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    SubjectRe: [PATCH v2 01/17] compat_ioctl: add generic_compat_ioctl_ptrarg()
    On Thu, Sep 13, 2018 at 4:07 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
    >
    > On Wed, Sep 12, 2018 at 05:01:02PM +0200, Arnd Bergmann wrote:
    > > Many drivers have ioctl() handlers that are completely compatible
    > > between 32-bit and 64-bit architectures, except for the argument
    > > that is passed down from user space and may have to be passed
    > > through compat_ptr() in order to become a valid 64-bit pointer.
    > >
    > > Using ".compat_ptr=generic_compat_ioctl_ptrarg" in file operations
    > > should let us simplify a lot of those drivers to avoid #ifdef
    > > checks, and convert additional drivers that don't have proper
    > > compat handling yet.
    >
    > Just keep in mind that this should *only* be used when all
    > ioctls implemented in a given instance do take pointers.
    > Because otherwise you are asking for trouble - e.g. if one of
    > them takes an u32 used as a bitmap, this will run into trouble
    > as soon as somebody uses bit 31. With no visible warnings.
    >
    > IOW, it shouldn't be used blindly and it should come with big
    > fat warning.

    I was hoping that the _ptrarg suffix gives enough warning here,
    but maybe not. I was careful to only use it in cases that I
    checked are safe, either using only pointer arguments, or
    no arguments.

    What we might do for further clarification (besides adding a
    comment next to the declaration), would be to add a
    complementary generic_compat_ioctl_intarg() that skips
    the compat_ptr(). There are only a handful of drivers that
    would use this though.

    Arnd

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