取消引用空指针时内核崩溃

问题描述

我有一个像这样的简单模块:

I have a simple module like this:

#define MODULE

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>

int init_module(void) {
    struct inode {
        int i_ino;
    };
    struct dentry {
        struct inode *d_inode;
    };
    struct dentry *f_dentry;
    f_dentry = NULL;
    struct inode * p = f_dentry->d_inode;
    return 0;
}

void cleanup_module(void) {
        printk("Goodbye world\n");
}

我的Makefile是这样的:

obj-m += oops.o

all:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
        make -C /lib/modules$(shell uname -r)/build M=$(PWD) clean

我希望内核会崩溃，因为struct inode * p = f_dentry->d_inode;取消引用了空指针，对吗?但事实并非如此.我的想法有什么问题吗?好吧，现在我再尝试一次.如果我的模块是这样的:

I expect that the kernel will crash because struct inode * p = f_dentry->d_inode; has dereferenced a null pointer, right? But it does not. Anything wrong with my idea?All right, now I'll have one more try. If my module is like this:

#define MODULE

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>

int init_module(void) {
    *(int *)0 = 0;
    return 0;
}

void cleanup_module(void) {
        printk("Goodbye world\n");
}

我的计算机确实崩溃了.我以前的例子有什么问题吗?它不会取消引用空指针吗?

My computer really crashes. Or anything wrong with my former example? It doesn't dereference a null pointer?

推荐答案

如果您查看汇编代码(例如，通过objdump -D oops.ko)，那么所有init_module()都会被优化，大概是因为它不会什么都不要做.

If you look at the assembly code(via e.g. objdump -D oops.ko), all of your init_module() is optimized away, presumably because it doesn't do anything.

例如做p->i_ino = 1;，您可能会看到不同的结果(尽管这是未定义的行为，所以并不能直接推断出代码将要做什么的原因-在这种情况下也最好检查一下汇编).

If you e.g. do p->i_ino = 1; , you'll likely see different results(Albeit this is undefined behavior, so it's not straight forward to reason about what the code is going to do - better check the assembly in this case too).

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