Re: Sec and Reset vector

[Andrew Fish <afish@apple.com>]

> On Oct 22, 2016, at 10:03 AM, Marvin H?user <Marvin.Haeuser@outlook.com> wrote:
> 
> Hey Rafael,
> 
> There actually is some generic SEC code in UefiCpuPkg you might want to take a look at. It's generic because it does not have "Intel NDA" code, such as CAR (Cache-As-RAM) etc.
> The Reset Vector may or may not be part of SecCore. It's either embedded within the SecCore module, or a separate file in the FFS. You can check the start/end address of the modules (e.g. with UEFITool) and find the Reset Vector file that way.
> 

Rafael,

There is some strange construction things going on with the SEC for X86. 

If you look in the FDF file you will see that the SEC is a PE/COFF (or TE) image and a raw binary for the 16-bit real mode reset vector code. 

https://github.com/tianocore/edk2/blob/master/Vlv2TbltDevicePkg/PlatformPkg.fdf#L876 <https://github.com/tianocore/edk2/blob/master/Vlv2TbltDevicePkg/PlatformPkg.fdf#L876>
[Rule.Common.SEC]
  FILE SEC = $(NAMED_GUID) RELOCS_STRIPPED {
    PE32  PE32    Align = 8       $(INF_OUTPUT)/$(MODULE_NAME).efi
    RAW BIN       Align = 16      |.com
  }

The .com files are constructed from *.nasmb, *.asm16, or *.S16 files. 
https://github.com/tianocore/edk2/tree/master/UefiCpuPkg/SecCore/Ia32 <https://github.com/tianocore/edk2/tree/master/UefiCpuPkg/SecCore/Ia32>

Special extensions are needed to have special build rules. The build rules are here:
https://github.com/tianocore/edk2/blob/master/BaseTools/Conf/build_rule.template#L480 <https://github.com/tianocore/edk2/blob/master/BaseTools/Conf/build_rule.template#L480>
Look at the [Masm16-Code-File] and [Nasm-to-Binary-Code-File] rules.

The build tools also do some magic to stitch the .com and PE/COFF (TE) file together. 
https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/SecCore/Ia32/ResetVec.nasmb#L46 <https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/SecCore/Ia32/ResetVec.nasmb#L46>
;
; Pointer to the entry point of the PEI core
; It is located at 0xFFFFFFE0, and is fixed up by some build tool
; So if the value 8..1 appears in the final FD image, tool failure occurs.
;
PeiCoreEntryPoint:       DD      87654321h


The reason you need special build rules is it is really hard to get code at the end of a PE/COFF file, so you need a stripped binary for the reset vector. 

The next problem is how do you get the FV File to be at the end of the FV (that is usually free space). The PI spec defines that if an FFS file has the File GUID of gEfiFirmwareVolumeTopFileGuid then it gets place at the end of the FV. Thus the X86 SEC must have this file guid. This also triggers the magic behavior to stitch the .com and PE/COFF together. 
https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/SecCore/SecCore.inf#L25 <https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/SecCore/SecCore.inf#L25>
  FILE_GUID                      = 1BA0062E-C779-4582-8566-336AE8F78F09



For ARM things are much simpler. The FV reserves 16-bytes at the start of the volume for the reset vector. If the build tools see an FV has an ARM SEC it can patch in a branch to the SEC PE/COFF (TE) entry point (going from memory hopefully I did not botch that).

https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Pi/PiFirmwareVolume.h#L110 <https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Pi/PiFirmwareVolume.h#L110>
  ///
  /// The first 16 bytes are reserved to allow for the reset vector of
  /// processors whose reset vector is at address 0.
  ///
  UINT8                     ZeroVector[16];



> PS.: Seems like inline images are not supported by the mailing list (or is it my error?). Either way, I do not see the image in my mail client (Outlook 2016).
> 

I don't see the image in my macOS Mail client. 

Thanks,

Andrew Fish


> Regards,
> Marvin.
> 
>> -----Original Message-----
>> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of
>> Rafael Machado
>> Sent: Saturday, October 22, 2016 6:28 PM
>> To: edk2-devel@lists.01.org
>> Subject: [edk2] Sec and Reset vector
>> 
>> Hi eveyrone
>> 
>> I'm doing some studies on edk2 and coreboot, but I'm having some questions
>> that I believe you can help.
>> 
>> On the journey to try to understand things since the beginning, so they make
>> sense in future, I'm trying to understand how does the Initial phases of UEFI
>> / PI firmware work. To do that I got a bios image and start to reverse it to
>> check the modules and everything present at that bios. Now I understand, at
>> least the basics, about DXE and PEI phase.
>> 
>> The main question that I have now is about the SEC phase.
>> To try to understand the SEC phase I tried to reverse this firmware so I could
>> check the reset vector's first jump or something like that.
>> The surprise I have is that I was not able to find this code.
>> 
>> To be sure I was reversing on the correct way I generated a coreboot image.
>> On the image below we can see the initial code of a firmware generated
>> using coreboot
>> 
>> [image: pasted1]
>> 
>> But at the UEFI firmware I'm studying I'm not able to find anything similar to
>> that.
>> My guess before starting this was that at least the SEC initial code should be
>> similar to the legacy way of doing things, a jmp at 0xfff:fff0 and after that the
>> magic should get started with all uefi phases.
>> 
>> Could someone please give me some light on that?
>> 
>> 
>> Thanks and Regards
>> Rafael R. Machado
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