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Answer pselect6's poll fallback as select does - #318
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jserv
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pselect6 leaves fd_set behind and waits in poll() when a host descriptor lands at or above FD_SETSIZE, and hands the guest poll()'s view of that wait in two places. macOS poll() answers POLLNVAL for every descriptor it will not put on a kqueue: /dev/null, /dev/zero, /dev/random, /dev/urandom, directories, and kqueue descriptors themselves. The fallback copies that revents into the request, where POLLNVAL maps to no fd_set bit, so a guest whose descriptor table is close to full is told a live /dev/null is not ready. Linux select() reports every one of them ready. poll_eval_unpollable() answers those entries for sys_ppoll already, so the fallback takes them out of the poll set on the pass that exposes them and hands them to the same function. A refused entry makes poll() return at once, so that single restart waits for nothing. The return value is the second place. poll() counts a descriptor once however many events it reports, while select() counts it once per set it is reported in. Measured against the qemu-system-aarch64 reference VM, a /dev/null in both readfds and writefds answers 2 there and 1 through the fallback, and a socket pair holding a byte answers the same way. The count comes from the bits the write-back lights. Reaching the fallback needs no descriptor pressure beyond a high number. host_fd_ref_open_io() borrows the host descriptor instead of duplicating it while one thread is active, so a single-threaded guest gets there as soon as a borrowed descriptor is numbered at or above FD_SETSIZE. The poll pass moves into pselect_fallback_pass() with its state in one struct, since sys_pselect6 crosses the 400-line clang-tidy advisory otherwise. An allocation failure there leaves the wait loop at once, ahead of the interrupt predicates that can overwrite errno. Verified: tests/test-poll.c carries three new cases and reports 22 passed, 0 failed both under elfuse and inside the reference VM, and its two high-descriptor cases fail against main as it stands. make check is clean, make lint reports no new finding, and tests/test-matrix.sh all stays within baseline with zero failures: elfuse-aarch64 255 passed, qemu-aarch64 234 passed, elfuse-x86_64 78 passed.
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pselect6 falls back to poll() when a host descriptor is out of the fd_set range, causing the guest to receive poll()'s behavior in two ways: descriptors refused by macOS poll() (/dev/null, /dev/zero, /dev/urandom, directories, and kqueue fds) arrive as not ready, whereas Linux select() reports them as ready; additionally, the return value counts each descriptor once, whereas select() counts it once per set. The refused entries are routed through poll_eval_unpollable(), the helper sys_ppoll() already uses for the same descriptor set, and the count comes from the bits set by the write-back.
Reproduction is covered by the two high-descriptor cases in
tests/test-poll.c, which fail on main and pass here.make checkis clean,make lintis unchanged, andtests/test-matrix.shpasses all within baseline with zero failures (elfuse-aarch64 255, qemu-aarch64 234, elfuse-x86_64 78).