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Diffstat (limited to 'results/scraper/launchpad-without-comments/1809075')
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diff --git a/results/scraper/launchpad-without-comments/1809075 b/results/scraper/launchpad-without-comments/1809075 new file mode 100644 index 000000000..03a94425c --- /dev/null +++ b/results/scraper/launchpad-without-comments/1809075 @@ -0,0 +1,31 @@ +Concurrency bug on keyboard events: capslock LED messing up keycode streams causes character misses at guest kernel + +Whenever capslock is pressed on host, both capslock keycode(0x3a 0xba) and capslock LED keycode(0xfa 0xfa) would be sent to the ps2 keycode stream. + +However, capslock LED is handled by another thread, confirmed by tracing `ps2_write_keyboard` with gdb. The keycode of casplock LED might divide + +For example, I sent AaBb but got ABa. I was using vncdotool, so it equals sending `capslock a capslock a capslock b capslock b`. In ps2_queue, I was expecting `3a fa fa ba 1e 9e 3a fa fa ba 1e 9e 3a fa fa ba 30 b0 3a fa fa ba 30 b0`. But actually once in a while, it might not receive such streams. In one case I got `3a fa fa ba 1e 9e 3a ba 1e fa fa 9e 3a ba 30 b0 3a ba 30 b0 fa fa` + +In this specific example, `a` was lost because LED keycode 'jumps in' its keycode. Kernel event device receives below streams +``` +# /dev/input/event receives what is sent from ps2_queue +# I use cap_1 to show capslock key down +cap_1 led caps_0, # 0x3a 0xfa 0xfa 0xba +a_1 a_0 # 0x1e 0x9e +caps_1 caps_0 # 0x3a 0xba +led # 0x1e 0xfa 0xfa 0x1e (we lost `a` here) +caps_1 caps_0 # 0x3a 0xba +b_1 led b_0 # 0x30 0xfa 0xfa 0xb0 +caps_1 caps_0 # 0x3a 0xba +led b_1 b_0 # 0xfa 0xfa 0x30 0xb0 +``` + +I made sure kernel receives the correct key stream as the qemu ps2_queue sends using /proc, ftrace and dynamic_debug. I explained the details in this [post](https://medium.com/@alapha23/quick-peek-into-kernel-land-keyboard-events-handling-with-ftrace-and-dynamic-debug-24a790056d5a) + +So it seems to be a concurrency issue. + +A hacky path on my mind is to skip all `0xfa` in ps2_queue. But I'm not sure if capslock LED is the only stink bug to our ps2 keycode queue as I've seen other keycodes handled by `ps2_write_keyboard` sent to ps2 queue. + +Another solution might be a memory barrier or a lock. Making key down and key up atomic will prevent another thread modifying the ps2 queue unwantedly. + +What do you think? \ No newline at end of file |