Add lesson 7

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diff --git a/Lesson_7/README.md b/Lesson_7/README.md
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+Let's print GUID for all the protocols that are exist in our `IMAGE_HANDLE`.
+
+First understand what `EFI_GUID` internally means in the edk2 codebase:
+
+https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Uefi/UefiBaseType.h
+```
+///
+/// 128-bit buffer containing a unique identifier value.
+///
+typedef GUID                      EFI_GUID;
+```
+`GUID` structure is defined in https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Base.h :
+```
+///
+/// 128 bit buffer containing a unique identifier value.
+/// Unless otherwise specified, aligned on a 64 bit boundary.
+///
+typedef struct {
+  UINT32  Data1;
+  UINT16  Data2;
+  UINT16  Data3;
+  UINT8   Data4[8];
+} GUID;
+```
+Fortunately we don't have to manually print all these fields by hand. `Print` function has a format option `%g` to print GUIDs, so we could simply print GUIDs with a code like this:
+```
+Print("GUID=%g\n", myGUID);
+```
+More information about `Print` formating options can be found at https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PrintLib.h
+
+
+We want to print `EFI_GUID` field from the `PROTOCOL_ENTRY` structure which are referred from the `PROTOCOL_INTERFACE` structures. So we need to define both of these structures in our file for the same reason we've defined `IHANDLE` earlier.
+
+```
+typedef struct {
+  UINTN               Signature;
+  /// Link Entry inserted to mProtocolDatabase
+  LIST_ENTRY          AllEntries;
+  /// ID of the protocol
+  EFI_GUID            ProtocolID;
+  /// All protocol interfaces
+  LIST_ENTRY          Protocols;
+  /// Registerd notification handlers
+  LIST_ENTRY          Notify;
+} PROTOCOL_ENTRY;
+
+typedef struct {
+  UINTN                       Signature;
+  /// Link on IHANDLE.Protocols
+  LIST_ENTRY                  Link;
+  /// Back pointer
+  IHANDLE                     *Handle;
+  /// Link on PROTOCOL_ENTRY.Protocols
+  LIST_ENTRY                  ByProtocol;
+  /// The protocol ID
+  PROTOCOL_ENTRY              *Protocol;
+  /// The interface value
+  VOID                        *Interface;
+  /// OPEN_PROTOCOL_DATA list
+  LIST_ENTRY                  OpenList;
+  UINTN                       OpenListCount;
+
+} PROTOCOL_INTERFACE;
+```
+
+`PROTOCOL_INTERFACE` structures that are connected to any HANDLE are interlinked with each other with a help of a `LIST_ENTRY Link` field that connects them to a double linked list.
+
+As you may remember `LIST_ENTRY` is defined like this:
+```
+///
+/// LIST_ENTRY structure definition.
+///
+typedef struct _LIST_ENTRY LIST_ENTRY;
+
+///
+/// _LIST_ENTRY structure definition.
+///
+struct _LIST_ENTRY {
+  LIST_ENTRY  *ForwardLink;
+  LIST_ENTRY  *BackLink;
+};
+```
+Each of these fields inside these structure points to another `LIST_ENTRY` structure that is placed in another `PROTOCOL_INTERFACE`.
+
+So this connection looks like this:
+
+```
+typedef struct {                                         typedef struct {
+  UINTN                       Signature;                   UINTN                       Signature;
+  struct LIST_ENTRY {                          |---------> struct LIST_ENTRY {
+    LIST_ENTRY  *ForwardLink; -----------------|             LIST_ENTRY  *ForwardLink;
+    LIST_ENTRY  *BackLink;                                   LIST_ENTRY  *BackLink;
+  } Link;                                                  } Link;
+  IHANDLE                     *Handle;                     IHANDLE                     *Handle;
+  LIST_ENTRY                  ByProtocol;                  LIST_ENTRY                  ByProtocol;
+  PROTOCOL_ENTRY              *Protocol;                   PROTOCOL_ENTRY              *Protocol;
+  VOID                        *Interface;                  VOID                        *Interface;
+  LIST_ENTRY                  OpenList;                    LIST_ENTRY                  OpenList;
+  UINTN                       OpenListCount;               UINTN                       OpenListCount;
+} PROTOCOL_INTERFACE;                                    } PROTOCOL_INTERFACE;
+```
+
+But in reality we want a pointer not to `Link` field of another `PROTOCOL_INTERFACE` structure, we want a pointer to another `PROTCOL_INTERFACE` structure inself.
+
+Therefore one more thing that we would need is a couple of macros:
+```
+#define offsetof(a,b) ((INTN)(&(((a*)(0))->b)))
+
+#define container_of(ptr, type, member) ({                      \
+        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
+        (type *)( (char *)__mptr - offsetof(type,member) );})
+```
+These macros can be familiar to you if you've investigated linux kernel programming. This is where I got them from anyway.
+
+`contianer_of` macro helps to get a pointer to a structure if you have a pointer to one of its fields.
+
+If you want to understand how it works internally in C language I suggest you to look at the https://stackoverflow.com/questions/15832301/understanding-container-of-macro-in-the-linux-kernel
+
+With all of this information final code for our `UefiMain` function would look like this:
+```
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  IHANDLE* MyHandle = ImageHandle;
+  Print(L"Signature: %c %c %c %c\n", (MyHandle->Signature >>  0) & 0xff,
+                                     (MyHandle->Signature >>  8) & 0xff,
+                                     (MyHandle->Signature >> 16) & 0xff,
+                                     (MyHandle->Signature >> 24) & 0xff);
+
+  Print(L"Back Protocol Interface Link: %p\n", MyHandle->Protocols.BackLink);
+  Print(L"Forward Protocol Interface Link: %p\n", MyHandle->Protocols.ForwardLink);
+
+  LIST_ENTRY *FirstLink = MyHandle->Protocols.ForwardLink;
+  LIST_ENTRY *CurrentLink = FirstLink;
+  do {
+        PROTOCOL_INTERFACE* MyProtocolInterface = container_of(CurrentLink, PROTOCOL_INTERFACE, Link);
+
+        Print(L"\n");
+        Print(L"Current Link: %p\n", CurrentLink);
+        Print(L"Signature: %c %c %c %c\n", (MyProtocolInterface->Signature >>  0) & 0xff,
+                                           (MyProtocolInterface->Signature >>  8) & 0xff,
+                                           (MyProtocolInterface->Signature >> 16) & 0xff,
+                                           (MyProtocolInterface->Signature >> 24) & 0xff);
+
+        Print(L"Back Link: %p\n", MyProtocolInterface->Link.BackLink);
+        Print(L"Forward Link: %p\n", MyProtocolInterface->Link.ForwardLink);
+        Print(L"GUID=%g\n", MyProtocolInterface->Protocol->ProtocolID);
+        CurrentLink = MyProtocolInterface->Link.ForwardLink;
+  } while (CurrentLink != FirstLink);
+
+  return EFI_SUCCESS;
+}
+```
+
+If we compile and run our app in OVMF (I hope at this time I don't need to repeat how to do it):
+```
+FS0:\> ImageInfo.efi
+h n d l
+Back Protocol Interface Link: 68D4320
+Forward Protocol Interface Link: 6891520
+
+Current Link: 6891520
+p i f c
+Back Link: 6891430
+Forward Link: 6891B20
+GUID=752F3136-4E16-4FDC-A22A-E5F46812F4CA
+
+Current Link: 6891B20
+p i f c
+Back Link: 6891520
+Forward Link: 68D4320
+GUID=BC62157E-3E33-4FEC-9920-2D3B36D750DF
+
+Current Link: 68D4320
+p i f c
+Back Link: 6891B20
+Forward Link: 6891430
+GUID=5B1B31A1-9562-11D2-8E3F-00A0C969723B
+
+Current Link: 6891430
+  ? ? ?
+Back Link: 68D4320
+Forward Link: 6891520
+GUID=00000000-0000-0000-0000-000000000000
+```
+
+Let's find first GUID by executing grep on the edk2 source:
+```
+$ grep -i 752F3136 -r ./ --exclude-dir=Build
+./MdePkg/Include/Protocol/ShellParameters.h:  0x752f3136, 0x4e16, 0x4fdc, { 0xa2, 0x2a, 0xe5, 0xf4, 0x68, 0x12, 0xf4, 0xca } \
+./MdePkg/MdePkg.dec:  gEfiShellParametersProtocolGuid      = { 0x752f3136, 0x4e16, 0x4fdc, {0xa2, 0x2a, 0xe5, 0xf4, 0x68, 0x12, 0xf4, 0xca }}
+```
+
+https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Protocol/ShellParameters.h
+```
+#define EFI_SHELL_PARAMETERS_PROTOCOL_GUID \
+  { \
+  0x752f3136, 0x4e16, 0x4fdc, { 0xa2, 0x2a, 0xe5, 0xf4, 0x68, 0x12, 0xf4, 0xca } \
+  }
+```
+You can also see in in a https://github.com/tianocore/edk2/blob/master/MdePkg/MdePkg.dec file:
+```
+  ## Include/Protocol/ShellParameters.h
+  gEfiShellParametersProtocolGuid      = { 0x752f3136, 0x4e16, 0x4fdc, {0xa2, 0x2a, 0xe5, 0xf4, 0x68, 0x12, 0xf4, 0xca }}
+```
+
+The next two GUIDs you can find in UEFI the specification.
+
+`EFI_LOADED_IMAGE_DEVICE_PATH_PROTOCOL` - When installed, the Loaded Image Device Path Protocol specifies the device path that was used when a PE/COFF image was loaded through the EFI Boot Service LoadImage().
+
+```
+#define EFI_LOADED_IMAGE_DEVICE_PATH_PROTOCOL_GUID \
+{0xbc62157e,0x3e33,0x4fec,\
+ {0x99,0x20,0x2d,0x3b,0x36,0xd7,0x50,0xdf}}
+```
+
+`EFI_LOADED_IMAGE_PROTOCOL` - Can be used on any image handle to obtain information about the loaded image.
+```
+#define EFI_LOADED_IMAGE_PROTOCOL_GUID\
+ {0x5B1B31A1,0x9562,0x11d2,\
+ {0x8E,0x3F,0x00,0xA0,0xC9,0x69,0x72,0x3B}}
+```
+
+The last `PROTOCOL_INTERFACE` structure doesn't have a valid "p i f c" signature (in my case it is 0x20 0x0E 0xED 0x06), so we don't need to look at its GUID.
+
diff --git a/README.md b/README.md
index faae71c..34dd7ad 100644
--- a/README.md
+++ b/README.md
@@ -9,4 +9,5 @@ Lessons description:
 - Lesson 4: Use edk2 libraries to simplify our `HelloWorld` app
 - Lesson 5: Simplify build command with the help of files in the `Conf` folder
 - Lesson 6: Handle/Protocol databases internals - Part 1: Theory and raw access to an app ImageHandle
+- Lesson 7: Handle/Protocol databases internals - Part 2: Raw access to app protocols