Re: How to get BBS_BBS_DEVICE_PATH from BOOTXXXX variable

["Carsey, Jaben" <jaben.carsey@intel.com>]

Have you examined the structure EFI_LOAD_OPTION from UefiSpec.h?

I think that you are traversing the structure incorrectly.  I cannot be sure since I am not certain what this mGetVariable function does specifically.

I start by getting the correct size (BufferSize is zero at this first call):
    Status = gRT->GetVariable(
        VariableName,
        (EFI_GUID*)&gEfiGlobalVariableGuid,
        NULL,
        &BufferSize,
        Buffer);
    if (Status == EFI_BUFFER_TOO_SMALL) {
      Buffer = AllocateZeroPool(BufferSize);
      Status = gRT->GetVariable(
          VariableName,
          (EFI_GUID*)&gEfiGlobalVariableGuid,
          NULL,
          &BufferSize,
          Buffer);
    }

To parse the structure I divide it up using the structure.  I think your +3 may be causing lots of issues.

    LoadOption      = (EFI_LOAD_OPTION *)Buffer;
    Description     = (CHAR16*)(Buffer + sizeof (EFI_LOAD_OPTION));
    DescriptionSize = StrSize (Description);
    if (LoadOption->FilePathListLength != 0) {
      FilePathList = (UINT8 *)Description + DescriptionSize;
   }

After this we can calculate the location of optional data.
    OptionalDataOffset = sizeof *LoadOption + DescriptionSize +
                         LoadOption->FilePathListLength;
The optional data would be found at:
    Buffer + OptionalDataOffset

Note that all the pointer math is based on using UINT8* or the raw Buffer variable which is a UINT8*.  Otherwise the size in bytes will not work out.

-Jaben


From: Saqib Khan [mailto:saqib.khan2011@gmail.com]
Sent: Thursday, October 20, 2016 12:30 AM
To: Carsey, Jaben <jaben.carsey@intel.com>
Cc: Andrew Fish <afish@apple.com>; edk2-devel@ml01.01.org; Laszlo Ersek <lersek@redhat.com>
Subject: Re: [edk2] How to get BBS_BBS_DEVICE_PATH from BOOTXXXX variable
Importance: High

BootVariable = mGetVariable(Name, &gEfiGlobalVariableGuid, &BootVariableSize, NULL);
Ptr = (BootVariable+3);
         Print(L" %s",(CHAR16 *)(Ptr));
         DevicePath =AllocateZeroPool     (*FilePathListLength)     ;
         Ptr+= StrSize((CHAR16 *)Ptr);
         CopyMem (DevicePath, Ptr, *FilePathListLength);
As Carsey suggested i used the same Math in order to get DevicePath but in my case it is not working
As documentation this data structure is not aligned so i am assuming that just after description there should be
FilePathList ,so if i CopyMem (*FilePathListLength) bytes to DevicePath i should get the right value but i am getting garbage.

On Thu, Oct 20, 2016 at 1:41 AM, Carsey, Jaben <jaben.carsey@intel.com<mailto:jaben.carsey@intel.com>> wrote:
Have you examined the BCFG command in the UEFI Shell?

It does this exact behavior and the code is at: ShellPkg\Library\UefiBcfgCommandLib\bcfg.c.

-Jaben

> -----Original Message-----
> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org<mailto:edk2-devel-bounces@lists.01.org>] On Behalf Of
> Saqib Khan
> Sent: Wednesday, October 19, 2016 11:18 AM
> To: Andrew Fish <afish@apple.com<mailto:afish@apple.com>>
> Cc: edk2-devel@ml01.01.org<mailto:edk2-devel@ml01.01.org>; Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>>
> Subject: Re: [edk2] How to get BBS_BBS_DEVICE_PATH from BOOTXXXX
> variable
>
> Hi, so i have parsed the structure as follow :
>
>
>
>
>
>
>
> *BootVariable = mGetVariable(Name, &gEfiGlobalVariableGuid,
> &BootVariableSize, NULL);LDAttr = BootVariable[0];FilePathListLength =
> (BootVariable+2);Ptr = (BootVariable+3); // for description *
>
>
>
> *for FilePathList[] Ptr+= StrSize((CHAR16 *)Ptr);         CopyMem (Ptr,
> DevicePath, *FilePathListLength);*
>
>
> I am able to get LDAttr ,FilePathListLength and description but
>
> when i try to check DevicePath->Type it always return 0x01( Hardware Device
> Path) for each device .
>
>
> I have hard drive with legacy OS(BBS_DEVICE_PATH). So it should not always
> return 0x01 .What i am doing wrong in parsing ?
>
> Thanks
>
> On Tue, Oct 18, 2016 at 11:37 PM, Andrew Fish <afish@apple.com<mailto:afish@apple.com>> wrote:
>
> >
> > > On Oct 18, 2016, at 10:40 AM, Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>> wrote:
> > >
> > > On 10/18/16 19:18, Saqib Khan wrote:
> > >> Hi,
> > >> I can not find structure of BOOTXXXX variable, Can any one help how to
> > >> extract device path from BOOTXXXX variable.
> > >
> > > In UEFI-related questions, the UEFI specification has a fair chance to
> > > provide useful information.
> > >
> > > Please refer to "3.1.3 Load Options" in the UEFI v2.6 spec, available
> > > for download from <http://www.uefi.org/specifications>.
> > >
> > > Locating the matching structure definition in the edk2 tree is left as
> > > an exercise to the reader ;)
> > >
> >
> > Chapter 3 Boot Manager.
> >
> > The structure is EFI_LOAD_OPTION but some of the fields are variable
> > length so you have to do math to figure stuff out.
> >
> > Thanks,
> >
> > Andrew Fish
> >
> >
> > > Laszlo
> > >
> > > _______________________________________________
> > > edk2-devel mailing list
> > > edk2-devel@lists.01.org<mailto:edk2-devel@lists.01.org>
> > > https://lists.01.org/mailman/listinfo/edk2-devel
> >
> >
>
>
> --
> Regards
> Saqib Ahmed Khanzada
> _______________________________________________
> edk2-devel mailing list
> edk2-devel@lists.01.org<mailto:edk2-devel@lists.01.org>
> https://lists.01.org/mailman/listinfo/edk2-devel



--
Regards
Saqib Ahmed Khanzada