objtool: Fix UNWIND_HINT_{SAVE,RESTORE} across basic blocks

If SAVE and RESTORE unwind hints are in different basic blocks, and
objtool sees the RESTORE before the SAVE, it errors out with:

  vmlinux.o: warning: objtool: vmw_port_hb_in+0x242: objtool isn't smart enough to handle this CFI save/restore combo

In such a case, defer following the RESTORE block until the
straight-line path gets followed later.

Fixes: 8faea26e ("objtool: Re-add UNWIND_HINT_{SAVE_RESTORE}")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402240702.zJFNmahW-lkp@intel.com/Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240227073527.avcm5naavbv3cj5s@trebleSigned-off-by: Kees Cook <keescook@chromium.org>

tools/objtool/check.c

@@ -3620,6 +3620,18 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 				}
 
 				if (!save_insn->visited) {
+					/*
+					 * If the restore hint insn is at the
+					 * beginning of a basic block and was
+					 * branched to from elsewhere, and the
+					 * save insn hasn't been visited yet,
+					 * defer following this branch for now.
+					 * It will be seen later via the
+					 * straight-line path.
+					 */
+					if (!prev_insn)
+						return 0;
+
 					WARN_INSN(insn, "objtool isn't smart enough to handle this CFI save/restore combo");
 					return 1;
 				}