Messages in this thread | | | Date | Sat, 27 Sep 2008 09:13:04 -0700 | From | Arjan van de Ven <> | Subject | Re: [PATCH 0/3][RFC] ioctl dispatcher |
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On Sat, 27 Sep 2008 18:43:59 +0300 Avi Kivity <avi@redhat.com> 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. > > The second patch addresses another problem with ioctls: they are > brittle. Once written, an ioctl cannot be extended, since the buffer > sizes used for transferring data are encoded in the ioctl number. > This is addressed by allowing the user-supplied size and the > kernel-visible size of the data buffer to be different; the kernel > will zero fill or truncate appropriately. With the new mechanism, it > is easy to write forward- and backward- compatible ioctl handlers. > > The third patch demonstrates the effectiveness of the first patch; it > converts some of kvm's ioctl handlers to the new mechanism, removing > around 90 lines in the process. >
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... 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.... 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.
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