[PATCH V2 2/3] MdeModulePkg/PartitionDxe: Skip the MBR that add for CD-ROM

["Gao, Zhichao" <zhichao.gao@intel.com>]

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2823

Refer to
http://manpages.ubuntu.com/manpages/bionic/man8/mkudffs.8.html.
Some Linux ISOs may have the MBR table for compatibility reasons
for Windows. The MBR tale would contain the partition entry with
start LBA0 and whole media size. There are two methods to check
the filesystem in the CD-ROM:
1. MBR partition check (Windows)
2. Whole disk check (MAC OS)

UEFI doesn't have the MBR check for UDF and Eltorito. But it may
pass the MBR check for such table and fail to detect the filesystem
of UDF. Skip the MBR check if the MBR is added for Windows
compatiblity so that the partition driver can continue UDF and
ElTorito check.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
---
 .../Universal/Disk/PartitionDxe/Mbr.c         | 38 ++++++++++++++++---
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c b/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c
index aa0b6cadcc..b5cb56842a 100644
--- a/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c
+++ b/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c
@@ -39,6 +39,7 @@ PartitionValidMbr (
   UINT32  StartingLBA;
   UINT32  EndingLBA;
   UINT32  NewEndingLBA;
+  UINT32  SizeInLBA;
   INTN    Index1;
   INTN    Index2;
   BOOLEAN MbrValid;
@@ -51,13 +52,35 @@ PartitionValidMbr (
   //
   MbrValid = FALSE;
   for (Index1 = 0; Index1 < MAX_MBR_PARTITIONS; Index1++) {
-    if (Mbr->Partition[Index1].OSIndicator == 0x00 || UNPACK_UINT32 (Mbr->Partition[Index1].SizeInLBA) == 0) {
+    StartingLBA = UNPACK_UINT32 (Mbr->Partition[Index1].StartingLBA);
+    SizeInLBA   = UNPACK_UINT32 (Mbr->Partition[Index1].SizeInLBA);
+
+    //
+    // If the MBR with partition entry contains itself, i.e. start with LBA0,
+    // and have the same size with the media, we treat it as a El Torito partition.
+    //
+    if ((StartingLBA == 0) &&
+        (SizeInLBA != 0) &&
+        (SizeInLBA == (LastLba + 1))) {
+      //
+      // Refer to the http://manpages.ubuntu.com/manpages/bionic/man8/mkudffs.8.html
+      // "WHOLE DISK VS PARTITION"
+      // Some linux ISOs may put the MBR table in the first 512 bytes for compatibility reasons with Windows.
+      // Linux  kernel  ignores MBR table if contains partition which starts at sector 0.
+      // Skip it because we don't have the partition check for UDF(El Torito compatible).
+      // It would continue to do the whole disk check in the UDF routine.
+      //
+      DEBUG ((DEBUG_INFO, "PartitionValidMbr: The MBR table has partition entry start at sector 0.\n"));
+
+      return FALSE;
+    }
+
+    if (Mbr->Partition[Index1].OSIndicator == 0x00 || SizeInLBA == 0) {
       continue;
     }
 
     MbrValid    = TRUE;
-    StartingLBA = UNPACK_UINT32 (Mbr->Partition[Index1].StartingLBA);
-    EndingLBA   = StartingLBA + UNPACK_UINT32 (Mbr->Partition[Index1].SizeInLBA) - 1;
+    EndingLBA   = StartingLBA + SizeInLBA - 1;
     if (EndingLBA > LastLba) {
       //
       // Compatibility Errata:
@@ -77,12 +100,15 @@ PartitionValidMbr (
     }
 
     for (Index2 = Index1 + 1; Index2 < MAX_MBR_PARTITIONS; Index2++) {
-      if (Mbr->Partition[Index2].OSIndicator == 0x00 || UNPACK_UINT32 (Mbr->Partition[Index2].SizeInLBA) == 0) {
+      StartingLBA = UNPACK_UINT32 (Mbr->Partition[Index2].StartingLBA);
+      SizeInLBA   = UNPACK_UINT32 (Mbr->Partition[Index2].SizeInLBA);
+
+      if (Mbr->Partition[Index2].OSIndicator == 0x00 || SizeInLBA == 0) {
         continue;
       }
 
-      NewEndingLBA = UNPACK_UINT32 (Mbr->Partition[Index2].StartingLBA) + UNPACK_UINT32 (Mbr->Partition[Index2].SizeInLBA) - 1;
-      if (NewEndingLBA >= StartingLBA && UNPACK_UINT32 (Mbr->Partition[Index2].StartingLBA) <= EndingLBA) {
+      NewEndingLBA = StartingLBA + SizeInLBA - 1;
+      if (NewEndingLBA >= StartingLBA && StartingLBA <= EndingLBA) {
         //
         // This region overlaps with the Index1'th region
         //
-- 
2.21.0.windows.1