poll: Write back only revents to userspace, matching Linux behavior#12851
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copybara-service[bot] merged 1 commit intogoogle:masterfrom Apr 9, 2026
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Reproduction# In gVisor (runsc) — CPU spikes to ~95%
docker run -d --name tmux-test --runtime=runsc ubuntu:22.04 sleep 3600
docker exec tmux-test bash -c "apt-get update -qq && apt-get install -y -qq tmux procps >/dev/null 2>&1"
docker exec tmux-test bash -c "tmux new-session -d -s test && sleep 30 && ps aux | grep tmux"
# tmux: 94.7% CPU
# In Linux (runc) — CPU stays at 0%
docker run -d --name tmux-ctrl --runtime=runc ubuntu:22.04 sleep 3600
docker exec tmux-ctrl bash -c "apt-get update -qq && apt-get install -y -qq tmux procps >/dev/null 2>&1"
docker exec tmux-ctrl bash -c "tmux new-session -d -s test && sleep 30 && ps aux | grep tmux"
# tmux: 0.0% CPU |
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doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). Signed-off-by: Tan Yifeng <yiftan@tencent.com>
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ayushr2
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Apr 8, 2026
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doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896548818
copybara-service bot
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Apr 8, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896548818
copybara-service bot
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Apr 8, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896548818
copybara-service bot
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Apr 8, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896548818
copybara-service bot
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Apr 8, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896650914
copybara-service bot
pushed a commit
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Apr 8, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896650914
copybara-service bot
pushed a commit
that referenced
this pull request
Apr 8, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896650914
copybara-service bot
pushed a commit
that referenced
this pull request
Apr 8, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896650914
copybara-service bot
pushed a commit
that referenced
this pull request
Apr 9, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896650914
copybara-service bot
pushed a commit
that referenced
this pull request
Apr 9, 2026
doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%). Fix: write back only the 2-byte revents field per fd in doPoll(). FUTURE_COPYBARA_INTEGRATE_REVIEW=#12851 from tanyifeng:poll-revents-writeback 3169ea6 PiperOrigin-RevId: 896650914
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doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%).
Fix: write back only the 2-byte revents field per fd in doPoll().