[FREELDR] i386/pc/pcdisk.c: Fix LBA reads retry loop (reactos#7367)

When the Int 13h AH=42h "Extended read" function fails, the disk address
packet's LBA block count is reset to the number of blocks that have been
successfully transferred. This is more or less fine, unless one wants to
ensure the exact number of sectors gets read.

If the function fails so that zero sectors were read, the retry loop is
restarted, but with the packet's LBA block count member reset, as per
the documentation. (In this example, it is reset to zero.) Then, at the
next retry attempt, zero sectors are requested to be read, and this time
of course, the call succeeds... Wrongly, of course, this is not what's
expected.

Therefore, for each retry, the LBA block count member should be set
again to the correct number of sectors to read. There are maximum 3
retries, so the retry loop will stop anyway, but the LBA read will now
correctly fail and return FALSE, as expected.

This problem doesn't exist in the retry loop for the Int 13h, AH=02h
"Read Disk Sectors" CHS function, because here, the call is made only
using registers, and we use a pair of RegsIn/RegsOut. RegsOut receives
the modified register values, but the input RegsIn stays unchanged.

Author: HBelusca

boot/freeldr/freeldr/arch/i386/pc/pcdisk.c

@@ -570,14 +570,13 @@ PcDiskReadLogicalSectorsLBA(
     RtlZeroMemory(Packet, sizeof(*Packet));
     Packet->PacketSize = sizeof(*Packet);
     Packet->Reserved = 0;
-    Packet->LBABlockCount = (USHORT)SectorCount;
-    ASSERT(Packet->LBABlockCount == SectorCount);
+    // Packet->LBABlockCount set in the loop.
     Packet->TransferBufferOffset = ((ULONG_PTR)Buffer) & 0x0F;
     Packet->TransferBufferSegment = (USHORT)(((ULONG_PTR)Buffer) >> 4);
     Packet->LBAStartBlock = SectorNumber;
 
     /*
-     * BIOS int 0x13, function 42h - IBM/MS INT 13 Extensions - EXTENDED READ
+     * BIOS Int 13h, function 42h - IBM/MS INT 13 Extensions - EXTENDED READ
      * Return:
      * CF clear if successful
      * AH = 00h
@@ -586,14 +585,20 @@ PcDiskReadLogicalSectorsLBA(
      * Disk address packet's block count field set to the
      * number of blocks successfully transferred.
      */
-    RegsIn.b.ah = 0x42;                 // Subfunction 42h
+    RegsIn.b.ah = 0x42;
     RegsIn.b.dl = DriveNumber;          // Drive number in DL (0 - floppy, 0x80 - harddisk)
     RegsIn.x.ds = BIOSCALLBUFSEGMENT;   // DS:SI -> disk address packet
     RegsIn.w.si = BIOSCALLBUFOFFSET;
 
     /* Retry 3 times */
     for (RetryCount = 0; RetryCount < 3; ++RetryCount)
     {
+        /* Restore the number of blocks to transfer, since it gets reset
+         * on failure with the number of blocks that were successfully
+         * transferred (and which could be zero). */
+        Packet->LBABlockCount = (USHORT)SectorCount;
+        ASSERT(Packet->LBABlockCount == SectorCount);
+
         Int386(0x13, &RegsIn, &RegsOut);
 
         /* If it worked return TRUE */