1=================================
2(How to avoid) Botching up ioctls
3=================================
4
5From: https://blog.ffwll.ch/2013/11/botching-up-ioctls.html
6
7By: Daniel Vetter, Copyright © 2013 Intel Corporation
8
9One clear insight kernel graphics hackers gained in the past few years is that
10trying to come up with a unified interface to manage the execution units and
11memory on completely different GPUs is a futile effort. So nowadays every
12driver 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, but
14actually only used once interfaces. But the clear downside is that there's much
15more potential to screw things up.
16
17To avoid repeating all the same mistakes again I've written up some of the
18lessons learned while botching the job for the drm/i915 driver. Most of these
19only cover technicalities and not the big-picture issues like what the command
20submission ioctl exactly should look like. Learning these lessons is probably
21something every GPU driver has to do on its own.
22
23
24Prerequisites
25-------------
26
27First the prerequisites. Without these you have already failed, because you
28will need to add a 32-bit compat layer:
29
30 * Only use fixed sized integers. To avoid conflicts with typedefs in userspace
31   the kernel has special types like __u32, __s64. Use them.
32
33 * Align everything to the natural size and use explicit padding. 32-bit
34   platforms don't necessarily align 64-bit values to 64-bit boundaries, but
35   64-bit platforms do. So we always need padding to the natural size to get
36   this right.
37
38 * Pad the entire struct to a multiple of 64-bits if the structure contains
39   64-bit types - the structure size will otherwise differ on 32-bit versus
40   64-bit. Having a different structure size hurts when passing arrays of
41   structures to the kernel, or if the kernel checks the structure size, which
42   e.g. the drm core does.
43
44 * Pointers are __u64, cast from/to a uintptr_t on the userspace side and
45   from/to a void __user * in the kernel. Try really hard not to delay this
46   conversion or worse, fiddle the raw __u64 through your code since that
47   diminishes the checking tools like sparse can provide. The macro
48   u64_to_user_ptr can be used in the kernel to avoid warnings about integers
49   and pointers of different sizes.
50
51
52Basics
53------
54
55With the joys of writing a compat layer avoided we can take a look at the basic
56fumbles. Neglecting these will make backward and forward compatibility a real
57pain. And since getting things wrong on the first attempt is guaranteed you
58will 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 ioctl
61   extension is supported on a given kernel. If you can't rely on old kernels
62   rejecting the new flags/modes or ioctls (since doing that was botched in the
63   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 of
66   the structure. The drm core checks the passed-in size for each ioctl call
67   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't
69   notice that the newly added fields at the end get ignored. So this still
70   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 for
74   future extensions is going right down the gutters since someone will submit
75   an ioctl struct with random stack garbage in the yet unused parts. Which
76   then bakes in the ABI that those fields can never be used for anything else
77   but garbage. This is also the reason why you must explicitly pad all
78   structures, even if you never use them in an array - the padding the compiler
79   might insert could contain garbage.
80
81 * Have simple testcases for all of the above.
82
83
84Fun with Error Paths
85--------------------
86
87Nowadays we don't have any excuse left any more for drm drivers being neat
88little root exploits. This means we both need full input validation and solid
89error handling paths - GPUs will die eventually in the oddmost corner cases
90anyway:
91
92 * The ioctl must check for array overflows. Also it needs to check for
93   over/underflows and clamping issues of integer values in general. The usual
94   example is sprite positioning values fed directly into the hardware with the
95   hardware just having 12 bits or so. Works nicely until some odd display
96   server doesn't bother with clamping itself and the cursor wraps around the
97   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 sure
101   that you only test for one single error path in each subtest by submitting
102   otherwise perfectly valid data. Without this an earlier check might reject
103   the ioctl already and shadow the codepath you actually want to test, hiding
104   bugs and regressions.
105
106 * Make all your ioctls restartable. First X really loves signals and second
107   this will allow you to test 90% of all error handling paths by just
108   interrupting your main test suite constantly with signals. Thanks to X's
109   love for signal you'll get an excellent base coverage of all your error
110   paths pretty much for free for graphics drivers. Also, be consistent with
111   how you handle ioctl restarting - e.g. drm has a tiny drmIoctl helper in its
112   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 and
114   userspace.
115
116 * If you can't make a given codepath restartable make a stuck task at least
117   killable. GPUs just die and your users won't like you more if you hang their
118   entire box (by means of an unkillable X process). If the state recovery is
119   still too tricky have a timeout or hangcheck safety net as a last-ditch
120   effort in case the hardware has gone bananas.
121
122 * Have testcases for the really tricky corner cases in your error recovery code
123   - it's way too easy to create a deadlock between your hangcheck code and
124   waiters.
125
126
127Time, Waiting and Missing it
128----------------------------
129
130GPUs do most everything asynchronously, so we have a need to time operations and
131wait for outstanding ones. This is really tricky business; at the moment none of
132the ioctls supported by the drm/i915 get this fully right, which means there's
133still tons more lessons to learn here.
134
135 * Use CLOCK_MONOTONIC as your reference time, always. It's what alsa, drm and
136   v4l use by default nowadays. But let userspace know which timestamps are
137   derived from different clock domains like your main system clock (provided
138   by the kernel) or some independent hardware counter somewhere else. Clocks
139   will mismatch if you look close enough, but if performance measuring tools
140   have this information they can at least compensate. If your userspace can
141   get at the raw values of some clocks (e.g. through in-command-stream
142   performance counter sampling instructions) consider exposing those also.
143
144 * Use __s64 seconds plus __u64 nanoseconds to specify time. It's not the most
145   convenient time specification, but it's mostly the standard.
146
147 * Check that input time values are normalized and reject them if not. Note
148   that the kernel native struct ktime has a signed integer for both seconds
149   and nanoseconds, so beware here.
150
151 * For timeouts, use absolute times. If you're a good fellow and made your
152   ioctl restartable relative timeouts tend to be too coarse and can
153   indefinitely extend your wait time due to rounding on each restart.
154   Especially if your reference clock is something really slow like the display
155   frame counter. With a spec lawyer hat on this isn't a bug since timeouts can
156   always be extended - but users will surely hate you if their neat animations
157   starts to stutter due to this.
158
159 * Consider ditching any synchronous wait ioctls with timeouts and just deliver
160   an asynchronous event on a pollable file descriptor. It fits much better
161   into event driven applications' main loop.
162
163 * Have testcases for corner-cases, especially whether the return values for
164   already-completed events, successful waits and timed-out waits are all sane
165   and suiting to your needs.
166
167
168Leaking Resources, Not
169----------------------
170
171A full-blown drm driver essentially implements a little OS, but specialized to
172the given GPU platforms. This means a driver needs to expose tons of handles
173for different objects and other resources to userspace. Doing that right
174entails its own little set of pitfalls:
175
176 * Always attach the lifetime of your dynamically created resources to the
177   lifetime of a file descriptor. Consider using a 1:1 mapping if your resource
178   needs to be shared across processes -  fd-passing over unix domain sockets
179   also simplifies lifetime management for userspace.
180
181 * Always have O_CLOEXEC support.
182
183 * Ensure that you have sufficient insulation between different clients. By
184   default pick a private per-fd namespace which forces any sharing to be done
185   explicitly. Only go with a more global per-device namespace if the objects
186   are truly device-unique. One counterexample in the drm modeset interfaces is
187   that the per-device modeset objects like connectors share a namespace with
188   framebuffer objects, which mostly are not shared at all. A separate
189   namespace, private by default, for framebuffers would have been more
190   suitable.
191
192 * Think about uniqueness requirements for userspace handles. E.g. for most drm
193   drivers it's a userspace bug to submit the same object twice in the same
194   command submission ioctl. But then if objects are shareable userspace needs
195   to know whether it has seen an imported object from a different process
196   already or not. I haven't tried this myself yet due to lack of a new class
197   of objects, but consider using inode numbers on your shared file descriptors
198   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 Least
203-------------------
204
205Not every problem needs a new ioctl:
206
207 * Think hard whether you really want a driver-private interface. Of course
208   it's much quicker to push a driver-private interface than engaging in
209   lengthy discussions for a more generic solution. And occasionally doing a
210   private interface to spearhead a new concept is what's required. But in the
211   end, once the generic interface comes around you'll end up maintaining two
212   interfaces. Indefinitely.
213
214 * Consider other interfaces than ioctls. A sysfs attribute is much better for
215   per-device settings, or for child objects with fairly static lifetimes (like
216   output connectors in drm with all the detection override attributes). Or
217   maybe only your testsuite needs this interface, and then debugfs with its
218   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 if
221your driver proves popular and your hardware platforms long-lived then you'll
222be stuck with a given ioctl essentially forever. You can try to deprecate
223horrible ioctls on newer iterations of your hardware, but generally it takes
224years to accomplish this. And then again years until the last user able to
225complain about regressions disappears, too.
226