[llvm-dev] lld-link /ALIGN option doesn't work for the "execute in place (XIP)"

[Rui Ueyama via llvm-dev]

I filed this bug as https://reviews.llvm.org/D65736 and created a patch so
that you can try to see if that could fix the problem.

On Mon, Aug 5, 2019 at 12:52 PM Rui Ueyama <ruiu at google.com> wrote:

> Hi Steven,
>
> I need to take a look in more detail, but I believe that magic number is
> hard-coded without any particular reason and we can change that to match
> the behavior of MSVC linker.
>
> On Sat, Aug 3, 2019 at 11:14 PM Shi, Steven <steven.shi at intel.com> wrote:
>
>> Hi Rui,
>>
>> We meet a problem when enable the *execute in place* (XIP,
>> https://en.wikipedia.org/wiki/Execute_in_place) for uefi firmware with
>> lld-link. We need to set the COFF executable file SectionAlignment through
>> the lld-link /ALIGN option, but we find the SectionAlignment is hardcoded
>> to 4096 which cause the /ALIGN option doesn’t work at all. Below is the
>> hardcode in lld:
>>
>>
>>
>> lld\COFF\Writer.h
>>
>>   static const int pageSize = 4096;
>>
>> lld\COFF\Writer.cpp
>>
>>   pe->SectionAlignment = pageSize;
>>
>>
>>
>> The uefi firmware need to directly run in ROM device (in XIP way) before
>> the system memory is ready. These XIP executable files are not loaded in
>> memory and their section running alignment is just same as the alignment in
>> file. With MSVC link.exe, we can use the /ALIGN option to set
>> SectionAlignment same as FileAlignment to ensure the XIP executable file
>> has correct running alignment. But with lld-link, we cannot change the XIP
>> file SectionAlignment value 4096 because it is hardcoded. So, we have to
>> change the FileAlignment to 4096 to match the SectionAlignment instead, but
>> the big FileAlignment value (4096) cause the firmware XIP code size is +10
>> times larger than normal, which is not affordable for firmware.
>>
>>
>>
>> Could we not hardcode the SectionAlignment as 4096 in COFF file and let
>> the lld-link respect the user’s /ALIGN option value? We can set the 4096 as
>> the default SectionAlignment value.
>>
>>
>>
>>
>>
>> Below are some steps to show the hardcode issue and code size impact:
>>
>> $ cat main.c
>>
>> int main()
>>
>> {
>>
>>   return 0;
>>
>> }
>>
>> $ "/home/jshi19/llvm/llvm-project/releaseinstall/bin/clang" -target
>> i686-unknown-windows -fno-builtin -c -o main.obj  main.c
>>
>> $ "/home/jshi19/llvm/llvm-project/releaseinstall/bin/lld-link"
>> /OUT:main.dll /MACHINE:X86 /DLL /ENTRY:main /ALIGN:32 /FILEALIGN:32 main.obj
>>
>> $ sudo apt install pev
>>
>> $ readpe -H main.dll
>>
>> …
>>
>>     Alignment of sections:        0x1000
>>
>> Alignment factor:                0x20
>>
>> …
>>
>> $ ll main.dll, see main.dll size is 544
>>
>> -rwxrwxr-x 1 jshi19 jshi19 544 Aug  3 21:58 main.dll*
>>
>>
>>
>> If have to change the FileAlignment to 4096 to match the SectionAlignment:
>>
>> $ "/home/jshi19/llvm/llvm-project/releaseinstall/bin/lld-link"
>> /OUT:main.dll /MACHINE:X86 /DLL /ENTRY:main /ALIGN:32 /FILEALIGN:4096
>> main.obj
>>
>> $ readpe -H main.dll
>>
>> …
>>
>>     Alignment of sections:        0x1000
>>
>> Alignment factor:                0x1000
>>
>> …
>>
>> $ ll main.dll, see main.dll size is 8192 which ~15 timer larger.
>>
>> -rwxrwxr-x 1 jshi19 jshi19 8192 Aug  3 22:05 main.dll*
>>
>>
>>
>>
>>
>>
>>
>> Thanks
>>
>> Steven
>>
>>
>>
>
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