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

["Ard Biesheuvel" <ardb@kernel.org>]

On Tue, 4 Apr 2023 at 13:10, Rebecca Cran <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> 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.