Re: [PATCH 7/8] x86/EFI: keep debug info in xen.efi

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.04.2021 17:30, Roger Pau Monné wrote:
> On Wed, Apr 21, 2021 at 03:06:36PM +0200, Jan Beulich wrote:
>> On 21.04.2021 13:15, Roger Pau Monné wrote:
>>> On Thu, Apr 01, 2021 at 11:47:03AM +0200, Jan Beulich wrote:
>>>> --- a/xen/arch/x86/xen.lds.S
>>>> +++ b/xen/arch/x86/xen.lds.S
>>>> @@ -312,10 +312,60 @@ SECTIONS
>>>>      *(.reloc)
>>>>      __base_relocs_end = .;
>>>>    }
>>>> -  /* Trick the linker into setting the image size to exactly 16Mb. */
>>>> -  . = ALIGN(__section_alignment__);
>>>> -  DECL_SECTION(.pad) {
>>>> -    . = ALIGN(MB(16));
>>>> +  .debug_abbrev ALIGN(1) (NOLOAD) : {
>>>> +     *(.debug_abbrev)
>>>> +  }
>>>> +  .debug_info ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_info)
>>>> +    *(.gnu.linkonce.wi.*)
>>>> +  }
>>>> +  .debug_types ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_types)
>>>> +  }
>>>> +  .debug_str ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_str)
>>>> +  }
>>>> +  .debug_line ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_line)
>>>> +    *(.debug_line.*)
>>>> +  }
>>>> +  .debug_line_str ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_line_str)
>>>> +  }
>>>> +  .debug_names ALIGN(4) (NOLOAD) : {
>>>> +    *(.debug_names)
>>>> +  }
>>>> +  .debug_frame ALIGN(4) (NOLOAD) : {
>>>> +    *(.debug_frame)
>>>> +  }
>>>> +  .debug_loc ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_loc)
>>>> +  }
>>>> +  .debug_loclists ALIGN(4) (NOLOAD) : {
>>>> +    *(.debug_loclists)
>>>> +  }
>>>> +  .debug_ranges ALIGN(8) (NOLOAD) : {
>>>> +    *(.debug_ranges)
>>>> +  }
>>>> +  .debug_rnglists ALIGN(4) (NOLOAD) : {
>>>> +    *(.debug_rnglists)
>>>> +  }
>>>> +  .debug_addr ALIGN(8) (NOLOAD) : {
>>>> +    *(.debug_addr)
>>>> +  }
>>>> +  .debug_aranges ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_aranges)
>>>> +  }
>>>> +  .debug_pubnames ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_pubnames)
>>>> +  }
>>>> +  .debug_pubtypes ALIGN(1) (NOLOAD) : {
>>>> +    *(.debug_pubtypes)
>>>> +  }
>>>> +  /* Trick the linker into setting the image size to no less than 16Mb. */
>>>> +  __image_end__ = .;
>>>> +  .pad ALIGN(__section_alignment__) : {
>>>> +    . = __image_end__ < __image_base__ + MB(16) ? ALIGN(MB(16)) : .;
>>>
>>> I think this is inside an ifdef EFI region, since this is DWARF info
>>> couldn't it also be present when building with EFI disabled?
>>
>> Of course (and it's not just "could" but "will"), yet the linker will
>> do fine (and perhaps even better) without when building ELF. Also
>> note that we'll be responsible for keeping the list of sections up-to-
>> date. The linker will recognize Dwarf sections by looking for a
>> .debug_ prefix. We can't use such here (or at least I'm not aware of
>> a suitable mechanism); .debug_* would mean munging together all the
>> different kinds of Dwarf sections. Hence by limiting the explicit
>> enumeration to PE, I'm trying to avoid anomalies in ELF down the road.
> 
> Right, so we will have to keep this list of debug_ sections updated
> manually if/when more of those appear as part of DWARF updates?

Yes.

> Do we have a way to get some kind of warning or error when a new
> section not explicitly handled here appears?

ld 2.37 will start warning about such sections, as they'd land at
VA 0 and hence below image base.

> Instead of adding DWARF debug info to the xen.efi binary, is there a
> way to translate the DWARF from the ELF build into the native debug
> format for PE/COFF?

I'm unaware of a single, "native" debug format (Dwarf also isn't
specifically tied to ELF). I further don't think all of the Dwarf
bits could reasonably be expressed in another debug format.

Jan