|(How to avoid) Botching up ioctls
|By: Daniel Vetter, Copyright © 2013 Intel Corporation
|One clear insight kernel graphics hackers gained in the past few years is that
|trying to come up with a unified interface to manage the execution units and
|memory on completely different GPUs is a futile effort. So nowadays every
|driver has its own set of ioctls to allocate memory and submit work to the GPU.
|Which is nice, since there's no more insanity in the form of fake-generic, but
|actually only used once interfaces. But the clear downside is that there's much
|more potential to screw things up.
|To avoid repeating all the same mistakes again I've written up some of the
|lessons learned while botching the job for the drm/i915 driver. Most of these
|only cover technicalities and not the big-picture issues like what the command
|submission ioctl exactly should look like. Learning these lessons is probably
|something every GPU driver has to do on its own.
|First the prerequisites. Without these you have already failed, because you
|will need to add a 32-bit compat layer:
|* Only use fixed sized integers. To avoid conflicts with typedefs in userspace
|the kernel has special types like __u32, __s64. Use them.
|* Align everything to the natural size and use explicit padding. 32-bit
|platforms don't necessarily align 64-bit values to 64-bit boundaries, but
|64-bit platforms do. So we always need padding to the natural size to get
|* Pad the entire struct to a multiple of 64-bits if the structure contains
|64-bit types - the structure size will otherwise differ on 32-bit versus
|64-bit. Having a different structure size hurts when passing arrays of
|structures to the kernel, or if the kernel checks the structure size, which
|e.g. the drm core does.
|* Pointers are __u64, cast from/to a uintprt_t on the userspace side and
|from/to a void __user * in the kernel. Try really hard not to delay this
|conversion or worse, fiddle the raw __u64 through your code since that
|diminishes the checking tools like sparse can provide. The macro
|u64_to_user_ptr can be used in the kernel to avoid warnings about integers
|and pointers of different sizes.
|With the joys of writing a compat layer avoided we can take a look at the basic
|fumbles. Neglecting these will make backward and forward compatibility a real
|pain. And since getting things wrong on the first attempt is guaranteed you
|will have a second iteration or at least an extension for any given interface.
|* Have a clear way for userspace to figure out whether your new ioctl or ioctl
|extension is supported on a given kernel. If you can't rely on old kernels
|rejecting the new flags/modes or ioctls (since doing that was botched in the
|past) then you need a driver feature flag or revision number somewhere.
|* Have a plan for extending ioctls with new flags or new fields at the end of
|the structure. The drm core checks the passed-in size for each ioctl call
|and zero-extends any mismatches between kernel and userspace. That helps,
|but isn't a complete solution since newer userspace on older kernels won't
|notice that the newly added fields at the end get ignored. So this still
|needs a new driver feature flags.
|* Check all unused fields and flags and all the padding for whether it's 0,
|and reject the ioctl if that's not the case. Otherwise your nice plan for
|future extensions is going right down the gutters since someone will submit
|an ioctl struct with random stack garbage in the yet unused parts. Which
|then bakes in the ABI that those fields can never be used for anything else
|but garbage. This is also the reason why you must explicitly pad all
|structures, even if you never use them in an array - the padding the compiler
|might insert could contain garbage.
|* Have simple testcases for all of the above.
|Fun with Error Paths
|Nowadays we don't have any excuse left any more for drm drivers being neat
|little root exploits. This means we both need full input validation and solid
|error handling paths - GPUs will die eventually in the oddmost corner cases
|* The ioctl must check for array overflows. Also it needs to check for
|over/underflows and clamping issues of integer values in general. The usual
|example is sprite positioning values fed directly into the hardware with the
|hardware just having 12 bits or so. Works nicely until some odd display
|server doesn't bother with clamping itself and the cursor wraps around the
|* Have simple testcases for every input validation failure case in your ioctl.
|Check that the error code matches your expectations. And finally make sure
|that you only test for one single error path in each subtest by submitting
|otherwise perfectly valid data. Without this an earlier check might reject
|the ioctl already and shadow the codepath you actually want to test, hiding
|bugs and regressions.
|* Make all your ioctls restartable. First X really loves signals and second
|this will allow you to test 90% of all error handling paths by just
|interrupting your main test suite constantly with signals. Thanks to X's
|love for signal you'll get an excellent base coverage of all your error
|paths pretty much for free for graphics drivers. Also, be consistent with
|how you handle ioctl restarting - e.g. drm has a tiny drmIoctl helper in its
|userspace library. The i915 driver botched this with the set_tiling ioctl,
|now we're stuck forever with some arcane semantics in both the kernel and
|* If you can't make a given codepath restartable make a stuck task at least
|killable. GPUs just die and your users won't like you more if you hang their
|entire box (by means of an unkillable X process). If the state recovery is
|still too tricky have a timeout or hangcheck safety net as a last-ditch
|effort in case the hardware has gone bananas.
|* Have testcases for the really tricky corner cases in your error recovery code
|- it's way too easy to create a deadlock between your hangcheck code and
|Time, Waiting and Missing it
|GPUs do most everything asynchronously, so we have a need to time operations and
|wait for outstanding ones. This is really tricky business; at the moment none of
|the ioctls supported by the drm/i915 get this fully right, which means there's
|still tons more lessons to learn here.
|* Use CLOCK_MONOTONIC as your reference time, always. It's what alsa, drm and
|v4l use by default nowadays. But let userspace know which timestamps are
|derived from different clock domains like your main system clock (provided
|by the kernel) or some independent hardware counter somewhere else. Clocks
|will mismatch if you look close enough, but if performance measuring tools
|have this information they can at least compensate. If your userspace can
|get at the raw values of some clocks (e.g. through in-command-stream
|performance counter sampling instructions) consider exposing those also.
|* Use __s64 seconds plus __u64 nanoseconds to specify time. It's not the most
|convenient time specification, but it's mostly the standard.
|* Check that input time values are normalized and reject them if not. Note
|that the kernel native struct ktime has a signed integer for both seconds
|and nanoseconds, so beware here.
|* For timeouts, use absolute times. If you're a good fellow and made your
|ioctl restartable relative timeouts tend to be too coarse and can
|indefinitely extend your wait time due to rounding on each restart.
|Especially if your reference clock is something really slow like the display
|frame counter. With a spec lawyer hat on this isn't a bug since timeouts can
|always be extended - but users will surely hate you if their neat animations
|starts to stutter due to this.
|* Consider ditching any synchronous wait ioctls with timeouts and just deliver
|an asynchronous event on a pollable file descriptor. It fits much better
|into event driven applications' main loop.
|* Have testcases for corner-cases, especially whether the return values for
|already-completed events, successful waits and timed-out waits are all sane
|and suiting to your needs.
|Leaking Resources, Not
|A full-blown drm driver essentially implements a little OS, but specialized to
|the given GPU platforms. This means a driver needs to expose tons of handles
|for different objects and other resources to userspace. Doing that right
|entails its own little set of pitfalls:
|* Always attach the lifetime of your dynamically created resources to the
|lifetime of a file descriptor. Consider using a 1:1 mapping if your resource
|needs to be shared across processes - fd-passing over unix domain sockets
|also simplifies lifetime management for userspace.
|* Always have O_CLOEXEC support.
|* Ensure that you have sufficient insulation between different clients. By
|default pick a private per-fd namespace which forces any sharing to be done
|explicitly. Only go with a more global per-device namespace if the objects
|are truly device-unique. One counterexample in the drm modeset interfaces is
|that the per-device modeset objects like connectors share a namespace with
|framebuffer objects, which mostly are not shared at all. A separate
|namespace, private by default, for framebuffers would have been more
|* Think about uniqueness requirements for userspace handles. E.g. for most drm
|drivers it's a userspace bug to submit the same object twice in the same
|command submission ioctl. But then if objects are shareable userspace needs
|to know whether it has seen an imported object from a different process
|already or not. I haven't tried this myself yet due to lack of a new class
|of objects, but consider using inode numbers on your shared file descriptors
|as unique identifiers - it's how real files are told apart, too.
|Unfortunately this requires a full-blown virtual filesystem in the kernel.
|Last, but not Least
|Not every problem needs a new ioctl:
|* Think hard whether you really want a driver-private interface. Of course
|it's much quicker to push a driver-private interface than engaging in
|lengthy discussions for a more generic solution. And occasionally doing a
|private interface to spearhead a new concept is what's required. But in the
|end, once the generic interface comes around you'll end up maintainer two
|* Consider other interfaces than ioctls. A sysfs attribute is much better for
|per-device settings, or for child objects with fairly static lifetimes (like
|output connectors in drm with all the detection override attributes). Or
|maybe only your testsuite needs this interface, and then debugfs with its
|disclaimer of not having a stable ABI would be better.
|Finally, the name of the game is to get it right on the first attempt, since if
|your driver proves popular and your hardware platforms long-lived then you'll
|be stuck with a given ioctl essentially forever. You can try to deprecate
|horrible ioctls on newer iterations of your hardware, but generally it takes
|years to accomplish this. And then again years until the last user able to
|complain about regressions disappears, too.