[PATCH 2/7] objtool: Extend UNWIND_HINT based ENDBR rules

[Peter Zijlstra <peterz@infradead.org>]

Extend the UNWIND hint driven rules for ENDBR placement. Currently
objtool expects an ENDBR at any UNWINT_HINT_IRET_REGS that is at +0 of
an STB_GLOBAL symbol, with the expectation that this is an exception
entry point.

Extend this to also expect ENDBR at UNWIND_HINT_EMPTY at +0 for
STB_GLOBAL symbols, with the expectation that these are also machine
entry points (SYSCALL et. al.).

This guarantees all machine entry points are covered by objtool rules at
the expense of a few additional false positives:

  vmlinux.o: warning: objtool: startup_64+0x0: UNWIND_HINT_EMPTY without ENDBR
  vmlinux.o: warning: objtool: start_cpu0+0x0: UNWIND_HINT_EMPTY without ENDBR

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/head_64.S |    2 ++
 tools/objtool/check.c     |   23 ++++++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -43,6 +43,7 @@ L3_START_KERNEL = pud_index(__START_KERN
 	.code64
 SYM_CODE_START_NOALIGN(startup_64)
 	UNWIND_HINT_EMPTY
+	ANNOTATE_NOENDBR
 	/*
 	 * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 0,
 	 * and someone has loaded an identity mapped page table
@@ -371,6 +372,7 @@ SYM_CODE_END(secondary_startup_64)
  */
 SYM_CODE_START(start_cpu0)
 	UNWIND_HINT_EMPTY
+	ANNOTATE_NOENDBR
 	movq	initial_stack(%rip), %rsp
 	jmp	.Ljump_to_C_code
 SYM_CODE_END(start_cpu0)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1947,14 +1947,27 @@ static int read_unwind_hints(struct objt
 
 		insn->hint = true;
 
-		if (opts.ibt && hint->type == UNWIND_HINT_TYPE_REGS_PARTIAL) {
+		if (opts.ibt) {
 			struct symbol *sym = find_symbol_by_offset(insn->sec, insn->offset);
 
-			if (sym && sym->bind == STB_GLOBAL &&
-			    insn->type != INSN_ENDBR && !insn->noendbr) {
-				WARN_FUNC("UNWIND_HINT_IRET_REGS without ENDBR",
-					  insn->sec, insn->offset);
+			if (!sym || sym->bind != STB_GLOBAL)
+				goto no_entry;
+
+			if (hint->type == UNWIND_HINT_TYPE_CALL && !hint->sp_reg) {
+				if (insn->type != INSN_ENDBR && !insn->noendbr) {
+					WARN_FUNC("UNWIND_HINT_EMPTY without ENDBR",
+							insn->sec, insn->offset);
+				}
+			}
+
+			if (hint->type == UNWIND_HINT_TYPE_REGS_PARTIAL) {
+				if (insn->type != INSN_ENDBR && !insn->noendbr) {
+					WARN_FUNC("UNWIND_HINT_IRET_REGS without ENDBR",
+							insn->sec, insn->offset);
+				}
 			}
+
+no_entry:		;
 		}
 
 		if (hint->type == UNWIND_HINT_TYPE_FUNC) {