LcRead fails for sub-page requests contained within the first 4KiB page

[shih6]

Hi, I found a possible issue in LcRead() when the requested range is fully contained in the first 4KiB page.

Reproduction:

• page-aligned pa
• LcRead(hLC, pa, 1, buffer) returns FALSE
• a valid 4KiB-aligned LcReadScatter() request for the same page succeeds

In LcRead(), for pa aligned and cb = 1:

fFirst = TRUE
fLast = FALSE
cbData = cb - (0x1000 - (pa & 0xfff))
       = 1 - 0x1000
       = 0xfffff001

LcAllocScatter3() then rejects cbData, so LcRead() fails before issuing the backend read.

Was the intended calculation to subtract the actual first-page bytes, e.g.:

cbFirst = min(cb, 0x1000 - (pa & 0xfff));
cbData = cb - cbFirst - cbLast;

Is LcRead() intended to support sub-page contiguous reads such as 1B, 64B, and 4095B? The public API appears to imply that it should, while MEM_SCATTER entries remain page/DWORD constrained.