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[Bugfix] Clamp remote_prefill_cached_tokens override to [0, prompt_len] - #58730
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kv_transfer_params is a client-visible request field on the OpenAI API. The remote_prefill_cached_tokens override it carries now respects the same bounds as the local count (see PrefillStats.finalize), so prompt_tokens_details and request metrics cannot report values outside the prompt length. Signed-off-by: ptimizeroracle <contact@binblok.com>
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following up after the 7-day review clock. state is unchanged: single-line clamp at output_processor.py mirroring PrefillStats.finalize (stats.py), three-case regression test, maintainer ack on #58728. one more supporting data point since filing: lmdeploy ships the identical accounting rule for its prefix-cache stats. finalize_match in pytorch/paging/block_trie/trie.py computes cached_tokens = max(0, min(cached_end, prompt_end) - max(match_start, prompt_start)) and deliberately zeroes the stat for recompute-preemption matches, so published reuse can never exceed the accepted prompt window. the clamp here brings vllm to the same invariant for the remote-prefill override. happy to rebase or adjust the test shape if a reviewer prefers a different seam. |
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closing this to take it off your queue, feel free to reopen if it is still useful |
Fixes #58728
Purpose
clamp the client-reachable
kv_transfer_params.remote_prefill_cached_tokensoverride to[0, prompt_len]soprompt_tokens_details.cached_tokensand request metrics cannot report values outside the prompt length.Scope
vllm/v1/engine/output_processor.py(RequestState.remote_prefill_cached_tokensapplication)kv_transfer_paramshardening tracked separately ([Bugfix] Validate kv_transfer_params before NIXL recv registration #54807, [Security] Validate kv_transfer_params role sub-dicts are dicts #55182, fix(security): bound kv_transfer_params to prevent OOM via unbounded tp_size #51137, [Security] Validate NixlPush remote prefill params before registration #56585, [Security] Reject reserved transfer keys in vllm_xargs #56814)Why not duplicate
searched open PRs for
remote_prefill_cached_tokens/prompt_tokens_detailsvalidation. the hardening PRs above cover registration, sub-dict types, tp_size and reserved keys, none clamps the accounting value. no issue existed, filed #58728.Validation
num_cached_tokens=9999; remote=-5 reports -5test_remote_prefill_cached_tokens_override_clampedcovers passthrough, upper clamp, lower clamptest_remote_prefill_cached_tokens_overridestays green (in-range values unaffected)--enable-prompt-tokens-details: forgedcached_tokens: 9999and-5in the API response, see issue transcriptcommand:
PYTHONPATH=. python <repro script>(OutputProcessor constructed directly, no GPU needed for the unit path)Interpretation
this proves and fixes the bounds violation on the accounting value. in-range forged values from a client remain indistinguishable from router-supplied truth at this layer, which is the trust question under discussion in the linked hardening PRs.
AI assistance: coded with an AI agent under my direction, evidence and claims reviewed by me.