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Detects possible FAT geometry in fixed VHDs too #4961

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Apr 14, 2024
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Detects possible FAT geometry in fixed VHDs too #4961

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9 changes: 8 additions & 1 deletion src/ints/bios_vhd.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -127,13 +127,20 @@ imageDiskVHD::ErrorCodes imageDiskVHD::Open(const char* fileName, const bool rea
uint8_t buf[512];
if(fseeko64(file, 0, SEEK_SET)) { fclose(file); return INVALID_DATA; }
if(fread(&buf, sizeof(uint8_t), 512, file) != 512) { fclose(file); return INVALID_DATA; }
//detect real DOS geometry (MBR, BPB or default X-255-63)
imageDiskVHD::scanMBR(buf, sizes, footer.currentSize);
vhd->cylinders = sizes[3];
vhd->heads = sizes[2];
vhd->sectors = sizes[1];
vhd->fixedDisk = new imageDisk(file, fileName, vhd->cylinders, vhd->heads, vhd->sectors, 512, true);
//now possible FAT geometry can be detected (in Windows 11, those two can differ!)
vhd->DetectGeometry(sizes);
vhd->cylinders = sizes[3];
vhd->heads = sizes[2];
vhd->sectors = sizes[1];
vhd->fixedDisk = new imageDisk(file, fileName, vhd->cylinders, vhd->heads, vhd->sectors, 512, true);
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Does calling new imageDisk() twice safely deletes the previous one?
I think better to check sizes[] and DetectGeometry before new imageDisk

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@maxpat78 maxpat78 Apr 13, 2024 •
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Obviously, it does not (deleting caused an obscure exception). [*]

DetectGeometry actually requires a valid disk image internally to properly read VHD sectors, that's the reason why I avoided that call in previous commits (having tought at least Windows 1x set MBR CHS == BPB CHS).

Perhaps I'll find another way, if you haven't suggestions.

EDIT:
[*] The exception cause: delete vhd->fixedDisk would call fclose(dskimg), where dskimg is an alias for file object, required by following new imageDisk call.

*disk = vhd;
return !readOnly && roflag ? UNSUPPORTED_WRITE : OPEN_SUCCESS;
return !readOnly && roflag ? UNSUPPORTED_WRITE : OPEN_SUCCESS;
}

//if not dynamic or differencing, fail
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