---
id: "bug-1824"
status: "open"
status_updated: "2026-09-24"
status_note: "repl_info.c bounds check uses zarray count instead of caps"
component: "scheduler"
severity: "crash"
timestamp: "2026-09-17T02:45:00Z"
last_seen: "2026-09-19T09:00:00Z"
occurrences: 1
files: ["src/scheduler/src/repl_info.c"]
tags: ["scheduler", "repl", "zarray", "bounds-check", "out-of-bounds-read", "latent", "discovered-during-scheduler-sim-diagnostics"]
related: []
legacy: true
---

# bug-1824: out-of-scope latent bug discovered while implementing scheduler-sim-diagnostics task 3.1 (new…

## Description
NOT A CRASH -- pre-existing, out-of-scope latent bug discovered while implementing scheduler-sim-diagnostics task 3.1 (new repl_info_limit() live-usage display). repl_info_resource_one() (src/scheduler/src/repl_info.c ~line 996) bounds-checks a resource's zarray index via `(r->idx < 0) || (r->idx >= sim->core.sharedInst.count)` before calling zarray_get() -- but zarray_t.count tracks the number of POPULATED slots (incremented only when a previously-NULL slot becomes non-NULL via zarray_put_ex/zarray_store), not the array's allocated capacity (.caps, grown by zarray_ensure_items). A sparsely-populated zarray (e.g. only a high index ever zarray_put() -- realistic if resources are registered out of index order, or an index is skipped) can have count=1 while a LOWER, unpopulated index is itself < count and would incorrectly pass this check and be read as though populated, or a HIGHER populated index can exceed .count and be incorrectly rejected as absent even though zarray_get()'s own .caps-based check would have safely returned it (or correctly returned NULL for a genuinely-unpopulated-but-in-capacity slot, since zarray_ensure_items() NULL-initializes newly-grown slots). Net effect: repl_info_resource_one() can report a resource as having no value ('value=(none)') when a real, populated value exists at that index but the array is sparse, OR (less likely) skip a real bounds-safety net zarray_get_ex() would otherwise provide.

## Environment
_Not recorded (migrated from buglog.json)._

## Reproduction
_Not recorded (migrated from buglog.json)._

## Root cause
repl_info_resource_one()'s pre-check compares r->idx against sim->core.sharedInst.count (populated-slot count) instead of relying on zarray_get_ex()'s own bounds check, which is against .caps (allocated capacity) -- these two counters diverge for any sparsely-populated zarray. Confirmed by direct reading of src/lib/zarray.c: zarray_get_ex() (~line 341-362) checks only `idx >= array->caps` before dereferencing array->data[idx]; zarray_put_ex()/zarray_store() are the only writers of .count, incrementing it exclusively when filling a previously-NULL slot -- so .count is a 'high-water mark of contiguous-from-zero fill,' not a true capacity bound. Separately, and more seriously: neither the .count-based check NOR zarray_get_ex()'s own .caps check safely rejects a NEGATIVE idx -- `idx >= caps` is false for any negative idx when caps >= 0, so a negative idx falls through to `array->data[idx]`, a genuine out-of-bounds read. A negative idx is reachable in this codebase: add_vres() (src/lib/xcelimit.c ~407-435) unconditionally sets a newly-allocated xce_resource_t's ->idx to -1 before key_add() runs, only overwriting it with the real slot index on success -- a caller that reaches a resource-value read before/without that overwrite succeeding would pass a negative idx through. repl_info_resource_one()'s existing `r->idx < 0` half of its check does happen to guard this specific negative case correctly (by luck of ordering, not by understanding zarray's real contract) -- the count-vs-caps confusion is the part that is a genuine, distinct latent bug.

## Fix
NOT FIXED -- discovered and characterized during scheduler-sim-diagnostics task 3.1 (limit-usage display), out of that task's scope (task 3.1 only touches its OWN new code path, repl_print_limit_usage(), which was written from scratch to avoid this exact mistake: it omits the imprecise .count pre-check entirely, relies on zarray_get_ex()'s own .caps bound for the upper end, and adds an explicit idx<0 guard of its own -- verified correct and reviewed independently, see this session's git history for scheduler-sim-diagnostics task 3.1). repl_info_resource_one() itself was deliberately left untouched to keep task 3.1 in its declared boundary. Recommended fix when picked up: replace the `r->idx >= sim->core.sharedInst.count` half of the pre-check with reliance on zarray_get()'s own .caps-based bound (mirroring task 3.1's new repl_print_limit_usage() pattern), keeping the `r->idx < 0` guard. Likely benign in practice today only because resource registration order in this codebase's current production paths tends to be dense/contiguous from index 0, so .count and .caps rarely diverge for sim->core.sharedInst specifically -- but this is an incidental property of current callers, not a guarantee, and worth a regression test with a deliberately sparse fixture (zarray_put() at a high index with no lower indices ever populated) before relying on it.

## Verification
_Not recorded (migrated from buglog.json)._

## References
_Not recorded (migrated from buglog.json)._