feat(system): source linux page_size from AT_PAGESZ (#336)#343
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page_size() on linux hardcoded 4096, wrong on aarch64 kernels running 16 KiB or 64 KiB pages. Capture AT_PAGESZ from the auxiliary vector once at startup into a shared OS-layer global (_pagesz) and have each arch's page_size() read it, falling back to 4096 only when the auxv genuinely lacks it. The entrypoint publishes the value via capture_pagesz() right after _envp, and (under --pie) after _rt_relocate has applied the relocations, so it is ordinary post-relocation code that may touch globals. std.runtime.linux.reloc keeps its own private pre-relocation read of AT_PAGESZ for the RELRO mprotect, since that runs before any global is safe to reference; the OS layer is the single consumer-facing page-size source of truth. Closes #336
page_size() fell back to 4096 when the captured _pagesz was 0. AT_PAGESZ is mandatory on linux, so a 0 at call time means the auxv lacked it (broken or nonstandard environment) or a caller ran before _rt_init published — and on a large-page kernel the fallback would hand that early caller 4096 while later callers see 16K, an inconsistency worse than either constant. Panic naming the invariant rather than fabricating a value; no in-tree caller runs before the startup capture, so the normal path is unaffected.
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Closes #336.
What
std.system.os.linux.{aarch64,riscv64,x86_64}.page_size()hardcodedret 4096, wrong on aarch64 kernels configured for 16 KiB or 64 KiB pages. This sources the page size fromAT_PAGESZin the auxiliary vector instead.src/system/os/linux/shared.mach): add_pagesz(the runtime page size,0= unavailable),pagesz_from_auxv(envp)(walks envp to its NULL terminator, then scans the auxv forAT_PAGESZ), andcapture_pagesz()(publishes the walk result into_pagesz, reading the already-set_envp).page_size(): readshared._pagesz, falling back to4096only when the auxv genuinely lackedAT_PAGESZ. All three linux arches converge on the one source.src/runtime/linux/*): each_rt_initcallsos.capture_pagesz()right afteros._envp = _rt_envp.Capture / ordering design
_rt_initruns, in order: (--pieonly)reloc._rt_relocate(...), thenos._envp = _rt_envp, thenos.capture_pagesz(). So the capture happens after relocation and after_envpis set — it is ordinary post-relocation code that may freely touch globals.std.runtime.linux.relockeeps its own private pre-relocation read ofAT_PAGESZ(for the RELROmprotect): that pass runs before any global is safe to reference and is deliberately position-independent, so it cannot consume the OS-layer global. The two reads serve different constraints at different times; the OS layer is the single consumer-facing page-size source of truth, which is what #336 asked for.Verification
mach build .andmach test .— 571 passed, 0 failed.test/riscv64/verify.sh(repo smoke test) — passes under qemu-riscv64.AT_PAGESZand assertsos.page_size() == AT_PAGESZ,shared._pagesz != 0(capture ran), andshared._pagesz == page_size()— exits 0 on x86_64 native, riscv64/qemu, aarch64/qemu, both default and--piebuilds. (This qemu-user build presents a fixed 4 KiB page and cannot override it, so a live 16 KiB/64 KiB run is not reproducible locally; the probe proves the value is auxv-sourced rather than a literal.)🤖 Generated with Claude Code