objtool: Introduce HINT_RET_OFFSET

Normally objtool ensures a function keeps the stack layout invariant.
But there is a useful exception, it is possible to stuff the return
stack in order to 'inject' a 'call':

	push $fun
	ret

In this case the invariant mentioned above is violated.

Add an objtool HINT to annotate this and allow a function exit with a
modified stack frame.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200416115118.690601403@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/orc_types.h

@@ -60,6 +60,7 @@
 #define ORC_TYPE_REGS_IRET		2
 #define UNWIND_HINT_TYPE_SAVE		3
 #define UNWIND_HINT_TYPE_RESTORE	4
+#define UNWIND_HINT_TYPE_RET_OFFSET	5
 
 #ifndef __ASSEMBLY__
 /*

arch/x86/include/asm/unwind_hints.h

@@ -94,6 +94,16 @@
 	UNWIND_HINT type=UNWIND_HINT_TYPE_RESTORE
 .endm
 
+
+/*
+ * RET_OFFSET: Used on instructions that terminate a function; mostly RETURN
+ * and sibling calls. On these, sp_offset denotes the expected offset from
+ * initial_func_cfi.
+ */
+.macro UNWIND_HINT_RET_OFFSET sp_offset=8
+	UNWIND_HINT type=UNWIND_HINT_TYPE_RET_OFFSET sp_offset=\sp_offset
+.endm
+
 #else /* !__ASSEMBLY__ */
 
 #define UNWIND_HINT(sp_reg, sp_offset, type, end)		\

tools/arch/x86/include/asm/orc_types.h

@@ -60,6 +60,7 @@
 #define ORC_TYPE_REGS_IRET		2
 #define UNWIND_HINT_TYPE_SAVE		3
 #define UNWIND_HINT_TYPE_RESTORE	4
+#define UNWIND_HINT_TYPE_RET_OFFSET	5
 
 #ifndef __ASSEMBLY__
 /*

tools/objtool/check.c

@@ -1261,6 +1261,9 @@ static int read_unwind_hints(struct objtool_file *file)
 		} else if (hint->type == UNWIND_HINT_TYPE_RESTORE) {
 			insn->restore = true;
 			insn->hint = true;
+
+		} else if (hint->type == UNWIND_HINT_TYPE_RET_OFFSET) {
+			insn->ret_offset = hint->sp_offset;
 			continue;
 		}
 
@@ -1424,20 +1427,25 @@ static bool is_fentry_call(struct instruction *insn)
 	return false;
 }
 
-static bool has_modified_stack_frame(struct insn_state *state)
+static bool has_modified_stack_frame(struct instruction *insn, struct insn_state *state)
 {
+	u8 ret_offset = insn->ret_offset;
 	int i;
 
-	if (state->cfa.base != initial_func_cfi.cfa.base ||
+	if (state->cfa.base != initial_func_cfi.cfa.base || state->drap)
-	    state->cfa.offset != initial_func_cfi.cfa.offset ||
-	    state->stack_size != initial_func_cfi.cfa.offset ||
-	    state->drap)
 		return true;
 
-	for (i = 0; i < CFI_NUM_REGS; i++)
+	if (state->cfa.offset != initial_func_cfi.cfa.offset + ret_offset)
+		return true;
+
+	if (state->stack_size != initial_func_cfi.cfa.offset + ret_offset)
+		return true;
+
+	for (i = 0; i < CFI_NUM_REGS; i++) {
 		if (state->regs[i].base != initial_func_cfi.regs[i].base ||
 		    state->regs[i].offset != initial_func_cfi.regs[i].offset)
 			return true;
+	}
 
 	return false;
 }
@@ -2014,7 +2022,7 @@ static int validate_call(struct instruction *insn, struct insn_state *state)
 
 static int validate_sibling_call(struct instruction *insn, struct insn_state *state)
 {
-	if (has_modified_stack_frame(state)) {
+	if (has_modified_stack_frame(insn, state)) {
 		WARN_FUNC("sibling call from callable instruction with modified stack frame",
 				insn->sec, insn->offset);
 		return 1;
@@ -2043,7 +2051,7 @@ static int validate_return(struct symbol *func, struct instruction *insn, struct
 		return 1;
 	}
 
-	if (func && has_modified_stack_frame(state)) {
+	if (func && has_modified_stack_frame(insn, state)) {
 		WARN_FUNC("return with modified stack frame",
 			  insn->sec, insn->offset);
 		return 1;

tools/objtool/check.h

@@ -33,9 +33,11 @@ struct instruction {
 	unsigned int len;
 	enum insn_type type;
 	unsigned long immediate;
-	bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
+	bool alt_group, dead_end, ignore, ignore_alts;
+	bool hint, save, restore;
 	bool retpoline_safe;
 	u8 visited;
+	u8 ret_offset;
 	struct symbol *call_dest;
 	struct instruction *jump_dest;
 	struct instruction *first_jump_src;