Documentation: add How to avoid botching up ioctls

I pointed some folks at this and they wondered why it wasn't in the
kernel Documentation directory. So now it is.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 219 insertions, 0 deletions

diff --git a/Documentation/ioctl/botching-up-ioctls.txt b/Documentation/ioctl/botching-up-ioctls.txt
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+(How to avoid) Botching up ioctls
+=================================
+
+From: http://blog.ffwll.ch/2013/11/botching-up-ioctls.html
+
+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.
+
+
+Prerequisites
+-------------
+
+First the prerequisites. Without these you have already failed, because you
+will need to add a 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
+   this right.
+
+ * Pad the entire struct to a multiple of 64-bits - 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.
+
+
+Basics
+------
+
+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.
+
+ * 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
+anyway:
+
+ * 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
+   screen.
+
+ * 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
+   userspace.
+
+ * 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
+   waiters.
+
+
+Time, Waiting and Missing it
+----------------------------
+
+GPUs do most everything asynchronously, so we have a need to time operations and
+wait for oustanding 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 laywer 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
+   explictly. 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
+   suitable.
+
+ * 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
+   interfaces. Indefinitely.
+
+ * 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.