[Regression] [PATCH] x86: fold sections in final binaries

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 01/03/2022 08:55, Jan Beulich wrote:
> Especially when linking a PE binary (xen.efi), standalone output
> sections are expensive: Often the linker will align the subsequent one
> on the section alignment boundary (2Mb) when the linker script doesn't
> otherwise place it. (I haven't been able to derive from observed
> behavior under what conditions it would not do so.)
>
> With gcov enabled (and with gcc11) I'm observing enough sections that,
> as of quite recently, the resulting image doesn't fit in 16Mb anymore,
> failing the final ASSERT() in the linker script. (That assertion is
> slated to go away, but that's a separate change.)
>
> Any destructor related sections can be discarded, as we never "exit"
> the hypervisor. This includes .text.exit, which is referenced from
> .dtors.*. Constructor related sections need to all be taken care of, not
> just those with historically used names: .ctors.* and .text.startup is
> what gcc11 populates. While there re-arrange ordering / sorting to match
> that used by the linker provided scripts.
>
> Finally, for xen.efi only, also discard .note.gnu.*. These are
> meaningless in a PE binary. Quite likely, while not meaningless there,
> the section is also of no use in ELF, but keep it there for now.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> TBD: We also use CONSTRUCTORS for an unknown reason. Documentation for
>      ld is quite clear that this is an a.out-only construct.
>      Implementation doesn't look to fully match this for ELF, but I'd
>      nevertheless be inclined to remove its use.
>
> --- a/xen/arch/x86/xen.lds.S
> +++ b/xen/arch/x86/xen.lds.S
> @@ -194,6 +194,7 @@ SECTIONS
>  #endif
>         _sinittext = .;
>         *(.init.text)
> +       *(.text.startup)
>         _einittext = .;
>         /*
>          * Here are the replacement instructions. The linker sticks them
> @@ -258,9 +259,10 @@ SECTIONS
>  
>         . = ALIGN(8);
>         __ctors_start = .;
> -       *(.ctors)
> +       *(SORT_BY_INIT_PRIORITY(.init_array.*))
> +       *(SORT_BY_INIT_PRIORITY(.ctors.*))
>         *(.init_array)
> -       *(SORT(.init_array.*))
> +       *(.ctors)
>         __ctors_end = .;
>    } PHDR(text)
>  
> @@ -404,16 +406,20 @@ SECTIONS
>  
>    /* Sections to be discarded */
>    /DISCARD/ : {
> +       *(.text.exit)
>         *(.exit.text)
>         *(.exit.data)
>         *(.exitcall.exit)
>         *(.discard)
>         *(.discard.*)
>         *(.eh_frame)
> +       *(.dtors)
> +       *(.dtors.*)
>  #ifdef EFI
>         *(.comment)
>         *(.comment.*)
>         *(.note.Xen)
> +       *(.note.gnu.*)
>  #endif
>    }

This breaks reliably in Gitlab CI.

https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/2159059956 (gcc 11)

~Andrew