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SubjectRe: __user annotations

On Sat, 7 Jun 2003, Ingo Oeser wrote:
>
> That's a big pity. How do I workaround this? I would like to
> help resolving this issues, if you are interested.

The solution to these things is to _always_ have a separate type for the
user thing than for the kernel thing.

In practice, a lot of code has ended up doing that _anyway_, since the
kernel usually wants to have a few extra fields for its internal use. The
classic unix example of this, of course, is "struct stat" vs "struct
inode".

But if your structures are 100% the same, then you can just share them.
There's nothing wrong with having

struct ioctl_arg {
int value;
int another_value;
..
};

and then in your ioctl routines you have

int my_ioctl_routine(struct ioctl_arg __user *ptr)
{
struct ioctl_arg arg;

if (copy_from_user(&arg, ptr, sizeof(*ptr))
return -EFAULT;
...
}

and that's fine.

You can even have user pointers _inside_ the structure: because "sparse"
really understands C types at a very fundamental level (like a compiler
would, not like some simpler source scanner), you can have

struct ioctl_arg {
int value;
void __user *buf;
};

and do

int my_ioctl_routine(struct ioctl_arg __user *ptr)
{
struct ioctl_arg arg;
char buffer[10];

if (copy_from_user(&arg, ptr, sizeof(*ptr))
return -EFAULT;

and sparse will be aware of the fact that "arg.buf" is a user pointer, and
it will properly warn if you pass it to a function that expects a kernel
pointer (or assign it to a normal non-user pointer thing).

Linus

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