Question

我得到 sys_call_table from /boot/System.map- uname-r 的地址，我的内核是2.6.32（x86-32）。

当我拦截了一些syscalls except 'read'时，我的模块做得很好。但是，当我截获'read' syscall,时，它发生了一个错误。重新出现如下：

(tty1)                           (tty2)

insmod mymodule.ko               opened
read some files
rmmod mymodule        
insmod mymodule.ko     
                                switch to this tty*

然后发生内核恐慌并且系统崩溃...... 这是我的代码：

#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/fs.h>
#include <linux/file.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_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(0);
#endif


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

unsigned int orig_cr0;

/**
 * * the original syscall functions
 * */
asmlinkage long (*old_read) (unsigned int fd, char __user *buf, size_t count);


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));



/**
 * * clear WP bit of CR0, and return the original value
 * */
unsigned int clear_and_return_cr0(void)
{
    unsigned int cr0 = 0;
    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 appearance 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 == 0)
        /* 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, 0))
        return NULL;

    for (begin = (const char *) haystack; begin <= last_possible;
         ++begin)
        if (begin[0] == ((const char *) needle)[0]
            && !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 0;

    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
}/*}}}*/

asmlinkage long new_read(unsigned int fd, char __user *buf, size_t count)
{
    long ret;
    struct file *file;
    ret = old_read(fd, buf, count);
    /*0 represent EOF
    * We just collect a 'read' when finished
    */
    if(ret == 0){
        file = fget(fd);
        if(file){
            fput(file);
        }
    }
    return ret;
}

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(read);

#undef REPLACE
    return 0;
}


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(read);
    setback_cr0(orig_cr0);

#undef RESTORE
}

module_init(this_init);
module_exit(this_fini);

PS：删除new_read函数中的if块，我的模块运行良好。

if(ret == 0){
        file = fget(fd);
        if(file){
            fput(file);
        }
    }

任何人都知道为什么以及如何解决它;我想要的就是获取文件结构的信息。

Answer 1

您的读取系统调用是否是实际读取的完美替代品？它是否以递归方式执行了某些操作？就像调用一个调用read的函数一样？ Read是一个非常低级别的系统调用。几乎所有东西都必须从文件描述符中读取信息才能完成任何工作......

在Linux内核中拦截读取系统调用时的错误

1 个答案: