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

From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH 7/8] x86/EFI: keep debug info in xen.efi
Date: Thu, 22 Apr 2021 13:03:13 +0200	[thread overview]
Message-ID: <a00cfe95-06bd-4999-05ab-cae737ab6f4c@suse.com> (raw)
In-Reply-To: <YIEwW3HQuTtgWH5M@Air-de-Roger>

On 22.04.2021 10:14, Roger Pau Monné wrote:
> On Wed, Apr 21, 2021 at 05:38:42PM +0200, Jan Beulich wrote:
>> 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.
> 
> That seems like a bug in ld?
> 
> The '--image-base' option description mentions: "This is the lowest
> memory location that will be used when your program or dll is
> loaded.", so I would expect that if the option is used the default VA
> should be >= image-base, or else the description of the option is not
> consistent, as ld will still place sections at addresses below
> image-base.

ld's "general" logic is pretty ELF-centric. Hence debugging sections
get placed at VA 0 by default, not matter what the (PE-specific)
--image-base says. Whether that's a bug though I'm not sure: There
are no really good alternatives that could be used by default. Doing
what we do here (and what e.g. Cygwin does) via linker script may not
be appropriate in the common case. In particular it is not generally
correct for debug info to be part of what gets loaded into memory.
Microsoft's CodeView / PDB debug info gets represented differently,
for example, such that it can be part of the file, but wouldn't get
loaded. But that's a vendor's choice, again not something that's
necessarily generalizable.

Jan