Move lessons to separate folder

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>

1 files changed, 359 insertions, 0 deletions

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+Let's print content of variables that define boot options in UEFI.
+
+To do it, we need to use `GetVariable()` API function from the EFI Runtime Services:
+```
+GetVariable()
+
+Summary:
+Returns the value of a variable.
+
+Prototype:
+typedef
+EFI_STATUS
+GetVariable (
+ IN CHAR16 *VariableName,
+ IN EFI_GUID *VendorGuid,
+ OUT UINT32 *Attributes OPTIONAL,
+ IN OUT UINTN *DataSize,
+ OUT VOID *Data OPTIONAL
+ );
+
+Parameters:
+VariableName    A Null-terminated string that is the name of the vendor’s variable.
+VendorGuid      A unique identifier for the vendor
+Attributes      If not NULL, a pointer to the memory location to return the
+                attributes bitmask for the variable.
+                If not NULL, then Attributes is set on output both when
+                EFI_SUCCESS and when EFI_BUFFER_TOO_SMALL is returned.
+DataSize        On input, the size in bytes of the return Data buffer.
+                On output the size of data returned in Data.
+Data            The buffer to return the contents of the variable. May be NULL
+                with a zero DataSize in order to determine the size buffer needed.
+
+Description:
+Each vendor may create and manage its own variables without the risk of name conflicts by using a
+unique VendorGuid. When a variable is set its Attributes are supplied to indicate how the data variable
+should be stored and maintained by the system. The attributes affect when the variable may be accessed
+and volatility of the data
+
+If the Data buffer is too small to hold the contents of the variable, the error EFI_BUFFER_TOO_SMALL is
+returned and DataSize is set to the required buffer size to obtain the data.
+
+Status Codes Returned:
+EFI_SUCCESS 		The function completed successfully.
+EFI_NOT_FOUND 		The variable was not found.
+EFI_BUFFER_TOO_SMALL 	The DataSize is too small for the result. DataSize has been
+			updated with the size needed to complete the request. If
+			Attributes is not NULL, then the attributes bitmask for the
+			variable has been stored to the memory location pointed-to by
+			Attributes.
+...
+
+```
+
+In previous lesson we've discovered that these options are present in our environment under GUID `EFI_GLOBAL_VARIABLE` (gEfiGlobalVariableGuid):
+```
+Boot0000
+Boot0001
+Boot0002
+Boot0003
+Boot0004
+BootCurrent
+BootOrder
+```
+
+If we look at the UEFI spec for these options description we would find:
+```
+Each Boot#### variable contains an EFI_LOAD_OPTION. Each Boot#### variable is the name “Boot”
+appended with a unique four digit hexadecimal number. For example, Boot0001, Boot0002, Boot0A02,
+etc.
+
+...
+
+The BootOrder variable contains an array of UINT16’s that make up an ordered list of the Boot####
+options. The first element in the array is the value for the first logical boot option, the second element is
+the value for the second logical boot option, etc. The BootOrder order list is used by the firmware’s
+boot manager as the default boot order.
+
+...
+
+The BootCurrent variable is a single UINT16 that defines the Boot#### option that was selected on
+the current boot.
+```
+
+Create `ShowBootVariables` for this lesson.
+
+First let's try to get simple `UINT16 BootCurrent` option.
+
+We would be getting variables a lot in this program, so it is best to create a function that abstracts all the necessities of the `GetVariable` API calls:
+```
+EFI_STATUS
+GetNvramVariable( CHAR16   *VariableName,
+                  EFI_GUID *VariableOwnerGuid,
+                  VOID     **Buffer,
+                  UINTN    *BufferSize)
+{
+    UINTN Size = 0;
+    *BufferSize = 0;
+
+    EFI_STATUS Status = gRT->GetVariable(VariableName, VariableOwnerGuid, NULL, &Size, NULL);
+    if (Status != EFI_BUFFER_TOO_SMALL) {
+        Print(L"Error! 'gRT->GetVariable' call returned %r\n", Status);
+        return Status;
+    }
+
+    *Buffer = AllocateZeroPool(Size);
+    if (!Buffer) {
+        Print(L"Error! 'AllocateZeroPool' call returned %r\n", Status);
+        return EFI_OUT_OF_RESOURCES;
+    }
+
+    Status = gRT->GetVariable(VariableName, VariableOwnerGuid, NULL, &Size, *Buffer);
+    if (Status == EFI_SUCCESS) {
+        *BufferSize = Size;
+    } else {
+        FreePool( *Buffer );
+        *Buffer = NULL;
+    }
+
+    return Status;
+}
+```
+
+As with many other different UEFI APIs first `GetVariable` call gives us `EFI_BUFFER_TOO_SMALL` error, but fills `Size` variable with the size of the array that we need to allocate and pass to this function for the correct execution.
+We allocate necessary size with the `AllocateZeroPool` edk2 library call, like we did it in previous lesson. After that we call `GetVariable` function for the second time, expecting `EFI_SUCCESS` status.
+
+
+The main function would look like this:
+```
+#include <Library/UefiRuntimeServicesTableLib.h>
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/UefiLib.h>
+#include <Library/MemoryAllocationLib.h>
+
+EFI_STATUS
+GetNvramVariable( CHAR16   *VariableName,
+                  EFI_GUID *VariableOwnerGuid,
+                  VOID     **Buffer,
+                  UINTN    *BufferSize)
+{
+  ...
+}
+
+INTN EFIAPI ShellAppMain(IN UINTN Argc, IN CHAR16 **Argv)
+{
+  UINTN OptionSize;
+  EFI_STATUS Status;
+
+  UINT16* BootCurrent;
+  Status = GetNvramVariable(L"BootCurrent", &gEfiGlobalVariableGuid, (VOID**)&BootCurrent, &OptionSize);
+  if (Status == EFI_SUCCESS) {
+    Print(L"BootCurrent=%d\n", *BootCurrent);
+  } else {
+    Print(L"Can't get BootCurrent variable\n");
+  }
+
+  return EFI_SUCCESS;
+}
+```
+
+If we execute this code under OVMF we would get:
+```
+FS0:\> ShowBootVariables.efi
+3
+```
+This means that `Boot0003` is active.
+
+
+Now let's get `UINT16 BootOrder[]` variable. It is an array of XXXX numbers that describe Boot option priority.
+So `{0,2,4,1,3}` for example would mean this order:
+```
+Boot0000
+Boot0002
+Boot0004
+Boot0001
+Boot0003
+```
+
+This is the code to get and print this array:
+```
+UINT16* BootOrderArray;
+Status = GetNvramVariable(L"BootOrder", &gEfiGlobalVariableGuid, (VOID**)&BootOrderArray, &OptionSize);
+if (Status == EFI_SUCCESS) {
+  for (UINTN i=0; i<(OptionSize/sizeof(UINT16)); i++) {
+    Print(L"Boot%04d%s\n", BootOrderArray[i], (BootOrderArray[i] == *BootCurrent)? L"*" : L"" );
+  }
+} else {
+  Print(L"Can't get BootOrder variable\n");
+}
+```
+
+If we execute this code under OVMF we would get:
+```
+Boot0000
+Boot0001
+Boot0002
+Boot0003*
+Boot0004
+```
+
+
+Now let's print information about every `Boot####` option.
+
+According to the UEFI the spec these options are stored in a `EFI_LOAD_OPTION` structure:
+```
+typedef struct _EFI_LOAD_OPTION {
+ UINT32 Attributes;
+ UINT16 FilePathListLength;
+ // CHAR16 Description[];
+ // EFI_DEVICE_PATH_PROTOCOL FilePathList[];
+ // UINT8 OptionalData[];
+} EFI_LOAD_OPTION;
+
+Parameters
+Attributes 		The attributes for this load option entry. All unused bits must be zero
+			and are reserved by the UEFI specification for future growth.
+FilePathListLength	Length in bytes of the FilePathList. OptionalData starts at
+			offset sizeof(UINT32) + sizeof(UINT16) +
+			StrSize(Description) + FilePathListLength of the
+			EFI_LOAD_OPTION descriptor.
+Description 		The user readable description for the load option. This field ends
+			with a Null character.
+FilePathList 		A packed array of UEFI device paths. The first element of the array is
+			a device path that describes the device and location of the Image for
+			this load option. The FilePathList[0] is specific to the device
+			type. Other device paths may optionally exist in the FilePathList,
+			but their usage is OSV specific. Each element in the array is variable
+			length, and ends at the device path end structure. Because the size
+			of Description is arbitrary, this data structure is not guaranteed
+			to be aligned on a natural boundary. This data structure may have to
+			be copied to an aligned natural boundary before it is used.
+OptionalData 		The remaining bytes in the load option descriptor are a binary data
+			buffer that is passed to the loaded image. If the field is zero bytes
+			long, a NULL pointer is passed to the loaded image. The number of
+			bytes in OptionalData can be computed by subtracting the
+			starting offset of OptionalData from total size in bytes of the
+			EFI_LOAD_OPTION. 
+```
+In edk2 it is defined in a file https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Uefi/UefiSpec.h
+
+Pay attention to the fact that some fields in this structure are commented:
+```
+// CHAR16 Description[];
+// EFI_DEVICE_PATH_PROTOCOL FilePathList[];
+// UINT8 OptionalData[];
+```
+It is because these fields are variable size arrays, so we need to calculate offsets to them dynamically.
+
+Create a function that accepts names of a Boot variable (such as "Boot0003") and outputs all the needed information about it:
+```
+VOID PrintBootOption(CHAR16* BootOptionName)
+{
+  UINTN OptionSize;
+  UINT8* Buffer;
+
+  EFI_STATUS Status = GetNvramVariable(BootOptionName, &gEfiGlobalVariableGuid, (VOID**)&Buffer, &OptionSize);
+  if (Status == EFI_SUCCESS) {
+      EFI_LOAD_OPTION* LoadOption = (EFI_LOAD_OPTION*) Buffer;
+      CHAR16* Description = (CHAR16*)(Buffer + sizeof (EFI_LOAD_OPTION));
+      UINTN DescriptionSize = StrSize(Description);
+
+      Print(L"%s\n", Description);
+      if (LoadOption->FilePathListLength != 0) {
+        VOID* FilePathList = (UINT8 *)Description + DescriptionSize;
+        CHAR16* DevPathString = ConvertDevicePathToText(FilePathList, TRUE, FALSE);
+        Print(L"%s\n", DevPathString);
+      }
+  } else {
+    Print(L"Can't get %s variable\n", BootOptionName);
+  }
+}
+```
+
+This code uses pointer arithmetics that we've discussed above. To get `Description` field size we simply use `StrSize(Description)` call as `Description` field always ends with Null.
+
+To print DevicePath as a string we use `ConvertDevicePathToText` call (we've used it earlier in our `ImageInfo` application). To use it we need to add `#include <Library/DevicePathLib.h>` to our program.
+
+Now we need to construct "BootXXXX" variables from a `BootOrder` numbers.
+
+To do so we would use `UnicodeSPrint` function from the https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PrintLib.h
+```
+/**
+  Produces a Null-terminated Unicode string in an output buffer based on a Null-terminated
+  Unicode format string and variable argument list.
+  This function is similar as snprintf_s defined in C11.
+
+  ...
+
+  @param  StartOfBuffer   A pointer to the output buffer for the produced Null-terminated
+                          Unicode string.
+  @param  BufferSize      The size, in bytes, of the output buffer specified by StartOfBuffer.
+  @param  FormatString    A Null-terminated Unicode format string.
+  @param  ...             Variable argument list whose contents are accessed based on the
+                          format string specified by FormatString.
+  @return The number of Unicode characters in the produced output buffer not including the
+          Null-terminator.
+**/
+
+UINTN
+EFIAPI
+UnicodeSPrint (
+  OUT CHAR16        *StartOfBuffer,
+  IN  UINTN         BufferSize,
+  IN  CONST CHAR16  *FormatString,
+  ...
+  );
+```
+
+Just in case in the same file there is a similar function for ASCII:
+```
+UINTN
+EFIAPI
+AsciiSPrint (
+  OUT CHAR8        *StartOfBuffer,
+  IN  UINTN        BufferSize,
+  IN  CONST CHAR8  *FormatString,
+  ...
+  );
+```
+
+
+So the code inside BootOrder loop now would look like this:
+```
+CHAR16 BootOptionStr[sizeof("Boot####")+1];
+UnicodeSPrint(BootOptionStr, (sizeof("Boot####")+1)*sizeof(CHAR16), L"Boot%04x", BootOrderArray[i]);
+Print(L"%s%s\n", BootOptionStr, (BootOrderArray[i] == *BootCurrent)? L"*" : L"" );
+PrintBootOption(BootOptionStr);
+Print(L"\n");
+```
+
+Don't forget to add `#include <Library/PrintLib.h>` to the start of the file.
+
+
+If we compile our app and execute it under OVMF we would get:
+```
+FS0:\> ShowBootVariables.efi
+Boot0000
+UiApp
+Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
+
+Boot0001
+UEFI QEMU DVD-ROM QM00003
+PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
+
+Boot0002
+UEFI QEMU HARDDISK QM00001
+PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
+
+Boot0003*
+EFI Internal Shell
+Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
+
+Boot0004
+UEFI PXEv4 (MAC:525400123456)
+PciRoot(0x0)/Pci(0x3,0x0)/MAC(525400123456,0x1)/IPv4(0.0.0.0)
+```
+
+We are definitely in the UEFI shell, so the `BootCurrent`(="*") is placed correctly.
+
+