Re: [edk2-devel] [PATCH 4/5] StandaloneMmPkg: generate position independent code for StMM core

["Ard Biesheuvel" <ard.biesheuvel@arm.com>]

On 6/10/20 8:21 PM, Sean Brogan wrote:
> Ard,
> 
> I see you are only doing this for GCC?
> 
> Is it not needed for VS or clang? Are these toolchains not supported for 
> StandaloneMmPkg?
> 

This works with Clang as well as GCC (Clang is part of the GCC toolchain 
family in EDK2)

None of the AArch64 platforms we have in EDK2 or edk2-platforms can be 
built with VS today: the problem is that the asm dialect is different, 
and we don't have .asm versions of the startup code that can be built 
with VS. I think some of the common pieces have been ported by Pete 
Batard (so you can build individual drivers for AArch64 using VS), but 
nothing else.

> Not trying to hold up your work and don't expect you to enable every 
> toolchain but also don't like the idea of only building out support for 
> a single toolchain given all the work that has gone into making modules 
> compatible with numerous toolchains.
> 
> thoughts?
> 

Leif and I tasked someone with adding this VS support when we were both 
at Linaro, but this never materialized, unfortunately. I don't have a 
Windows machine, and no bandwidth to educate myself on the use of Visual 
Studio (last version I used was 6 :-)), so I am going to leave this up 
to someone else.

That said, this alternative approach (suggested by Jiewen) was adopted 
to rely less on ELF particulars or GCC/binutils. I do think that 
building this code needs the attention of someone who understanfds the 
VS toolchain really well, though, since we need to build a standalone 
PE/COFF executable that is structured as a shared library (i.e., with no 
relocation records pointing into the executable segment), otherwise 
there is no way we can dispatch those running at S-EL0 under the Secure 
Partition Manager.

Do you have any candidates that could help me figure out the build 
options we need for VS here? Beyond that, I don't see any GCC 
dependencies in StandaloneMmPkg at all.



> 
> On 6/10/2020 1:17 AM, Ard Biesheuvel wrote:
>> The standalone MM core runs in a restricted environment that is set
>> up by a higher privilege level, and which may not allow memory regions
>> to be writable and executable at the same time.
>>
>> This means that making the StMM core self-relocatable requires that
>> all the targets of the relocation fixups are outside of the executable
>> region of the image, given that we cannot remap the executable code
>> writable from the executable code itself without losing those execute
>> permissions.
>>
>> So instead, use the existing toolchain support to ensure that position
>> independent code is used where possible, and that all the remaining
>> relocated quantities are emitted into the data section. (Note that
>> staticallly initialized const pointers will be emitted into the
>> .data.rel.ro section, which gets pulled into the .data section by
>> our linker script)
>>
>> To ensure that we don't pick up any absolute references in executable
>> code inadvertently (e.g., in assembler code), add the '-z text' linker
>> option which will force the build to fail in this case.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
>> ---
>>   
>> StandaloneMmPkg/Core/StandaloneMmCore.inf                                         
>> | 4 ++++
>>   
>> StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/StandaloneMmCoreEntryPoint.inf 
>> | 3 +++
>>   2 files changed, 7 insertions(+)
>>
>> diff --git a/StandaloneMmPkg/Core/StandaloneMmCore.inf 
>> b/StandaloneMmPkg/Core/StandaloneMmCore.inf
>> index d17ff9965bdc..87bf6e9440a7 100644
>> --- a/StandaloneMmPkg/Core/StandaloneMmCore.inf
>> +++ b/StandaloneMmPkg/Core/StandaloneMmCore.inf
>> @@ -75,3 +75,7 @@ [Guids]
>>     gEfiEventLegacyBootGuid
>>
>>     gEfiEventExitBootServicesGuid
>>
>>     gEfiEventReadyToBootGuid
>>
>> +
>>
>> +[BuildOptions]
>>
>> +  GCC:*_*_*_CC_FLAGS = -fpie
>>
>> +  GCC:*_*_*_DLINK_FLAGS = -Wl,-z,text,-Bsymbolic,-pie
>>
>> diff --git 
>> a/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/StandaloneMmCoreEntryPoint.inf 
>> b/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/StandaloneMmCoreEntryPoint.inf 
>>
>> index 891c292e92f8..7d6ee4e08ecb 100644
>> --- 
>> a/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/StandaloneMmCoreEntryPoint.inf 
>>
>> +++ 
>> b/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/StandaloneMmCoreEntryPoint.inf 
>>
>> @@ -48,3 +48,6 @@ [Guids]
>>     gEfiMmPeiMmramMemoryReserveGuid
>>
>>     gEfiStandaloneMmNonSecureBufferGuid
>>
>>     gEfiArmTfCpuDriverEpDescriptorGuid
>>
>> +
>>
>> +[BuildOptions]
>>
>> +  GCC:*_*_*_CC_FLAGS = -fpie
>>