fix(native-sidecar): close maxFilesystemBytes gaps on host-backed writes#1813
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Review found the mapped-host budget check barely constrained a real guest: - fs.writeFile/writeFileSync and fs.copyFile allocate no fd, so they never reached the fd-based check at all. writeFileSync is the most common Node write API: a single 8 GB write succeeded under a 64 MiB cap. Both now check before writing (O_TRUNC, so the resulting size is the payload size). - The fd check failed OPEN: metadata() errors defaulted the current size to 0, so an append was measured from offset 0 and any file size passed. An enforcement check that fails open is worse than none, because it reads as enforced. It now fails closed. - ftruncate: VERIFIED NOT a bypass, contrary to one review claim. The kernel truncate applies the quota and the single construction site always supplies a guest path. Added a fail-closed guard so a future `guest_path: None` caller cannot silently disable enforcement for truncation. All host-backed writes now funnel through one admission point, check_mapped_host_write_budget, so the per-VM aggregate has a single place to land. That aggregate is still MISSING: the budget remains per-file while the kernel side compares against whole-filesystem usage, so a guest can still exceed its VM budget by spreading bytes across many files. Documented on the function.
This was referenced Jul 20, 2026
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This was referenced Jul 20, 2026
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Review found the mapped-host budget check barely constrained a real guest:
fs.writeFile/writeFileSync and fs.copyFile allocate no fd, so they never
reached the fd-based check at all. writeFileSync is the most common Node
write API: a single 8 GB write succeeded under a 64 MiB cap. Both now
check before writing (O_TRUNC, so the resulting size is the payload size).
The fd check failed OPEN: metadata() errors defaulted the current size to
0, so an append was measured from offset 0 and any file size passed. An
enforcement check that fails open is worse than none, because it reads as
enforced. It now fails closed.
ftruncate: VERIFIED NOT a bypass, contrary to one review claim. The kernel
truncate applies the quota and the single construction site always supplies
a guest path. Added a fail-closed guard so a future
guest_path: Nonecaller cannot silently disable enforcement for truncation.
All host-backed writes now funnel through one admission point,
check_mapped_host_write_budget, so the per-VM aggregate has a single place
to land. That aggregate is still MISSING: the budget remains per-file while
the kernel side compares against whole-filesystem usage, so a guest can still
exceed its VM budget by spreading bytes across many files. Documented on the
function.