LD_PRELOAD具有可能的静态共享库函数

时间:2017-04-15 18:01:41

标签: c linux gcc ld dlopen

我的目标是挂钩dlopen on linux使用的open函数。出于某种原因,这段代码没有挂钩dlopen-> open,但它确实挂了我的open main.c-> open版本。 dlopen是不是以某种方式使用我的符号?

编译过程如下:

  1. gcc main.c -ldl -ggdb
  2. gcc fake-open.c -o libexample.so -fPIC -shared
  3. export LD_PRELOAD="$PWD/libexample.so"
  4. 当我运行程序时,一切正常。确保设置LD_PRELOAD变量..等等。

    这是问题所在,当我尝试直接或间接地通过dlopen调用open函数时,我的版本没有解析/重定向/挂起open的这个“版本”。

    [main.c]
    #include <dlfcn.h>
    #include <stdio.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    int main()
    {
        puts("calling open");
        int fd = open("/tmp/test.so", O_RDONLY|O_CLOEXEC);
    
        puts("calling dlopen");
        int *handle = dlopen("/tmp/test.so", RTLD_LAZY);
    }
    
    
    [fake-open.c]
    #define _GNU_SOURCE
    #include <stdio.h>
    #include <dlfcn.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    //#include <fcntl.h>
    
    int open(const char *pathname, int flags)
    {
        puts("from hooked..");
    
        return 1;
    }
    

    控制台输出:

    调用open

    来自迷茫..

    调用dlopen

    我知道dlopen因为strace而以某种方式打开。

    write(1, "calling open\n", 13calling open
    )          = 13
    write(1, "from hooked..\n", 14from hooked..
    )         = 14
    write(1, "calling dlopen\n", 15calling dlopen
    )        = 15
    brk(0)                                  = 0x804b000
    brk(0x806c000)                          = 0x806c000
    open("/tmp/test.so", O_RDONLY|O_CLOEXEC) = 3
    read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\2\0\3\0\1\0\0\0`\205\4\0104\0\0\0"..., 512) = 512
    

    但是,出于某种原因,当dlopen调用打开时,它没有使用我的open版本。这必须是运行时符号解析问题的某种链接,或者dlopen使用静态版本的open并且不需要在运行或加载时解析任何符号?

1 个答案:

答案 0 :(得分:3)

首先,与@ usr的回答相反,dlopen确实open了库。

我们可以通过在GDB下运行一个简单的测试来确认这一点:

// main.c
#include <dlfcn.h>
int main()
{
   void *h = dlopen("./foo.so", RTLD_LAZY);
   return 0;
}

// foo.c; compile with "gcc -fPIC -shared -o foo.so foo.c"
int foo() { return 0; }

让我们编译并运行它:

gdb -q ./a.out
(gdb) start
Temporary breakpoint 1 at 0x400605: file main.c, line 4.
Starting program: /tmp/a.out

Temporary breakpoint 1, main () at main.c:4
4          void *h = dlopen("./foo.so", RTLD_LAZY);
(gdb) catch syscall open
Catchpoint 2 (syscall 'open' [2])
(gdb) c
Continuing.

Catchpoint 2 (call to syscall open), 0x00007ffff7df3497 in open64 () at ../sysdeps/unix/syscall-template.S:81
81  ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) bt
#0  0x00007ffff7df3497 in open64 () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007ffff7ddf5bd in open_verify (name=0x602010 "./foo.so", fbp=0x7fffffffd568, loader=<optimized out>, whatcode=<optimized out>, found_other_class=0x7fffffffd550, free_name=<optimized out>) at dl-load.c:1930
#2  0x00007ffff7de2d6f in _dl_map_object (loader=loader@entry=0x7ffff7ffe1c8, name=name@entry=0x4006a4 "./foo.so", type=type@entry=2, trace_mode=trace_mode@entry=0, mode=mode@entry=-1879048191, nsid=0) at dl-load.c:2543
#3  0x00007ffff7deea14 in dl_open_worker (a=a@entry=0x7fffffffdae8) at dl-open.c:235
#4  0x00007ffff7de9fc4 in _dl_catch_error (objname=objname@entry=0x7fffffffdad8, errstring=errstring@entry=0x7fffffffdae0, mallocedp=mallocedp@entry=0x7fffffffdad0, operate=operate@entry=0x7ffff7dee960 <dl_open_worker>, args=args@entry=0x7fffffffdae8) at dl-error.c:187
#5  0x00007ffff7dee37b in _dl_open (file=0x4006a4 "./foo.so", mode=-2147483647, caller_dlopen=<optimized out>, nsid=-2, argc=1, argv=0x7fffffffde28, env=0x7fffffffde38) at dl-open.c:661
#6  0x00007ffff7bd702b in dlopen_doit (a=a@entry=0x7fffffffdd00) at dlopen.c:66
#7  0x00007ffff7de9fc4 in _dl_catch_error (objname=0x7ffff7dd9110 <last_result+16>, errstring=0x7ffff7dd9118 <last_result+24>, mallocedp=0x7ffff7dd9108 <last_result+8>, operate=0x7ffff7bd6fd0 <dlopen_doit>, args=0x7fffffffdd00) at dl-error.c:187
#8  0x00007ffff7bd762d in _dlerror_run (operate=operate@entry=0x7ffff7bd6fd0 <dlopen_doit>, args=args@entry=0x7fffffffdd00) at dlerror.c:163
#9  0x00007ffff7bd70c1 in __dlopen (file=<optimized out>, mode=<optimized out>) at dlopen.c:87
#10 0x0000000000400614 in main () at main.c:4

这告诉您,在64位系统上,dlopen调用open64而不是open,因此您的插入器无法工作(您需要插入open64而不是)。

但您使用的是32位系统(由0x806c000打印的strace等地址证明),堆栈跟踪如下所示:

#0  0xf7ff3774 in open () at ../sysdeps/unix/syscall-template.S:81
#1  0xf7fe1211 in open_verify (name=0x804b008 "./foo.so", fbp=fbp@entry=0xffffc93c, loader=0xf7ffd938, whatcode=whatcode@entry=0, found_other_class=found_other_class@entry=0xffffc933, free_name=free_name@entry=true) at dl-load.c:1930
#2  0xf7fe4114 in _dl_map_object (loader=loader@entry=0xf7ffd938, name=name@entry=0x8048590 "./foo.so", type=type@entry=2, trace_mode=trace_mode@entry=0, mode=mode@entry=-1879048191, nsid=0) at dl-load.c:2543
#3  0xf7feec14 in dl_open_worker (a=0xffffccdc) at dl-open.c:235
#4  0xf7feac06 in _dl_catch_error (objname=objname@entry=0xffffccd4, errstring=errstring@entry=0xffffccd8, mallocedp=mallocedp@entry=0xffffccd3, operate=operate@entry=0xf7feeb50 <dl_open_worker>, args=args@entry=0xffffccdc) at dl-error.c:187
#5  0xf7fee644 in _dl_open (file=0x8048590 "./foo.so", mode=-2147483647, caller_dlopen=0x80484ea <main+29>, nsid=<optimized out>, argc=1, argv=0xffffcf74, env=0xffffcf7c) at dl-open.c:661
#6  0xf7fafcbc in dlopen_doit (a=0xffffce90) at dlopen.c:66
#7  0xf7feac06 in _dl_catch_error (objname=0xf7fb3070 <last_result+12>, errstring=0xf7fb3074 <last_result+16>, mallocedp=0xf7fb306c <last_result+8>, operate=0xf7fafc30 <dlopen_doit>, args=0xffffce90) at dl-error.c:187
#8  0xf7fb037c in _dlerror_run (operate=operate@entry=0xf7fafc30 <dlopen_doit>, args=args@entry=0xffffce90) at dlerror.c:163
#9  0xf7fafd71 in __dlopen (file=0x8048590 "./foo.so", mode=1) at dlopen.c:87
#10 0x080484ea in main () at main.c:4

那么为什么open_verify未将open的电话重定向到您的open插播工具?

首先,让我们看一下第1帧中的实际调用指令:

(gdb) fr 1
#1  0xf7fe1211 in open_verify (name=0x804b008 "./foo.so", fbp=fbp@entry=0xffffc93c, loader=0xf7ffd938, whatcode=whatcode@entry=0, found_other_class=found_other_class@entry=0xffffc933, free_name=free_name@entry=true) at dl-load.c:1930
1930    dl-load.c: No such file or directory.
(gdb) x/i $pc-5
   0xf7fe120c <open_verify+60>: call   0xf7ff3760 <open>

将此与第10帧中的调用指令进行比较:

(gdb) fr 10
#10 0x080484ea in main () at main.c:4
4          void *h = dlopen("./foo.so", RTLD_LAZY);
(gdb) x/i $pc-5
   0x80484e5 <main+24>: call   0x80483c0 <dlopen@plt>

注意有什么不同吗?

这是正确的:来自main的调用通过过程链接表(PLT),动态加载器(ld-linux.so.2)解析为适当的定义。

但是第1帧中对open的调用不通过PLT(因此不是可插入的)。

为什么?因为该调用必须在 之前完成,所以open的任何其他定义都可用 - 它是,而 libc.so.6(通常提供定义) open)是本身被加载(由动态加载器)。

出于这个原因,动态加载器必须完全自包含(事实上,它包含静态链接的libc子集的副本。)

  

我的目标是挂钩dlopen on linux使用的open函数。

由于上述原因,无法通过LD_PRELOAD实现此目标。您需要使用其他一些挂钩机制,例如在运行时修补加载程序可执行代码。