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1 | (How to avoid) Botching up ioctls |
2 | ================================= | |
3 | ||
4 | From: http://blog.ffwll.ch/2013/11/botching-up-ioctls.html | |
5 | ||
6 | By: Daniel Vetter, Copyright © 2013 Intel Corporation | |
7 | ||
8 | One clear insight kernel graphics hackers gained in the past few years is that | |
9 | trying to come up with a unified interface to manage the execution units and | |
10 | memory on completely different GPUs is a futile effort. So nowadays every | |
11 | driver has its own set of ioctls to allocate memory and submit work to the GPU. | |
12 | Which is nice, since there's no more insanity in the form of fake-generic, but | |
13 | actually only used once interfaces. But the clear downside is that there's much | |
14 | more potential to screw things up. | |
15 | ||
16 | To avoid repeating all the same mistakes again I've written up some of the | |
17 | lessons learned while botching the job for the drm/i915 driver. Most of these | |
18 | only cover technicalities and not the big-picture issues like what the command | |
19 | submission ioctl exactly should look like. Learning these lessons is probably | |
20 | something every GPU driver has to do on its own. | |
21 | ||
22 | ||
23 | Prerequisites | |
24 | ------------- | |
25 | ||
26 | First the prerequisites. Without these you have already failed, because you | |
27 | will need to add a a 32-bit compat layer: | |
28 | ||
29 | * Only use fixed sized integers. To avoid conflicts with typedefs in userspace | |
30 | the kernel has special types like __u32, __s64. Use them. | |
31 | ||
32 | * Align everything to the natural size and use explicit padding. 32-bit | |
33 | platforms don't necessarily align 64-bit values to 64-bit boundaries, but | |
34 | 64-bit platforms do. So we always need padding to the natural size to get | |
35 | this right. | |
36 | ||
37 | * Pad the entire struct to a multiple of 64-bits - the structure size will | |
38 | otherwise differ on 32-bit versus 64-bit. Having a different structure size | |
39 | hurts when passing arrays of structures to the kernel, or if the kernel | |
40 | checks the structure size, which e.g. the drm core does. | |
41 | ||
42 | * Pointers are __u64, cast from/to a uintprt_t on the userspace side and | |
43 | from/to a void __user * in the kernel. Try really hard not to delay this | |
44 | conversion or worse, fiddle the raw __u64 through your code since that | |
45 | diminishes the checking tools like sparse can provide. | |
46 | ||
47 | ||
48 | Basics | |
49 | ------ | |
50 | ||
51 | With the joys of writing a compat layer avoided we can take a look at the basic | |
52 | fumbles. Neglecting these will make backward and forward compatibility a real | |
53 | pain. And since getting things wrong on the first attempt is guaranteed you | |
54 | will have a second iteration or at least an extension for any given interface. | |
55 | ||
56 | * Have a clear way for userspace to figure out whether your new ioctl or ioctl | |
57 | extension is supported on a given kernel. If you can't rely on old kernels | |
58 | rejecting the new flags/modes or ioctls (since doing that was botched in the | |
59 | past) then you need a driver feature flag or revision number somewhere. | |
60 | ||
61 | * Have a plan for extending ioctls with new flags or new fields at the end of | |
62 | the structure. The drm core checks the passed-in size for each ioctl call | |
63 | and zero-extends any mismatches between kernel and userspace. That helps, | |
64 | but isn't a complete solution since newer userspace on older kernels won't | |
65 | notice that the newly added fields at the end get ignored. So this still | |
66 | needs a new driver feature flags. | |
67 | ||
68 | * Check all unused fields and flags and all the padding for whether it's 0, | |
69 | and reject the ioctl if that's not the case. Otherwise your nice plan for | |
70 | future extensions is going right down the gutters since someone will submit | |
71 | an ioctl struct with random stack garbage in the yet unused parts. Which | |
72 | then bakes in the ABI that those fields can never be used for anything else | |
73 | but garbage. | |
74 | ||
75 | * Have simple testcases for all of the above. | |
76 | ||
77 | ||
78 | Fun with Error Paths | |
79 | -------------------- | |
80 | ||
81 | Nowadays we don't have any excuse left any more for drm drivers being neat | |
82 | little root exploits. This means we both need full input validation and solid | |
83 | error handling paths - GPUs will die eventually in the oddmost corner cases | |
84 | anyway: | |
85 | ||
86 | * The ioctl must check for array overflows. Also it needs to check for | |
87 | over/underflows and clamping issues of integer values in general. The usual | |
88 | example is sprite positioning values fed directly into the hardware with the | |
89 | hardware just having 12 bits or so. Works nicely until some odd display | |
90 | server doesn't bother with clamping itself and the cursor wraps around the | |
91 | screen. | |
92 | ||
93 | * Have simple testcases for every input validation failure case in your ioctl. | |
94 | Check that the error code matches your expectations. And finally make sure | |
95 | that you only test for one single error path in each subtest by submitting | |
96 | otherwise perfectly valid data. Without this an earlier check might reject | |
97 | the ioctl already and shadow the codepath you actually want to test, hiding | |
98 | bugs and regressions. | |
99 | ||
100 | * Make all your ioctls restartable. First X really loves signals and second | |
101 | this will allow you to test 90% of all error handling paths by just | |
102 | interrupting your main test suite constantly with signals. Thanks to X's | |
103 | love for signal you'll get an excellent base coverage of all your error | |
104 | paths pretty much for free for graphics drivers. Also, be consistent with | |
105 | how you handle ioctl restarting - e.g. drm has a tiny drmIoctl helper in its | |
106 | userspace library. The i915 driver botched this with the set_tiling ioctl, | |
107 | now we're stuck forever with some arcane semantics in both the kernel and | |
108 | userspace. | |
109 | ||
110 | * If you can't make a given codepath restartable make a stuck task at least | |
111 | killable. GPUs just die and your users won't like you more if you hang their | |
112 | entire box (by means of an unkillable X process). If the state recovery is | |
113 | still too tricky have a timeout or hangcheck safety net as a last-ditch | |
114 | effort in case the hardware has gone bananas. | |
115 | ||
116 | * Have testcases for the really tricky corner cases in your error recovery code | |
117 | - it's way too easy to create a deadlock between your hangcheck code and | |
118 | waiters. | |
119 | ||
120 | ||
121 | Time, Waiting and Missing it | |
122 | ---------------------------- | |
123 | ||
124 | GPUs do most everything asynchronously, so we have a need to time operations and | |
d53a7b8f | 125 | wait for outstanding ones. This is really tricky business; at the moment none of |
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126 | the ioctls supported by the drm/i915 get this fully right, which means there's |
127 | still tons more lessons to learn here. | |
128 | ||
129 | * Use CLOCK_MONOTONIC as your reference time, always. It's what alsa, drm and | |
130 | v4l use by default nowadays. But let userspace know which timestamps are | |
131 | derived from different clock domains like your main system clock (provided | |
132 | by the kernel) or some independent hardware counter somewhere else. Clocks | |
133 | will mismatch if you look close enough, but if performance measuring tools | |
134 | have this information they can at least compensate. If your userspace can | |
135 | get at the raw values of some clocks (e.g. through in-command-stream | |
136 | performance counter sampling instructions) consider exposing those also. | |
137 | ||
138 | * Use __s64 seconds plus __u64 nanoseconds to specify time. It's not the most | |
139 | convenient time specification, but it's mostly the standard. | |
140 | ||
141 | * Check that input time values are normalized and reject them if not. Note | |
142 | that the kernel native struct ktime has a signed integer for both seconds | |
143 | and nanoseconds, so beware here. | |
144 | ||
145 | * For timeouts, use absolute times. If you're a good fellow and made your | |
146 | ioctl restartable relative timeouts tend to be too coarse and can | |
147 | indefinitely extend your wait time due to rounding on each restart. | |
148 | Especially if your reference clock is something really slow like the display | |
d53a7b8f | 149 | frame counter. With a spec lawyer hat on this isn't a bug since timeouts can |
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150 | always be extended - but users will surely hate you if their neat animations |
151 | starts to stutter due to this. | |
152 | ||
153 | * Consider ditching any synchronous wait ioctls with timeouts and just deliver | |
154 | an asynchronous event on a pollable file descriptor. It fits much better | |
155 | into event driven applications' main loop. | |
156 | ||
157 | * Have testcases for corner-cases, especially whether the return values for | |
158 | already-completed events, successful waits and timed-out waits are all sane | |
159 | and suiting to your needs. | |
160 | ||
161 | ||
162 | Leaking Resources, Not | |
163 | ---------------------- | |
164 | ||
165 | A full-blown drm driver essentially implements a little OS, but specialized to | |
166 | the given GPU platforms. This means a driver needs to expose tons of handles | |
167 | for different objects and other resources to userspace. Doing that right | |
168 | entails its own little set of pitfalls: | |
169 | ||
170 | * Always attach the lifetime of your dynamically created resources to the | |
171 | lifetime of a file descriptor. Consider using a 1:1 mapping if your resource | |
172 | needs to be shared across processes - fd-passing over unix domain sockets | |
173 | also simplifies lifetime management for userspace. | |
174 | ||
175 | * Always have O_CLOEXEC support. | |
176 | ||
177 | * Ensure that you have sufficient insulation between different clients. By | |
178 | default pick a private per-fd namespace which forces any sharing to be done | |
d53a7b8f | 179 | explicitly. Only go with a more global per-device namespace if the objects |
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180 | are truly device-unique. One counterexample in the drm modeset interfaces is |
181 | that the per-device modeset objects like connectors share a namespace with | |
182 | framebuffer objects, which mostly are not shared at all. A separate | |
183 | namespace, private by default, for framebuffers would have been more | |
184 | suitable. | |
185 | ||
186 | * Think about uniqueness requirements for userspace handles. E.g. for most drm | |
187 | drivers it's a userspace bug to submit the same object twice in the same | |
188 | command submission ioctl. But then if objects are shareable userspace needs | |
189 | to know whether it has seen an imported object from a different process | |
190 | already or not. I haven't tried this myself yet due to lack of a new class | |
191 | of objects, but consider using inode numbers on your shared file descriptors | |
192 | as unique identifiers - it's how real files are told apart, too. | |
193 | Unfortunately this requires a full-blown virtual filesystem in the kernel. | |
194 | ||
195 | ||
196 | Last, but not Least | |
197 | ------------------- | |
198 | ||
199 | Not every problem needs a new ioctl: | |
200 | ||
201 | * Think hard whether you really want a driver-private interface. Of course | |
202 | it's much quicker to push a driver-private interface than engaging in | |
203 | lengthy discussions for a more generic solution. And occasionally doing a | |
204 | private interface to spearhead a new concept is what's required. But in the | |
205 | end, once the generic interface comes around you'll end up maintainer two | |
206 | interfaces. Indefinitely. | |
207 | ||
208 | * Consider other interfaces than ioctls. A sysfs attribute is much better for | |
209 | per-device settings, or for child objects with fairly static lifetimes (like | |
210 | output connectors in drm with all the detection override attributes). Or | |
211 | maybe only your testsuite needs this interface, and then debugfs with its | |
212 | disclaimer of not having a stable ABI would be better. | |
213 | ||
214 | Finally, the name of the game is to get it right on the first attempt, since if | |
215 | your driver proves popular and your hardware platforms long-lived then you'll | |
216 | be stuck with a given ioctl essentially forever. You can try to deprecate | |
217 | horrible ioctls on newer iterations of your hardware, but generally it takes | |
218 | years to accomplish this. And then again years until the last user able to | |
219 | complain about regressions disappears, too. |