无法使用 dlsym 获取统计信息 [英] Unable to get stat with dlsym

问题描述

我正在尝试为 stat/lstat/fstat 编写一个钩子，但我似乎无法从dlsym.我正在使用以下代码获取原始指针.

orig_stat = dlsym(RTLD_NEXT, "stat");orig_lstat = dlsym(RTLD_NEXT, "lstat");orig_fstat = dlsym(RTLD_NEXT, "fstat");

然而，所有三个变量都设置为 null 并且调用 dlerror 也会返回 null.

我正在创建共享对象:

clang fakestat.c -shared -fPIC -ldl -o fakestat.so

并通过运行带有 LD_PRELOAD=fakestat.so ./test

的测试程序来使用我的库

解决方案

stat 系列函数实际上是 glibc 中内部函数的包装器.您可以在调用任何标准 [lf]stat 调用的二进制文件上查看 objdump 的输出，您将看到二进制文件中这些函数的实际符号.所以你必须挂钩这些功能.以下是您要挂钩/dlsym 的函数:

__xstat &__xstat64 用于 stat.

__lxstat &__lxstat64 用于 lstat.

__fxstat &__fxstat64 用于 fstat.

64 后缀函数用于 LFS 支持.请参阅功能测试宏.>

I'm trying to write a hook for stat/lstat/fstat but I can't seem to get the original version from dlsym. I'm using the following code to obtain the original pointers.

orig_stat = dlsym(RTLD_NEXT, "stat");
orig_lstat = dlsym(RTLD_NEXT, "lstat");
orig_fstat = dlsym(RTLD_NEXT, "fstat");

However, all three variables are set to null and calling dlerror also returns null.

I'm creating the shared object with:

clang fakestat.c -shared -fPIC -ldl -o fakestat.so

and using my library by running a test program with LD_PRELOAD=fakestat.so ./test

解决方案

The stat family functions are actually wrappers to internal functions in glibc. You can look at the output of objdump on your binary that calls any of the standard [lf]stat calls and you'll see the actual symbols in the binary for these functions. So you'll have to hook those functions. The following are the functions you want to hook/dlsym on:

__xstat & __xstat64 for stat.

__lxstat & __lxstat64 for lstat.

__fxstat & __fxstat64 for fstat.

The 64 suffixed functions are for LFS support. See the feature test macros.

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