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[fat] align start of data region to MiB #2387

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[fat] align start of data region to MiB #2387

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11 changes: 10 additions & 1 deletion src/format_fat32.c
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,8 @@
FormatStatus = ERROR_SEVERITY_ERROR|FAC(FACILITY_STORAGE)|err; \
goto out; } while(0)

#define USER_AREA_ALIGN_BYTES (1 * 1024 * 1024)

extern BOOL write_as_esp;

/* Large FAT32 */
Expand Down Expand Up @@ -175,6 +177,7 @@ BOOL FormatLargeFAT32(DWORD DriveIndex, uint64_t PartitionOffset, DWORD ClusterS
DWORD BytesPerSect = 0;
DWORD SectorsPerCluster = 0;
DWORD TotalSectors = 0;
DWORD AlignSectors = 0;
DWORD SystemAreaSize = 0;
DWORD UserAreaSize = 0;
ULONGLONG qTotalSectors = 0;
Expand Down Expand Up @@ -295,7 +298,6 @@ BOOL FormatLargeFAT32(DWORD DriveIndex, uint64_t PartitionOffset, DWORD ClusterS
SectorsPerCluster = ClusterSize / BytesPerSect;

pFAT32BootSect->bSecPerClus = (BYTE)SectorsPerCluster;
pFAT32BootSect->wRsvdSecCnt = (WORD)ReservedSectCount;
pFAT32BootSect->bNumFATs = (BYTE)NumFATs;
pFAT32BootSect->wRootEntCnt = 0;
pFAT32BootSect->wTotSec16 = 0;
Expand All @@ -310,6 +312,13 @@ BOOL FormatLargeFAT32(DWORD DriveIndex, uint64_t PartitionOffset, DWORD ClusterS
FatSize = GetFATSizeSectors(pFAT32BootSect->dTotSec32, pFAT32BootSect->wRsvdSecCnt,
pFAT32BootSect->bSecPerClus, pFAT32BootSect->bNumFATs, BytesPerSect);

// Recalculate reserved sector count after FAT size is known
SystemAreaSize = ReservedSectCount + NumFATs * FatSize; // We round up from here to integral MiB
AlignSectors = USER_AREA_ALIGN_BYTES / BytesPerSect;
SystemAreaSize = (SystemAreaSize + AlignSectors - 1) / AlignSectors * AlignSectors;
ReservedSectCount = SystemAreaSize - NumFATs * FatSize;

pFAT32BootSect->wRsvdSecCnt = (WORD)ReservedSectCount;
pFAT32BootSect->dFATSz32 = FatSize;
pFAT32BootSect->wExtFlags = 0;
pFAT32BootSect->wFSVer = 0;
Expand Down