_string()+0x1c8: unreachable instruction

We weren't able to figure out a way to get rid of the double UD2s, so
just silence the warning.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/6653ad73c6b59c049211bd7c11ed3809c20ee9f5.1585761021.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -2364,14 +2364,27 @@ static bool ignore_unreachable_insn(struct instruction *insn)
 	    !strcmp(insn->sec->name, ".altinstr_aux"))
 		return true;
 
+	if (!insn->func)
+		return false;
+
+	/*
+	 * CONFIG_UBSAN_TRAP inserts a UD2 when it sees
+	 * __builtin_unreachable().  The BUG() macro has an unreachable() after
+	 * the UD2, which causes GCC's undefined trap logic to emit another UD2
+	 * (or occasionally a JMP to UD2).
+	 */
+	if (list_prev_entry(insn, list)->dead_end &&
+	    (insn->type == INSN_BUG ||
+	     (insn->type == INSN_JUMP_UNCONDITIONAL &&
+	      insn->jump_dest && insn->jump_dest->type == INSN_BUG)))
+		return true;
+
 	/*
 	 * Check if this (or a subsequent) instruction is related to
 	 * CONFIG_UBSAN or CONFIG_KASAN.
 	 *
 	 * End the search at 5 instructions to avoid going into the weeds.
 	 */
-	if (!insn->func)
-		return false;
 	for (i = 0; i < 5; i++) {
 
 		if (is_kasan_insn(insn) || is_ubsan_insn(insn))