Friday, July 10

bounds

daily challenge12

today's brief

July 19, 2024. CrowdStrike pushes a tiny config update to its Falcon sensor. The file described 21 fields, but the sensor only had 20, so the code reached for one slot past the end of an array. That read happened inside the Windows kernel, so instead of a caught error it was an instant blue screen, 8.5 million machines at once: airlines grounded, hospitals, banks, checkouts frozen. All of it from reading one index too far. Put the bounds check back.

Objective

Read an array element only when the index is in range, the check CrowdStrike's update skipped.

Steps

arr holds len elements, at positions 0 to len minus 1.
If idx is in range, return arr[idx].
If idx is negative, or equal to or greater than len, return -1. Never read outside the array.
In the checker, arr has 20 elements where element i holds the value i.

examples

$ read_at(arr, 20, 10)

$ read_at(arr, 20, 20)

-1

$ read_at(arr, 20, -1)

-1

Constraints

Expected files

bounds.c

Allowed functions

None. Write every helper yourself.

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Solutions

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