我可以将整数作为第二个参数传递给`access_ok()`吗? [英] Can I pass an integer to `access_ok()` as it's second argument?

问题描述

在 LDD3 的示例中，将access_ok()放在内核模块的ioctl方法的开头，以检查从用户空间传递的指针是否有效.当用户空间应用程序调用ioctl()系统调用并将变量的地址传递给它时，它是正确的.但是，在某些情况下，内核模块中的ioctl()系统调用会使用值而不是指针作为第三个参数，最后是access_ok()的第二个参数来调用.

In LDD3's example, access_ok() is placed at the beginning of ioctl method of a kernel module to check whether a pointer passed from userspace is valid. It is correct when userspace application calls ioctl() system call, and passes it an address of a variable. In some cases, however, ioctl() system call is invoked with a value instead of a pointer as third argument and finally the second argument of access_ok() in kernel module.

我试图传递一个整数作为access_ok()的第二个参数，并且它可以正常工作.没有错误的报告.但是我不太确定这种用法是否正确?

I've tried to pass an integer as access_ok()'s second argument and it works fine. No error was reported. But I don't very sure that is this usage correct?

例如，如果我在用户空间中调用ioctl()并将其第三个参数设为'3'.然后，在struct file_operations的ioctl()方法中，access_ok()将收到3作为第二个参数.因为access_ok()需要一个指针，所以它将3转换为用户空间指针.显然，这是错误的...

For example, if I invoke ioctl() in userspace with it's third argument to be '3'. Then, in ioctl() method of struct file_operations, access_ok() will receive 3 as it's second argument. Because the access_ok() expects a pointer, so it translates 3 to be a userspace pointer. Obversely, it's wrong...

推荐答案

实际上，access_ok的检查很粗糙.函数描述(在源文件中)说:

Actually, access_ok's check is rough. Description of the function (in the source file) say:

请注意，根据架构的不同，此功能可能只是 调用后检查指针是否在用户空间范围内 此功能，内存访问功能可能仍会返回-EFAULT.

Note that, depending on architecture, this function probably just checks that the pointer is in the user space range - after calling this function, memory access functions may still return -EFAULT.

例如，根据来源arch/x86/include/asm/uaccess.h，在x86 access_ok上仅检查给定的地址指向 lower 区域(因为内核除了在上部区域之外).因此，对于等于3的地址，它返回true.

E.g., according to source arch/x86/include/asm/uaccess.h, on x86 access_ok just checks that given address points to the lower area (because kernel besides in the upper area). So, it returns true for address equal to 3.

copy_from_user/copy_to_user返回有关用户内存可访问性的最终结论.

It is copy_from_user/copy_to_user who return a final verdict about user memory accessibility.

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