fsh-example

This repository shows how to use fsh syscall hooking library.

First, set MODULE_NAME and FSH_CUSTOM_OBJS variables in makefile:

# Makefile
MODULE_NAME := fsh_example
FSH_CUSTOM_OBJS := fsh_example_mod.o

In this example, we only have one source fsh_example_mod.c, thus FSH_CUSTOM_OBJS is fsh_example_mod.o.

In addition, we have to declare which system calls will be hooked. In this example, two syscalls are hooked: sys_execve and sys_open:

$ cat overrided_syscalls.tbl
sys_execve
sys_openat

You can use the filename whatever you prefer, just remember to provide it later when executing the header generation script.

The hooking function of sys_openat, defined in fsh_example_mod.c, prints the filename whenever the user who opened the file matches a specific uid:

FSH_SYSCALL_OVERRIDE(sys_openat)
{
    char filename[256] = {0};
    const uid_t uid = 1000;

    if (current->cred->uid.val != uid) return true;

    if (strncpy_from_user(filename, (const char *) *FSH_FUNC_ARG2,
                          sizeof(filename) - 1) < 0) {
        return true;
    }

    pr_info("fsh: file %s opened by uid %u\n", filename,
            current->cred->uid.val);
    return true;
}

Generate the fsh header before compiling the kernel module:

./fsh/scripts/gen_syscall_hdr.sh overrided_syscalls.tbl

To play with the module, use make to compile and make install to install it:

make && make install

Observe the kernel messages with demsg:

[6439379.577902] fsh: file userdb opened by uid 1000
[6439379.579052] fsh: file / opened by uid 1000
[6439379.579057] fsh: file usr opened by uid 1000
[6439379.579063] fsh: file lib opened by uid 1000
[6439379.579067] fsh: file userdb opened by uid 1000
[6439379.579287] fsh: file /etc/passwd opened by uid 1000
[6439379.579319] fsh: file /etc/security/pam_env.conf opened by uid 1000

You can see we’ve successfully hooked our open syscall!