Linux.com

First doubt here: memcpy freezes

Link to this post 17 Aug 11

Hi everyone,

I am a Linux noob and have only recently started playing around with it. So I apologize if this sounds too immature.

I am trying to "hijack" some syscalls by modifying their function pointers in the syscall table to redirect access to my own functions where I preprocess the arguments before calling the original functions with possibly modified arguments. I am doing this by loading an LKM in a user-mode linux running off kernel version 2.6.39.1 runnning in x86_64 mode. To get to the heart of the problem, I'll give an example of what happens for 2 syscalls - open and lstat.

All versions of sys_lstat accept two userland pointers - file-name and a struct where stat information is to be stored. What my function does is it checks the file-name and for some match, passes a different file-name (which is a kernel pointer) to the original sys_lstat version. A simplified version of the code is (ignore syntactic mistakes, if any):

asmlinkage long my_sys_lstat(const char __user *userland_filename,
struct __old_kernel_stat __user *statbuf)
{
int err;
struct path path;
bool modify_fs = false;
char* filename_param_to_orig = userland_filename;
mm_segment_t old_fs;

err = user_path_at(AT_FDCWD, userland_filename, 0, &path);
if (err) return err;

if (file_match(path)) { //compare inodes
// Modify DS before calling the original function with a kernel pointer
// as a param
modify_fs = true;
old_fs = = get_fs();
set_fs(KERNEL_DS);
filename_param_to_orig = my_mod_filename;
}

err = orig_sys_lstat(filename_param_to_orig, statbuf); // doesn't return if file matches
if (modify_fs)
set_fs(old_fs);
return err;
}

On a file match, it all goes fine until control reaches memcpy called from copy_to_user method (eventually called from sys_lstat), where the kernel tries to copy into the userland struct from its own struct which is filled with (hopefully) the right info. In memcpy (arch/x86/lib/memcpy_64.S) the kernel completely freezes. If gdb is to be trusted, this freeze happens on line 70 in the instruction:

movq 0*8(%rsi), %r8

Unfortunately I am not very well versed with x86 architecture either, and I have not been able to figure why this freeze happens.

A similar implementation for sys_open is working for me (passing a kernel pointer to the original function after changing the data segment).

Could anybody point out what's going wrong here or at least point me in a direction where I could look for it myself? Any help is much appreciated. Thanks a lot!

Who we are ?

The Linux Foundation is a non-profit consortium dedicated to the growth of Linux.

More About the foundation...

Frequent Questions

Join / Linux Training / Board