iOS Swift逆向——被编译优化后的函数参数调用约定修复

头文件导入：

typedef long long s64;
typedef unsigned long long u64;typedef s64 Int;
typedef u64 Bool;struct Swift::String
{u64 _countAndFlagsBits;void *_object;
};union Swift_ElementAny {Swift::String stringElement;
};struct Swift_Any {Swift_ElementAny element;u64 unknown;s64 type;
};struct Swift_ArrayAny {s64 length;Swift_Any *items;
};

 https://github.com/doronz88/swift_reversing

 https://github.com/doronz88/ida-scripts/blob/main/swift.py

Swift <=> OC的兼容层

小gadget（片段）

; void sub_101A34B60()
sub_101A34B60
MOV             X0, X20 ; id
B               _objc_release
; End of function sub_101A34B60

因为_objc_release的参数只要X0。修复为：

void __usercall sub_101A34B60(__int64 a1@<X20>)

根据swift官方文档，X20是self。所以这是内存引用计数-1。

https://github.com/swiftlang/swift/blob/13da4c2ad3077282f7c8922b293a74c32e79e428/docs/ABI/CallConvSummary.rst#L151

---

字符串操作

字符串对象由两个寄存器构成。

以String.append为例进行修复调用约定： 

X20是self。

Swift::Void __usercall String.append(_:)(void *a1@<X20>, Swift::String a2@<X0:X1>)
{__imp__$sSS6appendyySSF(a2._object, a2._countAndFlagsBits);
}

Swift::String __usercall dispatch thunk of CustomStringConvertible.description.getter@<X0:X1>(void *a1@<X20>,__int64 *a2@<X0>,__int64 *a3@<X1>)

 a=100;print("myIntVariable1= \(a)")的整理后IDA结果：

  v11 = 0xD000000000000010LL;                   // myIntVariable1= v12 = (id)0x800000010000CDF0LL;v10 = 100LL;v3 = dispatch thunk of CustomStringConvertible.description.getter(&v10,&type metadata for Int,&protocol witness table for Int);String.append(_:)(&v11, (Swift::String)__PAIR128__(v3._countAndFlagsBits, (unsigned __int64)v3._object));swift_bridgeObjectRelease(v3._object);v4 = v11;v5 = v12;*(_QWORD *)(v2 + 56) = &type metadata for String;*(_QWORD *)(v2 + 32) = v4;*(_QWORD *)(v2 + 40) = v5;v15._countAndFlagsBits = 32LL;v15._object = (void *)0xE100000000000000LL;v18._countAndFlagsBits = 10LL;v18._object = (void *)0xE100000000000000LL;print(_:separator:terminator:)((Swift_ArrayAny *)v2, v15, v18);swift_release(v2);

字符串Array

Swift::String __usercall BidirectionalCollection___joined_separator__@<X0:X1>(Swift_ArrayAny@<X20:X21>, Swift::String@<X0:X1>, __int64@<X2>, __int64@<X3>)

---

栈内存偏移计算相关小函数 

原先的函数点开里面是空的。 

sub_101A348B8
LDUR            X8, [X0,#-8]
LDR             X8, [X8,#0x40]
MOV             X9, SP
ADD             X8, X8, #0xF
AND             X8, X8, #0xFFFFFFFFFFFFFFF0
SUB             X19, X9, X8
MOV             SP, X19
RET
; End of function sub_101A348B8

 定义结构体：

00000000 stack_offset_struc struc ; (sizeof=0x18, copyof_7619)
00000000 offset          DCQ ?
00000008 sp              DCQ ?
00000010 addr            DCQ ?
00000018 stack_offset_struc ends

 定义调用约定。F5之后，函数体原来出现内容。

stack_offset_struc __usercall __spoils<> sub_101A348B8@<0:X8, 8:X9, 16:X19>(__int64 a1@<X0>)
{__int64 *v1; // x9__int64 v2; // x8char *v3; // x19__int64 v4; // [xsp+0h] [xbp+0h] BYREFstack_offset_struc result; // 0:x8.16,16:x19.8v1 = &v4;v2 = (*(_QWORD *)(*(_QWORD *)(a1 - 8) + 64LL) + 15LL) & 0xFFFFFFFFFFFFFFF0LL;v3 = (char *)&v4 - v2;result.addr = (__int64)v3;result.sp = (__int64)v1;result.offset = v2;return result;
}

另一个函数也进行修复：

sub_101A34B84
LDUR            X28, [X0,#-8]
LDR             X8, [X28,#0x40]
MOV             X9, SP
ADD             X8, X8, #0xF
AND             X8, X8, #0xFFFFFFFFFFFFFFF0
RET

stack_offset_struc __usercall __spoils<> sub_101A34B84@<0:X8, 8:X9, 16:X28>(__int64 a1@<X0>)
{__int64 v1; // x28__int64 *v2; // x9__int64 v3; // x8__int64 v4; // [xsp+0h] [xbp+0h] BYREFstack_offset_struc result; // 0:x8.16,16:x28.8v1 = *(_QWORD *)(a1 - 8);v2 = &v4;v3 = (*(_QWORD *)(v1 + 64) + 15LL) & 0xFFFFFFFFFFFFFFF0LL;result.addr = v1;result.sp = (__int64)v2;result.offset = v3;return result;
}

这两个小函数，其实是完全一样的。

可以发现第二个函数sub_101A34B84相比较于sub_101A348B8少了最后的SUB。其实这个SUB在上层调用者函数里面，看最后两行汇编：

STP             X28, X27, [SP,#-0x10+var_50]!
STP             X26, X25, [SP,#0x50+var_40]
STP             X24, X23, [SP,#0x50+var_30]
STP             X22, X21, [SP,#0x50+var_20]
STP             X20, X19, [SP,#0x50+var_10]
STP             X29, X30, [SP,#0x50+var_s0]
ADD             X29, SP, #0x50
SUB             SP, SP, #0x70
MOV             X20, X3
MOV             X23, X2
MOV             X24, X1
MOV             X25, X0
ADRL            X21, unk_1039839C0
MOV             X0, X21
BL              sub_1000B6ED0
BL              sub_101A348B8
BL              sub_101A34D54
MOV             X22, X0
BL              sub_101A34B84
SUB             X27, X9, X8
MOV             SP, X27

修复完成后，返回调用该函数的上层函数，在按F5。可以看到恢复之后的代码。

参考IDA官网博客里面的内容： 

 https://hex-rays.com/blog/author/igor-skochinsky

---

 https://github.com/swiftlang/swift/blob/65f7c9eeb50ebb2586fce4c4808f069cfb584b79/stdlib/public/core/EmbeddedRuntime.swift#L326

大部分函数的调用约定可以在源代码里面直接看到：

@_cdecl("swift_bridgeObjectRetain")
public func swift_bridgeObjectRetain(object: Builtin.RawPointer) -> Builtin.RawPointer {return swift_bridgeObjectRetain_n(object: object, n: 1)
}