Re: [PATCH 0/3][RFC] ioctl dispatcher

[Avi Kivity <avi@redhat.com>]

Arjan van de Ven wrote:
>   
>> While ioctls are officially ugly, they are the best choice for some
>> use cases, not to mention compatibility issues.  Currently ioctl
>> writers face the following hurdles:
>>
>> - if the ioctl uses a data buffer, the ioctl handler must allocate
>>   kernel memory for this buffer
>>   - the memory may be allocated on the heap or on the stack,
>> depending on the buffer size
>> - handle any errors from the operation
>> - copy the data from userspace, if necessary
>> - handle any errors from the operation
>> - actually perform the operation
>> - copy the data back to userspace, if necessary
>> - handle any errors from the operation
>> - free the buffer, if allocated from the heap
>>
>> The first patch automates these operations, only requiring the caller
>> to supply the ioctl number and a callback in a table.
>>
>>     
> this doesn't seem to be much different from the way the DRM code deals
> with ioctls. Or the V4L code.
> Personally I hate that code though.....
>
> There is a fine balance here; between driver writers screwing something
> up they shouldn't be doing in the first place and us being able to
> clearly see what the code is doing; your patch kinda hides some key
> elements of the ioctl path... 

Which key elements?

It hides the big switch (ioctl), memory allocation, and error handling, 
and exposes the actual ioctl-specific code, which I thought was the key 
element.

Why are we interested in boilerplate?

> I'm afraid it gives a false sense of
> security though. Not having to deal with one aspect of security just to
> have to deal with the rest....
>   

It reduces the number of potential mistakes a driver author can make.

> Lets put it this way: if the driver author has to type "copy_from_user"
> there's a chance that he'll remember that the data comes from the user
> and isn't to be trusted on face value.
>   

I'll rename the argp variable to argp_user_supplied.

I can't believe you think writing the copy code from scratch (or worse, 
copy/paste) each time helps security.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.