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1=================================2(How to avoid) Botching up ioctls3=================================4 5From: https://blog.ffwll.ch/2013/11/botching-up-ioctls.html6 7By: Daniel Vetter, Copyright © 2013 Intel Corporation8 9One clear insight kernel graphics hackers gained in the past few years is that10trying to come up with a unified interface to manage the execution units and11memory on completely different GPUs is a futile effort. So nowadays every12driver has its own set of ioctls to allocate memory and submit work to the GPU.13Which is nice, since there's no more insanity in the form of fake-generic, but14actually only used once interfaces. But the clear downside is that there's much15more potential to screw things up.16 17To avoid repeating all the same mistakes again I've written up some of the18lessons learned while botching the job for the drm/i915 driver. Most of these19only cover technicalities and not the big-picture issues like what the command20submission ioctl exactly should look like. Learning these lessons is probably21something every GPU driver has to do on its own.22 23 24Prerequisites25-------------26 27First the prerequisites. Without these you have already failed, because you28will need to add a 32-bit compat layer:29 30 * Only use fixed sized integers. To avoid conflicts with typedefs in userspace31 the kernel has special types like __u32, __s64. Use them.32 33 * Align everything to the natural size and use explicit padding. 32-bit34 platforms don't necessarily align 64-bit values to 64-bit boundaries, but35 64-bit platforms do. So we always need padding to the natural size to get36 this right.37 38 * Pad the entire struct to a multiple of 64-bits if the structure contains39 64-bit types - the structure size will otherwise differ on 32-bit versus40 64-bit. Having a different structure size hurts when passing arrays of41 structures to the kernel, or if the kernel checks the structure size, which42 e.g. the drm core does.43 44 * Pointers are __u64, cast from/to a uintptr_t on the userspace side and45 from/to a void __user * in the kernel. Try really hard not to delay this46 conversion or worse, fiddle the raw __u64 through your code since that47 diminishes the checking tools like sparse can provide. The macro48 u64_to_user_ptr can be used in the kernel to avoid warnings about integers49 and pointers of different sizes.50 51 52Basics53------54 55With the joys of writing a compat layer avoided we can take a look at the basic56fumbles. Neglecting these will make backward and forward compatibility a real57pain. And since getting things wrong on the first attempt is guaranteed you58will have a second iteration or at least an extension for any given interface.59 60 * Have a clear way for userspace to figure out whether your new ioctl or ioctl61 extension is supported on a given kernel. If you can't rely on old kernels62 rejecting the new flags/modes or ioctls (since doing that was botched in the63 past) then you need a driver feature flag or revision number somewhere.64 65 * Have a plan for extending ioctls with new flags or new fields at the end of66 the structure. The drm core checks the passed-in size for each ioctl call67 and zero-extends any mismatches between kernel and userspace. That helps,68 but isn't a complete solution since newer userspace on older kernels won't69 notice that the newly added fields at the end get ignored. So this still70 needs a new driver feature flags.71 72 * Check all unused fields and flags and all the padding for whether it's 0,73 and reject the ioctl if that's not the case. Otherwise your nice plan for74 future extensions is going right down the gutters since someone will submit75 an ioctl struct with random stack garbage in the yet unused parts. Which76 then bakes in the ABI that those fields can never be used for anything else77 but garbage. This is also the reason why you must explicitly pad all78 structures, even if you never use them in an array - the padding the compiler79 might insert could contain garbage.80 81 * Have simple testcases for all of the above.82 83 84Fun with Error Paths85--------------------86 87Nowadays we don't have any excuse left any more for drm drivers being neat88little root exploits. This means we both need full input validation and solid89error handling paths - GPUs will die eventually in the oddmost corner cases90anyway:91 92 * The ioctl must check for array overflows. Also it needs to check for93 over/underflows and clamping issues of integer values in general. The usual94 example is sprite positioning values fed directly into the hardware with the95 hardware just having 12 bits or so. Works nicely until some odd display96 server doesn't bother with clamping itself and the cursor wraps around the97 screen.98 99 * Have simple testcases for every input validation failure case in your ioctl.100 Check that the error code matches your expectations. And finally make sure101 that you only test for one single error path in each subtest by submitting102 otherwise perfectly valid data. Without this an earlier check might reject103 the ioctl already and shadow the codepath you actually want to test, hiding104 bugs and regressions.105 106 * Make all your ioctls restartable. First X really loves signals and second107 this will allow you to test 90% of all error handling paths by just108 interrupting your main test suite constantly with signals. Thanks to X's109 love for signal you'll get an excellent base coverage of all your error110 paths pretty much for free for graphics drivers. Also, be consistent with111 how you handle ioctl restarting - e.g. drm has a tiny drmIoctl helper in its112 userspace library. The i915 driver botched this with the set_tiling ioctl,113 now we're stuck forever with some arcane semantics in both the kernel and114 userspace.115 116 * If you can't make a given codepath restartable make a stuck task at least117 killable. GPUs just die and your users won't like you more if you hang their118 entire box (by means of an unkillable X process). If the state recovery is119 still too tricky have a timeout or hangcheck safety net as a last-ditch120 effort in case the hardware has gone bananas.121 122 * Have testcases for the really tricky corner cases in your error recovery code123 - it's way too easy to create a deadlock between your hangcheck code and124 waiters.125 126 127Time, Waiting and Missing it128----------------------------129 130GPUs do most everything asynchronously, so we have a need to time operations and131wait for outstanding ones. This is really tricky business; at the moment none of132the ioctls supported by the drm/i915 get this fully right, which means there's133still tons more lessons to learn here.134 135 * Use CLOCK_MONOTONIC as your reference time, always. It's what alsa, drm and136 v4l use by default nowadays. But let userspace know which timestamps are137 derived from different clock domains like your main system clock (provided138 by the kernel) or some independent hardware counter somewhere else. Clocks139 will mismatch if you look close enough, but if performance measuring tools140 have this information they can at least compensate. If your userspace can141 get at the raw values of some clocks (e.g. through in-command-stream142 performance counter sampling instructions) consider exposing those also.143 144 * Use __s64 seconds plus __u64 nanoseconds to specify time. It's not the most145 convenient time specification, but it's mostly the standard.146 147 * Check that input time values are normalized and reject them if not. Note148 that the kernel native struct ktime has a signed integer for both seconds149 and nanoseconds, so beware here.150 151 * For timeouts, use absolute times. If you're a good fellow and made your152 ioctl restartable relative timeouts tend to be too coarse and can153 indefinitely extend your wait time due to rounding on each restart.154 Especially if your reference clock is something really slow like the display155 frame counter. With a spec lawyer hat on this isn't a bug since timeouts can156 always be extended - but users will surely hate you if their neat animations157 starts to stutter due to this.158 159 * Consider ditching any synchronous wait ioctls with timeouts and just deliver160 an asynchronous event on a pollable file descriptor. It fits much better161 into event driven applications' main loop.162 163 * Have testcases for corner-cases, especially whether the return values for164 already-completed events, successful waits and timed-out waits are all sane165 and suiting to your needs.166 167 168Leaking Resources, Not169----------------------170 171A full-blown drm driver essentially implements a little OS, but specialized to172the given GPU platforms. This means a driver needs to expose tons of handles173for different objects and other resources to userspace. Doing that right174entails its own little set of pitfalls:175 176 * Always attach the lifetime of your dynamically created resources to the177 lifetime of a file descriptor. Consider using a 1:1 mapping if your resource178 needs to be shared across processes - fd-passing over unix domain sockets179 also simplifies lifetime management for userspace.180 181 * Always have O_CLOEXEC support.182 183 * Ensure that you have sufficient insulation between different clients. By184 default pick a private per-fd namespace which forces any sharing to be done185 explicitly. Only go with a more global per-device namespace if the objects186 are truly device-unique. One counterexample in the drm modeset interfaces is187 that the per-device modeset objects like connectors share a namespace with188 framebuffer objects, which mostly are not shared at all. A separate189 namespace, private by default, for framebuffers would have been more190 suitable.191 192 * Think about uniqueness requirements for userspace handles. E.g. for most drm193 drivers it's a userspace bug to submit the same object twice in the same194 command submission ioctl. But then if objects are shareable userspace needs195 to know whether it has seen an imported object from a different process196 already or not. I haven't tried this myself yet due to lack of a new class197 of objects, but consider using inode numbers on your shared file descriptors198 as unique identifiers - it's how real files are told apart, too.199 Unfortunately this requires a full-blown virtual filesystem in the kernel.200 201 202Last, but not Least203-------------------204 205Not every problem needs a new ioctl:206 207 * Think hard whether you really want a driver-private interface. Of course208 it's much quicker to push a driver-private interface than engaging in209 lengthy discussions for a more generic solution. And occasionally doing a210 private interface to spearhead a new concept is what's required. But in the211 end, once the generic interface comes around you'll end up maintaining two212 interfaces. Indefinitely.213 214 * Consider other interfaces than ioctls. A sysfs attribute is much better for215 per-device settings, or for child objects with fairly static lifetimes (like216 output connectors in drm with all the detection override attributes). Or217 maybe only your testsuite needs this interface, and then debugfs with its218 disclaimer of not having a stable ABI would be better.219 220Finally, the name of the game is to get it right on the first attempt, since if221your driver proves popular and your hardware platforms long-lived then you'll222be stuck with a given ioctl essentially forever. You can try to deprecate223horrible ioctls on newer iterations of your hardware, but generally it takes224years to accomplish this. And then again years until the last user able to225complain about regressions disappears, too.226