最近有一个读取 FFS 的需求，具体来说就是在 DXE 阶段需要读取指定的 FFS 文件的内容。针对这个需求，使用QEMU 进行了一番研究。
OVMF 中继承了 Shell，读取这个的动作和我们需求非常类似，于是在 QEMU 的Log中搜索可以看到如下字样：
[Bds]Booting EFI Internal Shell
[Bds] Expand Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1) -> Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
InstallProtocolInterface: 5B1B31A1-9562-11D2-8E3F-00A0C969723B 6C14040
Loading driver at 0x000068AE000 EntryPoint=0x000068AE4E8 Shell.efi
InstallProtocolInterface: BC62157E-3E33-4FEC-9920-2D3B36D750DF 70F9418
对应在代码中位于\mdemodulepkg\library\uefibootmanagerlib\BmLoadOption.c
DEBUG ((DEBUG_INFO, "[Bds] Expand "));
BmPrintDp (FilePath);
DEBUG ((DEBUG_INFO, " -> "));
BmPrintDp (CurFullPath);
DEBUG ((DEBUG_INFO, "\n"));
经过追踪可以看到具体工作的是位于 \mdepkg\library\dxeserviceslib\DxeServicesLib.c的如下函数：
/**
Get the image file buffer data and buffer size by its device path.
Access the file either from a firmware volume, from a file system interface,
or from the load file interface.
Allocate memory to store the found image. The caller is responsible to free memory.
If FilePath is NULL, then NULL is returned.
If FileSize is NULL, then NULL is returned.
If AuthenticationStatus is NULL, then NULL is returned.
@param[in] BootPolicy Policy for Open Image File.If TRUE, indicates
that the request originates from the boot
manager, and that the boot manager is
attempting to load FilePath as a boot
selection. If FALSE, then FilePath must
match an exact file to be loaded.
@param[in] FilePath The pointer to the device path of the file
that is abstracted to the file buffer.
@param[out] FileSize The pointer to the size of the abstracted
file buffer.
@param[out] AuthenticationStatus Pointer to the authentication status.
@retval NULL FilePath is NULL, or FileSize is NULL, or AuthenticationStatus is NULL, or the file can't be found.
@retval other The abstracted file buffer. The caller is responsible to free memory.
**/
VOID *
EFIAPI
GetFileBufferByFilePath (
IN BOOLEAN BootPolicy,
IN CONST EFI_DEVICE_PATH_PROTOCOL *FilePath,
OUT UINTN *FileSize,
OUT UINT32 *AuthenticationStatus
)
进一步研究，上述函数通过 EFI_FIRMWARE_VOLUME2_PROTOCOL 来完成读取。这个 Protocol 在 PI Spec （并不是 UEFI Spec）中有描述：
进一步追踪，可以看到是通过FwVol->ReadSection() 将Shell读取出来的：
SectionType = EFI_SECTION_PE32;
ImageBuffer = NULL;
Status = FwVol->ReadSection (
FwVol,
FvNameGuid,
SectionType,
0,
(VOID **)&ImageBuffer,
&ImageBufferSize,
AuthenticationStatus
);
ReadSection() 原型定义如下：
/**
Locates the requested section within a file and returns it in a buffer.
ReadSection() is used to retrieve a specific section from a file
within a firmware volume. The section returned is determined
using a depth-first, left-to-right search algorithm through all
sections found in the specified file. The output buffer is specified by a double indirection
of the Buffer parameter. The input value of Buffer is used to
determine if the output buffer is caller allocated or is
dynamically allocated by ReadSection(). If the input value of
Buffer!=NULL, it indicates that the output buffer is caller
allocated. In this case, the input value of *BufferSize
indicates the size of the caller-allocated output buffer. If
the output buffer is not large enough to contain the entire
requested output, it is filled up to the point that the output
buffer is exhausted and EFI_WARN_BUFFER_TOO_SMALL is returned,
and then BufferSize is returned with the size that is required
to successfully complete the read. All other
output parameters are returned with valid values. If the input
value of *Buffer==NULL, it indicates the output buffer is to
be allocated by ReadSection(). In this case, ReadSection()
will allocate an appropriately sized buffer from boot services
pool memory, which will be returned in *Buffer. The size of
the new buffer is returned in *BufferSize and all other output
parameters are returned with valid values. ReadSection() is
callable only from TPL_NOTIFY and below. Behavior of
ReadSection() at any EFI_TPL above TPL_NOTIFY is
undefined.
@param This Indicates the EFI_FIRMWARE_VOLUME2_PROTOCOL instance.
@param NameGuid Pointer to an EFI_GUID, which indicates the
file name from which the requested section
will be read.
@param SectionType Indicates the section type to return.
SectionType in conjunction with
SectionInstance indicates which section to
return.
@param SectionInstance Indicates which instance of sections
with a type of SectionType to return.
SectionType in conjunction with
SectionInstance indicates which
section to return. SectionInstance is
zero based.
@param Buffer Pointer to a pointer to a buffer in which the
section contents are returned, not including
the section header.
@param BufferSize Pointer to a caller-allocated UINTN. It
indicates the size of the memory
represented by Buffer.
@param AuthenticationStatus Pointer to a caller-allocated
UINT32 in which the authentication
status is returned.
@retval EFI_SUCCESS The call completed successfully.
@retval EFI_WARN_BUFFER_TOO_SMALL The caller-allocated
buffer is too small to
contain the requested
output. The buffer is
filled and the output is
truncated.
@retval EFI_OUT_OF_RESOURCES An allocation failure occurred.
@retval EFI_NOT_FOUND The requested file was not found in
the firmware volume. EFI_NOT_FOUND The
requested section was not found in the
specified file.
@retval EFI_DEVICE_ERROR A hardware error occurred when
attempting to access the firmware
volume.
@retval EFI_ACCESS_DENIED The firmware volume is configured to
disallow reads. EFI_PROTOCOL_ERROR
The requested section was not found,
but the file could not be fully
parsed because a required
GUIDED_SECTION_EXTRACTION_PROTOCOL
was not found. It is possible the
requested section exists within the
file and could be successfully
extracted once the required
GUIDED_SECTION_EXTRACTION_PROTOCOL
is published.
**/
typedef
EFI_STATUS
(EFIAPI * EFI_FV_READ_SECTION)(
IN CONST EFI_FIRMWARE_VOLUME2_PROTOCOL *This,
IN CONST EFI_GUID *NameGuid,
IN EFI_SECTION_TYPE SectionType,
IN UINTN SectionInstance,
IN OUT VOID **Buffer,
IN OUT UINTN *BufferSize,
OUT UINT32 *AuthenticationStatus
);
在 OVMF 中实际动作是 \mdemodulepkg\core\dxe\fwvol\FwVolRead.c 中的FvReadFileSection()函数完成的（通过输出 Log 的方法可以确认）：
/**
Locates a section in a given FFS File and
copies it to the supplied buffer (not including section header).
@param This Indicates the calling context.
@param NameGuid Pointer to an EFI_GUID, which is the
filename.
@param SectionType Indicates the section type to return.
@param SectionInstance Indicates which instance of sections with a
type of SectionType to return.
@param Buffer Buffer is a pointer to pointer to a buffer
in which the file or section contents or are
returned.
@param BufferSize BufferSize is a pointer to caller allocated
UINTN.
@param AuthenticationStatus AuthenticationStatus is a pointer to a
caller allocated UINT32 in which the
authentication status is returned.
@retval EFI_SUCCESS Successfully read the file section into
buffer.
@retval EFI_WARN_BUFFER_TOO_SMALL Buffer too small.
@retval EFI_NOT_FOUND Section not found.
@retval EFI_DEVICE_ERROR Device error.
@retval EFI_ACCESS_DENIED Could not read.
@retval EFI_INVALID_PARAMETER Invalid parameter.
**/
EFI_STATUS
EFIAPI
FvReadFileSection (
IN CONST EFI_FIRMWARE_VOLUME2_PROTOCOL *This,
IN CONST EFI_GUID *NameGuid,
IN EFI_SECTION_TYPE SectionType,
IN UINTN SectionInstance,
IN OUT VOID **Buffer,
IN OUT UINTN *BufferSize,
OUT UINT32 *AuthenticationStatus
)
运行结果如下：
oading driver at 0x00007AC4000 EntryPoint=0x00007AC4494 PcRtc.efi
InstallProtocolInterface: BC62157E-3E33-4FEC-9920-2D3B36D750DF 7097A98
ProtectUefiImageCommon - 0x7097740
- 0x0000000007AC4000 - 0x0000000000005300
Image - c:\buildbs\stable202108\Build\OvmfX64\DEBUG_VS2015x86\X64\PcAtChipsetPkg\PcatRealTimeClockRuntimeDxe\PcatRealTimeClockRuntimeDxe\DEBUG\PcRtc.pdb
!!!!!!!! ProtectUefiImageCommon - Section Alignment(0x20) is incorrect !!!!!!!!
!!!!!!!! Image - c:\buildbs\stable202108\Build\OvmfX64\DEBUG_VS2015x86\X64\PcAtChipsetPkg\PcatRealTimeClockRuntimeDxe\PcatRealTimeClockRuntimeDxe\DEBUG\PcRtc.pdb !!!!!!!!
[EFI_FIRMWARE_VOLUME2_PROTOCOL Success 944416]
4D 5A 0 0 0 0 0 0 0 0 0 0 0 0 0 0
A5 A0 A5 C0 A5 D8 A5 F0 A5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
InstallProtocolInterface: 27CFAC87-46CC-11D4-9A38-0090273FC14D 0
使用工具可以看到 Shell.efi 的大小是 94416Bytes，读取到Buffer 中的内容也是 Shell 的内容。
总结:BIOS 工程师可以通过 EFI_FIRMWARE_VOLUME2_PROTOCOL 来获取指定 FFS 的内容。