From b66140e102982eebde4b4478bd8339710baf3b13 Mon Sep 17 00:00:00 2001
From: Devedse <davydavidse@gmail.com>
Date: Sat, 4 Jul 2026 13:25:37 +0200
Subject: [PATCH] Fix GPT protective MBR to start at LBA 1 per UEFI spec

The protective MBR was created with CreatePrimaryByCylinder, which cylinder-aligns the 0xEE record so its StartingLBA is 63 instead of 1 and its SizeInLBA ends short of the disk end. The UEFI spec (Protective MBR) requires the 0xEE record to have StartingLBA exactly 1 and SizeInLBA equal to the disk size minus one, capped at 0xFFFFFFFF.

The Linux kernel enforces this in block/partitions/efi.c pmbr_part_valid() (starting_lba must equal 1) and EDK2 firmware (MdeModulePkg PartitionDxe, UNPACK_UINT32(StartingLBA) == 1) does the same, so disks initialized by DiscUtils were rejected by both and could not be EFI-booted; gdisk also warned the 0xEE partition did not start on sector 1.

Replace the cylinder-based call with CreatePrimaryBySector(1, min(sectorCount - 1, uint.MaxValue), ...), which already handles the CHS fields and caps oversized CHS at 1023/254/63. Adds a ProtectiveMbrIsSpecCompliant unit test asserting the record starts at LBA 1 and spans the disk.
---
 .../Partitions/GuidPartitionTable.cs          | 12 ++++-
 .../Partitions/GuidPartitionTableTest.cs      | 52 +++++++++++++++++++
 2 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/Library/DiscUtils.Core/Partitions/GuidPartitionTable.cs b/Library/DiscUtils.Core/Partitions/GuidPartitionTable.cs
index 31186b526..54b86d555 100644
--- a/Library/DiscUtils.Core/Partitions/GuidPartitionTable.cs
+++ b/Library/DiscUtils.Core/Partitions/GuidPartitionTable.cs
@@ -108,7 +108,17 @@ public static GuidPartitionTable Initialize(Stream disk, Geometry diskGeometry)
     {
         // Create the protective MBR partition record.
         var pt = BiosPartitionTable.Initialize(disk, diskGeometry);
-        pt.CreatePrimaryByCylinder(0, diskGeometry.Cylinders - 1, BiosPartitionTypes.GptProtective, false);
+
+        // The UEFI spec ("Protective MBR") requires the 0xEE record to have
+        // StartingLBA exactly 1 (the LBA of the GPT header) and SizeInLBA equal
+        // to the size of the disk minus one, capped at 0xFFFFFFFF. A cylinder
+        // aligned record starts at LBA 63 instead, which the Linux kernel
+        // (block/partitions/efi.c pmbr_part_valid, requires starting_lba == 1)
+        // and EDK2 firmware (MdeModulePkg PartitionDxe, UNPACK_UINT32(StartingLBA) == 1)
+        // reject, causing the whole GPT to be ignored. Use a sector-based record
+        // starting at LBA 1 covering the rest of the disk instead.
+        var sectorCount = disk.Length / diskGeometry.BytesPerSector;
+        pt.CreatePrimaryBySector(1, Math.Min(sectorCount - 1, uint.MaxValue), BiosPartitionTypes.GptProtective, false);
 
         // Create the GPT headers, and blank-out the entry areas
         const int EntryCount = 128;
diff --git a/Tests/LibraryTests/Partitions/GuidPartitionTableTest.cs b/Tests/LibraryTests/Partitions/GuidPartitionTableTest.cs
index 6caa3dda1..06ebf7e4f 100644
--- a/Tests/LibraryTests/Partitions/GuidPartitionTableTest.cs
+++ b/Tests/LibraryTests/Partitions/GuidPartitionTableTest.cs
@@ -20,7 +20,9 @@
 // DEALINGS IN THE SOFTWARE.
 //
 
+using System;
 using System.IO;
+using DiscUtils;
 using DiscUtils.Partitions;
 using DiscUtils.Streams;
 using DiscUtils.Vdi;
@@ -181,4 +183,54 @@ public void Delete()
         Assert.Equal(sectorCount[2], table[1].SectorCount);
     }
 
+    [Fact]
+    public void ProtectiveMbrIsSpecCompliant()
+    {
+        var ms = new MemoryStream();
+        ms.SetLength(3 * 1024 * 1024);
+        GuidPartitionTable.Initialize(ms, Geometry.FromCapacity(ms.Length));
+
+        ms.Position = 0;
+        var sector = new byte[512];
+        ms.ReadExactly(sector, 0, sector.Length);
+
+        // Boot signature.
+        Assert.Equal(0x55, sector[510]);
+        Assert.Equal(0xAA, sector[511]);
+
+        // Exactly one of the four partition records must be the 0xEE protective
+        // record; the other three must be empty (type 0x00).
+        var protectiveIndex = -1;
+        var protectiveCount = 0;
+        var emptyCount = 0;
+        for (var i = 0; i < 4; ++i)
+        {
+            var type = sector[0x1BE + 16 * i + 4];
+            if (type == 0xEE)
+            {
+                protectiveIndex = i;
+                ++protectiveCount;
+            }
+            else if (type == 0x00)
+            {
+                ++emptyCount;
+            }
+        }
+
+        Assert.Equal(1, protectiveCount);
+        Assert.Equal(3, emptyCount);
+
+        var record = sector.AsSpan(0x1BE + 16 * protectiveIndex, 16);
+
+        // Status byte must be non-bootable.
+        Assert.Equal(0, record[0]);
+
+        // UEFI spec: StartingLBA == 1 and SizeInLBA == disk size in sectors - 1.
+        var startingLba = EndianUtilities.ToUInt32LittleEndian(record.Slice(8));
+        var sizeInLba = EndianUtilities.ToUInt32LittleEndian(record.Slice(12));
+
+        Assert.Equal(1U, startingLba);
+        Assert.Equal((uint)(ms.Length / 512 - 1), sizeInLba);
+    }
+
 }