objtool: Allow symbol range comparisons for IBT/ENDBR

A semi common pattern is where code checks if a code address is
within a specific range. All text addresses require either ENDBR or
ANNOTATE_ENDBR, however the ANNOTATE_NOENDBR past the range is
unnatural.

Instead, suppress this warning when this is exactly at the end of a
symbol that itself starts with either ENDBR/ANNOTATE_ENDBR.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111146.434642471@infradead.org

arch/x86/entry/entry_64_compat.S

@@ -128,7 +128,6 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
 	popfq
 	jmp	.Lsysenter_flags_fixed
 SYM_INNER_LABEL(__end_entry_SYSENTER_compat, SYM_L_GLOBAL)
-	ANNOTATE_NOENDBR // is_sysenter_singlestep
 SYM_CODE_END(entry_SYSENTER_compat)
 
 /*

tools/objtool/check.c

@@ -4033,6 +4033,24 @@ static void mark_endbr_used(struct instruction *insn)
 		list_del_init(&insn->call_node);
 }
 
+static bool noendbr_range(struct objtool_file *file, struct instruction *insn)
+{
+	struct symbol *sym = find_symbol_containing(insn->sec, insn->offset-1);
+	struct instruction *first;
+
+	if (!sym)
+		return false;
+
+	first = find_insn(file, sym->sec, sym->offset);
+	if (!first)
+		return false;
+
+	if (first->type != INSN_ENDBR && !first->noendbr)
+		return false;
+
+	return insn->offset == sym->offset + sym->len;
+}
+
 static int validate_ibt_insn(struct objtool_file *file, struct instruction *insn)
 {
 	struct instruction *dest;
@@ -4105,9 +4123,19 @@ static int validate_ibt_insn(struct objtool_file *file, struct instruction *insn
 			continue;
 		}
 
+		/*
+		 * Accept anything ANNOTATE_NOENDBR.
+		 */
 		if (dest->noendbr)
 			continue;
 
+		/*
+		 * Accept if this is the instruction after a symbol
+		 * that is (no)endbr -- typical code-range usage.
+		 */
+		if (noendbr_range(file, dest))
+			continue;
+
 		WARN_FUNC("relocation to !ENDBR: %s",
 			  insn->sec, insn->offset,
 			  offstr(dest->sec, dest->offset));