关于截获execve等系统调用，很久以来存在一个问题：新函数不能直接调旧函数，
否则导致stack不平衡，出错。

曾经有高人用一串的汇编代码去平衡堆栈， 但对于偶们这些汇编菜鸟来说， 连阅读都很困难。
而且， 好像gcc4.x下不支持 它使用的一种寻址方式了。

这次使用kprobes来直接找do_execve，直接照搬sys_execve的实现代码。
至于LICENSE的问题，谁用谁负责，我只是从技术角度上说可以这样做。(我的代码是GPL的)

只截获了open和execve作为示例，其它的syscall都是一样的道理。

README:

Can work at i386, linux 2.6.22.

We only intercepted  1) sys_open, and 2) sys_execve,  for an illustrating purpose.


New Feautres:

        ->  Check and verify the WP bit of CR0.
        ->  Intercept execve with the help of kprobes. Likewise, fork/vfork/clone
            can be intercepted in the same way.

源代码：

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kprobes.h>
#include <linux/kallsyms.h>
#include <linux/sched.h>
#include <linux/ptrace.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/user.h>
#include <linux/errno.h>
#include <linux/cpu.h>
#include <asm/uaccess.h>
#include <asm/fcntl.h>
#include <asm/unistd.h>

MODULE_DESCRIPTION("Intercept the system call table in Linux");
MODULE_AUTHOR("alert7 (alert7@xfocus.org) \n\t\talbcamus <albcamus@gmail.com>");
MODULE_LICENSE("GPL");


/* comment the following line to shut me up */
#define INTERCEPT_DEBUG

#ifdef INTERCEPT_DEBUG
    #define dbgprint(format,args...) \
        printk("intercept: function:%s-L%d: "format, __FUNCTION__, __LINE__, ##args);
#else
    #define dbgprint(format,args...)  do {} while();
#endif


/**
* the system call table
*/
void **my_table;

unsigned int orig_cr0;

/**
* the original syscall functions
*/
asmlinkage long (*old_open) (char __user *filename, int flags, int mode);
asmlinkage int  (*old_execve) (struct pt_regs regs);



/** do_execve and do_fork */
unsigned int can_exec_fork = ;
int    (*new_do_execve) (char * filename,
            char __user *__user *argv,
            char __user *__user *envp,
            struct pt_regs * regs);


struct idtr {
    unsigned short limit;
    unsigned int base;
} __attribute__ ((packed));


struct idt {
    unsigned short off1;
    unsigned short sel;
    unsigned char none, flags;
    unsigned short off2;
} __attribute__ ((packed));



/**
*  check if we can intercept fork/vfork/clone/execve or not
*
*  return : 0 for no, 1 for yes
*/
struct kprobe kp_exec;
unsigned int can_intercept_fork_exec(void)
{
    int ret = ;

#ifndef CONFIG_KPROBES
    return ret;
#endif

    kp_exec.symbol_name = "do_execve";

    ret = register_kprobe(&kp_exec);
    if (ret !=  ) {
        dbgprint("cannot find do_execve by kprobe.\n");
        return ;
    }
    new_do_execve = ( int (*) 
             (char *,
              char __user * __user *,
              char __user * __user *,
              struct pt_regs *
             ) 
            ) kp_exec.addr;

    dbgprint("do_execve at %p\n", (void *)kp_exec.addr);
    unregister_kprobe(&kp_exec);


    return 1;
}



/** 
* clear WP bit of CR0, and return the original value
*/
unsigned int clear_and_return_cr0(void)
{
    unsigned int cr0 = ;
    unsigned int ret;

    asm volatile ("movl %%cr0, %%eax"
              : "=a"(cr0)
              );
    ret = cr0;

    /* clear the 20 bit of CR0, a.k.a WP bit */
    cr0 &= 0xfffeffff;

    asm volatile ("movl %%eax, %%cr0"
              :
              : "a"(cr0)
              );
    return ret;
}

/** set CR0 with new value
*
* @val : new value to set in cr0
*/
void setback_cr0(unsigned int val)
{
    asm volatile ("movl %%eax, %%cr0"
              :
              : "a"(val)
              );
}


/**
* Return the first appearence of NEEDLE in HAYSTACK.  
* */
static void *memmem(const void *haystack, size_t haystack_len,
            const void *needle, size_t needle_len)
{/*{{{*/
    const char *begin;
    const char *const last_possible
        = (const char *) haystack + haystack_len - needle_len;

    if (needle_len == )
        /* The first occurrence of the empty string is deemed to occur at
           the beginning of the string.  */
        return (void *) haystack;

    /* Sanity check, otherwise the loop might search through the whole
       memory.  */
    if (__builtin_expect(haystack_len < needle_len, ))
        return NULL;

    for (begin = (const char *) haystack; begin <= last_possible;
         ++begin)
        if (begin[] == ((const char *) needle)[]
            && !memcmp((const void *) &begin[1],
                   (const void *) ((const char *) needle + 1),
                   needle_len - 1))
            return (void *) begin;

    return NULL;
}/*}}}*/


/**
* Find the location of sys_call_table
*/
static unsigned long get_sys_call_table(void)
{/*{{{*/
/* we'll read first 100 bytes of int $0x80 */
#define OFFSET_SYSCALL 100        

    struct idtr idtr;
    struct idt idt;
    unsigned sys_call_off;
    unsigned retval;
    char sc_asm[OFFSET_SYSCALL], *p;

    /* well, let's read IDTR */
    asm("sidt %0":"=m"(idtr)
             :
             :"memory" );

    dbgprint("idtr base at 0x%X\n", (unsigned int)idtr.base);

    /* Read in IDT for vector 0x80 (syscall) */
    memcpy(&idt, (char *) idtr.base + 8 * 0x80, sizeof(idt));

    sys_call_off = (idt.off2 << 16) | idt.off1;

    dbgprint("idt80: flags=%X sel=%X off=%X\n",
                 (unsigned) idt.flags, (unsigned) idt.sel, sys_call_off);

    /* we have syscall routine address now, look for syscall table
       dispatch (indirect call) */
    memcpy(sc_asm, (void *)sys_call_off, OFFSET_SYSCALL);

    /**
     * Search opcode of `call sys_call_table(,eax,4)'
     */
    p = (char *) memmem(sc_asm, OFFSET_SYSCALL, "\xff\x14\x85", 3);
    if (p == NULL)
        return ;

    retval = *(unsigned *) (p + 3);
    if (p) {
        dbgprint("sys_call_table at 0x%x, call dispatch at 0x%x\n",
             retval, (unsigned int) p);
    }
    return retval;
#undef OFFSET_SYSCALL
}/*}}}*/



/**
* new_open - replace the original sys_open when initilazing, 
*           as well as be got rid of when removed
*/
asmlinkage long new_open(char *filename, int flags, int mode)
{
    dbgprint("hello\n");
    return old_open (filename, flags, mode);
}


/**
* new_execve - you should change this function whenever the kernel's sys_execve() 
* changes
*/
asmlinkage int new_execve(struct pt_regs regs)
{
    int error;
    char *filename;

    dbgprint("Hello\n");

    filename = getname( (char __user *) regs.ebx );
    error = PTR_ERR(filename);
    if ( IS_ERR(filename) )
        goto out;
    dbgprint("file to execve: %s\n", filename);
    error = new_do_execve(filename,
                  (char __user * __user *) regs.ecx,
                  (char __user * __user *) regs.edx,
                  ®s);
    if (error == ) {
        task_lock(current);
        current->ptrace &= ~PT_DTRACE;
        task_unlock(current);
        set_thread_flag(TIF_IRET);
    }
    putname (filename);
out:
    return error;
}

static int intercept_init(void)
{
    my_table = (void **)get_sys_call_table();
    if (my_table == NULL)
        return -1;

    dbgprint("sys call table address %p\n", (void *) my_table);

#define REPLACE(x) old_##x = my_table[__NR_##x];\
    my_table[__NR_##x] = new_##x

    
    REPLACE(open);
    can_exec_fork = can_intercept_fork_exec();
    if(can_exec_fork == 1)
        REPLACE(execve);


#undef REPLACE
    return ;
}





static int __init this_init(void)
{
    int ret;
    printk("syscall intercept: Hi, poor linux!\n");

    orig_cr0 = clear_and_return_cr0();    
    ret = intercept_init();
    setback_cr0(orig_cr0);

    return ret;
}

static void __exit this_fini(void)
{
    printk("syscall intercept: bye, poor linux!\n");

#define RESTORE(x) my_table[__NR_##x] = old_##x

    orig_cr0 = clear_and_return_cr0();    
    RESTORE(open);
    if(can_exec_fork == 1)
        RESTORE(execve);
    setback_cr0(orig_cr0);

#undef RESTORE
}

module_init(this_init);
module_exit(this_fini);

文章来源CU社区：使用kprobes，截获execve系统调用