Re: Linker scripts use of "-z common-page-size=0x20" etc.

["Marvin Häuser" <mhaeuser@posteo.de>]

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> On 4. Apr 2023, at 16:03, Ard Biesheuvel <ardb@kernel.org> wrote:
> 
> On Tue, 4 Apr 2023 at 13:10, Rebecca Cran <rebecca@bsdio.com <mailto:rebecca@bsdio.com>> wrote:
>> 
>> On 4/4/23 1:22 AM, Ard Biesheuvel wrote:
>>> On Mon, 3 Apr 2023 at 22:33, Rebecca Cran <rebecca@bsdio.com <mailto:rebecca@bsdio.com>> wrote:
>>>> As part of my work on the toolchain definitions, I've come across a
>>>> situation where ld.lld fails to align sections correctly, due to it
>>>> being invoked via clang with the '-n' option, which causes GenFw to fail
>>>> with "Section address not aligned to its own alignment.".
>>>> 
>>> Stupid question: if it breaks stuff, why do you use -n ?
>> 
>> As far as I can see, clang adds it automatically when it invokes ld.lld.
>> 
> 
> Ah right, fair enough.
> 
>>>> The following messages are printed:
>>>> 
>>>> 
>>>> ld.lld: warning: -z common-page-size set, but paging disabled by omagic
>>>> or nmagic ld.lld: warning: address (0x558) of section .data is not a
>>>> multiple of alignment (16)
>>>> 
>>>> I tracked the problem down to GccBase.lds and ClangBase.lds, which have:
>>>> 
>>>> /* * The alignment of the .data section should be less than or equal to
>>>> the * alignment of the .text section. This ensures that the relative
>>>> offset * between these sections is the same in the ELF and the PE/COFF
>>>> versions of * this binary. */ .data ALIGN(ALIGNOF(.text)) :
>>>> ALIGN(CONSTANT(COMMONPAGESIZE)) { *(.data .data.* .gnu.linkonce.d.*)
>>>> *(.bss .bss.*) }
>>>> 
>>>> I can work around the problem by removing "ALIGN(ALIGNOF(.text)", but
>>>> I'm wondering if our use of COMMONPAGESIZE/MAXPAGESIZE is correct.
>>>> 
>>> What do you mean by 'correct'? The intent is clearly to declare the
>>> mapping granule size, and for SEC/PEI binaries that execute in place
>>> from flash, the MMU page size is not the most useful quantum here.
>> 
>> On Discord, I think Marvin was saying that we shouldn't be using
>> COMMONPAGESIZE, but MAXPAGESIZE.
>> 
> 
> I don't have a preference for one over the other, although I fail to
> see how this fixes the -n problem above.
> 
>> Since .text is aligned to COMMONPAGESIZE, I think I can remove the
>> ALIGN(ALIGNOF(.text)) and keep the ALIGN(CONSTANT(COMMONPAGESIZE))?
>> 
> 
> No. The alignment of .text could exceed COMMONPAGESIZE; this happens
> e.g., with AArch64 vector tables which are 2k aligned, when they are
> emitted into a SEC or PEIM module which only has 32 byte alignment by
> default.
> 
> In this case, the PE/COFF section alignment will be 2k, and so .data
> should start at a 2k aligned address as well.
> 
>> 
>>> 'Page size' is highly context specific, and toolchain people are
>>> (imho) usually quite quick to call something abuse if it does not
>>> match their narrow definition of how a compiler or linker should be
>>> used. For the same reason, it has been so difficult to get a compiler
>>> to understand that the desire for position independent code does not
>>> imply that we want GOTs, or care about ELF symbol preemption or
>>> copy-on-write footprint of relocatable segments. In general, the bare
>>> metal use case (which includes EFI) is quite poorly understood by many
>>> people working on toolchains.
>>> 
>>>> I'm wondering what the correct approach is here: should we do something
>>>> similar to how we set PECOFF_HEADER_SIZE and define a SECTION_ALIGNMENT
>>>> symbol?
>>> We cannot, that is the reason for using the page size switches here:
>>> using a symbol to set the location pointer is fine, but using a symbol
>>> to set the alignment of a section is not.
>>> 
>>>> Or, as discussed on Discord should we just use
>>>> CONSTANT(MAXPAGESIZE) and ignore how it's normally used to specify the
>>>> maximum allowable page size for an executable?
>>>> 
>>> Note that (when I last checked), the only effect of setting -z
>>> xxx-page-size is that those macros assume the associated value in the
>>> linker script. Nothing else in the linker changes behavior (with the
>>> exception of the warning you are seeing)
>>> 
>>> So claiming abuse because the provided value does not match the page
>>> size of an OS that might also run on the same system is strenuous, and
>>> I think our use of it is fine. AIUI, the reason for having
>>> ClangBase.lds in addition to GccBase.lds is the fact that LLD does not
>>> support common but only max page size, so I think it should be fine to
>>> merge the two, and use max-page-size everywhere.
>> 
>> Thanks for the explanation. This is probably an error on my part, but
>> when I tried to use max-page-size everywhere, the AARCH64 build of
>> ArmVirtQemu failed, with GenFw saying "AARCH64 small code model requires
>> identical ELF and PE/COFF section offsets modulo 4 KB."
>> 
> 
> Let me know if you manage to reproduce that - I'd like to understand
> what exactly is happening there.
> 
>> LLD added support for common-page-size in 2019
>> (https://reviews.llvm.org/D56205), so I think we can keep the current
>> usage of common and max page size and just combine GccBase.lds and
>> ClangBase.lds.
>> 
> 
> Yes, that would be preferable.

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