Multiboot2 on aarch64: Alignment of ELF Headers

Multiboot2 on aarch64: Alignment of ELF Headers

[Chris Plant]

Hi,

I'm continuing to work on Multiboot2 support on aarch64, and I'm
looking at the alignment of the ELF headers which are passed through
MB2.

The alignment of the MB2 header's isn't the problem, the problem is in
the ELF headers which are linked through.  Some of the 8 byte wide
fields (specifically addr and size) in the ELF headers are not aligned
on 8 byte boundaries, and instead can aligned on 4-byte boundaries.

My question is, is it preferable to state that it is possible that the
headers may be aligned on 4 byte boundaries (and manage that through
reading two 4-byte chunks), or to change grub code to ensure that the
ELF headers are aligned on 8 byte boundaries and state that 8 byte
alignment is always the case?

I think logically that it should be 8 byte aligned so that's my
preference, but I thought I'd ask.

Thanks,

Chris

Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Hans Ulrich Niedermann]

On Fri, 22 May 2020 17:23:35 +0100
Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:

> I'm continuing to work on Multiboot2 support on aarch64, and I'm
> looking at the alignment of the ELF headers which are passed through
> MB2.

At the risk of me sounding stupid...

Having read the MB2 specs quite thoroughly in the past few months, I
still have no idea what "ELF headers" being "passed through MB2" could
be about. The MB2 spec defines a MB2 header consisting of a four MB2
header magic fields and a set of MB2 header tags.

The only thing ELF related I could find there is the MB2 header address
tag which duplicates some information from ELF (if OS image is in ELF
format) or contains address information (if OS image is in non-ELF
format). Is that what you are talking about?

> The alignment of the MB2 header's isn't the problem, the problem is in
> the ELF headers which are linked through.  Some of the 8 byte wide
> fields (specifically addr and size) in the ELF headers are not aligned
> on 8 byte boundaries, and instead can aligned on 4-byte boundaries.
> 
> My question is, is it preferable to state that it is possible that the
> headers may be aligned on 4 byte boundaries (and manage that through
> reading two 4-byte chunks), or to change grub code to ensure that the
> ELF headers are aligned on 8 byte boundaries and state that 8 byte
> alignment is always the case?

As it is, the largest values in the MB2 header defined in the OS image
are u32, there are no u64 values. Sorry if I am asking stupid questions,
but where would u64 values come into play here in the MB2 spec? Are you
redefining some MB2 header tags with u64 values instead of u32 values?

> I think logically that it should be 8 byte aligned so that's my
> preference, but I thought I'd ask.

If you happen to be speaking about MB2 Header Tags when you say "linked
through ELF headers" and you have been modeling your boot code off the
example boot.S and boot_mips.S and run into alignment issues there, be
advised that the current example boot code does violate the MB2 spec:

The MB2 spec and the grub2 code both agree on the MB2 header tags being
aligned to 8 byte boundaries, while the example code (both for ia32 and
for mips) does not align the MB2 header tags at all.

Uli

Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Chris Plant]

On Sat, 2020-05-23 at 12:43 +0200, Hans Ulrich Niedermann wrote:
> On Fri, 22 May 2020 17:23:35 +0100
> Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> 
> > I'm continuing to work on Multiboot2 support on aarch64, and I'm
> > looking at the alignment of the ELF headers which are passed
> > through
> > MB2.
> 
> At the risk of me sounding stupid...
> 
> Having read the MB2 specs quite thoroughly in the past few months, I
> still have no idea what "ELF headers" being "passed through MB2"
> could
> be about. The MB2 spec defines a MB2 header consisting of a four MB2
> header magic fields and a set of MB2 header tags.
> 
> The only thing ELF related I could find there is the MB2 header
> address
> tag which duplicates some information from ELF (if OS image is in ELF
> format) or contains address information (if OS image is in non-ELF
> format). Is that what you are talking about?

Yes, this is what I'm talking about.  I'm using the MB2 tag (type 9) to
see where the ELF section headers are.

Apologies, I've not been clear enough and rambled too much.  The issue
I see is nothing to do with grub's MB2 implementation, but instead in
the way grub is loading the ELF section headers.

The ELF section headers appear to be loaded on a 4 byte alignment not
an 8 byte alignment.  You can read the 4 byte members of the ELF header
structure fine, but not the 8 byte members as they are aligned to 4
bytes.

So, my question is, should grub load the ELF headers to an 8 byte
alignment, or should the OS be prepared for them to be aligned to 4
byte?

> 
> > The alignment of the MB2 header's isn't the problem, the problem is
> > in
> > the ELF headers which are linked through.  Some of the 8 byte wide
> > fields (specifically addr and size) in the ELF headers are not
> > aligned
> > on 8 byte boundaries, and instead can aligned on 4-byte boundaries.
> > 
> > My question is, is it preferable to state that it is possible that
> > the
> > headers may be aligned on 4 byte boundaries (and manage that
> > through
> > reading two 4-byte chunks), or to change grub code to ensure that
> > the
> > ELF headers are aligned on 8 byte boundaries and state that 8 byte
> > alignment is always the case?
> 
> As it is, the largest values in the MB2 header defined in the OS
> image
> are u32, there are no u64 values. Sorry if I am asking stupid
> questions,
> but where would u64 values come into play here in the MB2 spec? Are
> you
> redefining some MB2 header tags with u64 values instead of u32
> values?
> 
> > I think logically that it should be 8 byte aligned so that's my
> > preference, but I thought I'd ask.
> 
> If you happen to be speaking about MB2 Header Tags when you say
> "linked
> through ELF headers" and you have been modeling your boot code off
> the
> example boot.S and boot_mips.S and run into alignment issues there,
> be
> advised that the current example boot code does violate the MB2 spec:
> 
> The MB2 spec and the grub2 code both agree on the MB2 header tags
> being
> aligned to 8 byte boundaries, while the example code (both for ia32
> and
> for mips) does not align the MB2 header tags at all.

> 
> Uli
> 
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel

Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Hans Ulrich Niedermann]

On Sat, 23 May 2020 12:21:27 +0100
Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:

> On Sat, 2020-05-23 at 12:43 +0200, Hans Ulrich Niedermann wrote:
> > On Fri, 22 May 2020 17:23:35 +0100
> > Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> >   
> > > I'm continuing to work on Multiboot2 support on aarch64, and I'm
> > > looking at the alignment of the ELF headers which are passed
> > > through
> > > MB2.  
> > 
> > At the risk of me sounding stupid...
> > 
> > Having read the MB2 specs quite thoroughly in the past few months, I
> > still have no idea what "ELF headers" being "passed through MB2"
> > could
> > be about. The MB2 spec defines a MB2 header consisting of a four MB2
> > header magic fields and a set of MB2 header tags.
> > 
> > The only thing ELF related I could find there is the MB2 header
> > address
> > tag which duplicates some information from ELF (if OS image is in
> > ELF format) or contains address information (if OS image is in
> > non-ELF format). Is that what you are talking about?  
> 
> Yes, this is what I'm talking about.  I'm using the MB2 tag (type 9)
> to see where the ELF section headers are.

Ah. So the type 9 you are referring to is the "ELF-Symbols" boot
information tag (i.e. information the bootloader communicates to the OS
image), not an MB2 header tag (information the OS image communicates to
the bootloader).

I am catching up with you, finally.

> Apologies, I've not been clear enough and rambled too much.  The issue
> I see is nothing to do with grub's MB2 implementation, but instead in
> the way grub is loading the ELF section headers.
> 
> The ELF section headers appear to be loaded on a 4 byte alignment not
> an 8 byte alignment.  You can read the 4 byte members of the ELF
> header structure fine, but not the 8 byte members as they are aligned
> to 4 bytes.
> 
> So, my question is, should grub load the ELF headers to an 8 byte
> alignment, or should the OS be prepared for them to be aligned to 4
> byte?

The 64bit part of the ELF spec appears to specifically made such that
64bit values are aligned to 64bit boundaries (I have taken a look at
https://en.wikipedia.org/wiki/Executable_and_Linkable_Format for an
overview).

Therefore, loading an ELF64 image with its many 64bit values to an
address not aligned to 8 byte boundaries sounds to me like a probable
oversight going back to ELF32 days and thus a bug for ELF64, unless
someone has a very good argument to the contrary.

Uli

Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Chris Plant]

On Sat, 2020-05-23 at 14:33 +0200, Hans Ulrich Niedermann wrote:
> On Sat, 23 May 2020 12:21:27 +0100
> Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> 
> > On Sat, 2020-05-23 at 12:43 +0200, Hans Ulrich Niedermann wrote:
> > > On Fri, 22 May 2020 17:23:35 +0100
> > > Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> > >   
> > > > I'm continuing to work on Multiboot2 support on aarch64, and
> > > > I'm
> > > > looking at the alignment of the ELF headers which are passed
> > > > through
> > > > MB2.  
> > > 
> > > At the risk of me sounding stupid...
> > > 
> > > Having read the MB2 specs quite thoroughly in the past few
> > > months, I
> > > still have no idea what "ELF headers" being "passed through MB2"
> > > could
> > > be about. The MB2 spec defines a MB2 header consisting of a four
> > > MB2
> > > header magic fields and a set of MB2 header tags.
> > > 
> > > The only thing ELF related I could find there is the MB2 header
> > > address
> > > tag which duplicates some information from ELF (if OS image is in
> > > ELF format) or contains address information (if OS image is in
> > > non-ELF format). Is that what you are talking about?  
> > 
> > Yes, this is what I'm talking about.  I'm using the MB2 tag (type
> > 9)
> > to see where the ELF section headers are.
> 
> Ah. So the type 9 you are referring to is the "ELF-Symbols" boot
> information tag (i.e. information the bootloader communicates to the
> OS
> image), not an MB2 header tag (information the OS image communicates
> to
> the bootloader).
> 
> I am catching up with you, finally.
> 
> > Apologies, I've not been clear enough and rambled too much.  The
> > issue
> > I see is nothing to do with grub's MB2 implementation, but instead
> > in
> > the way grub is loading the ELF section headers.
> > 
> > The ELF section headers appear to be loaded on a 4 byte alignment
> > not
> > an 8 byte alignment.  You can read the 4 byte members of the ELF
> > header structure fine, but not the 8 byte members as they are
> > aligned
> > to 4 bytes.
> > 
> > So, my question is, should grub load the ELF headers to an 8 byte
> > alignment, or should the OS be prepared for them to be aligned to 4
> > byte?
> 
> The 64bit part of the ELF spec appears to specifically made such that
> 64bit values are aligned to 64bit boundaries (I have taken a look at
> https://en.wikipedia.org/wiki/Executable_and_Linkable_Format for an
> overview).
> 
> Therefore, loading an ELF64 image with its many 64bit values to an
> address not aligned to 8 byte boundaries sounds to me like a probable
> oversight going back to ELF32 days and thus a bug for ELF64, unless
> someone has a very good argument to the contrary.

On this basis, is the logical change to fix the definition of the "ELF-
Symbols" boot information tag, which is currently:

struct multiboot_tag_elf_sections
{
  multiboot_uint32_t type;
  multiboot_uint32_t size;
  multiboot_uint32_t num;
  multiboot_uint32_t entsize;
  multiboot_uint32_t shndx;
  char sections[0];
};

grub aligns this to an 8 byte boundary, but then the odd number of
uint32_t mean that sections is on a 4 byte boundary and is designed
within it's ELF defintion to be on 8 byte boundary.

So I would propose adding a 4 byte pad between shndx and sections, so
that sections on an 8 byte boundary.  This might break things until
they're recompiled but won't require any changes to code elsewhere and
I can't see how else to fix it without a new information defintion.

I can include this in my patch (first one, taking time and will be
terrible) to enable multiboot2 on ARM64 if this is the approach
required/desired?

Chris

> 
> Uli
> 
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel

Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Krystian Hebel]

On 21.09.2020 21:14, Chris Plant wrote:
> On Sat, 2020-05-23 at 14:33 +0200, Hans Ulrich Niedermann wrote:
>> On Sat, 23 May 2020 12:21:27 +0100
>> Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
>>
>>> On Sat, 2020-05-23 at 12:43 +0200, Hans Ulrich Niedermann wrote:
>>>> On Fri, 22 May 2020 17:23:35 +0100
>>>> Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
>>>>    
>>>>> I'm continuing to work on Multiboot2 support on aarch64, and
>>>>> I'm
>>>>> looking at the alignment of the ELF headers which are passed
>>>>> through
>>>>> MB2.
>>>> At the risk of me sounding stupid...
>>>>
>>>> Having read the MB2 specs quite thoroughly in the past few
>>>> months, I
>>>> still have no idea what "ELF headers" being "passed through MB2"
>>>> could
>>>> be about. The MB2 spec defines a MB2 header consisting of a four
>>>> MB2
>>>> header magic fields and a set of MB2 header tags.
>>>>
>>>> The only thing ELF related I could find there is the MB2 header
>>>> address
>>>> tag which duplicates some information from ELF (if OS image is in
>>>> ELF format) or contains address information (if OS image is in
>>>> non-ELF format). Is that what you are talking about?
>>> Yes, this is what I'm talking about.  I'm using the MB2 tag (type
>>> 9)
>>> to see where the ELF section headers are.
>> Ah. So the type 9 you are referring to is the "ELF-Symbols" boot
>> information tag (i.e. information the bootloader communicates to the
>> OS
>> image), not an MB2 header tag (information the OS image communicates
>> to
>> the bootloader).
>>
>> I am catching up with you, finally.
>>
>>> Apologies, I've not been clear enough and rambled too much.  The
>>> issue
>>> I see is nothing to do with grub's MB2 implementation, but instead
>>> in
>>> the way grub is loading the ELF section headers.
>>>
>>> The ELF section headers appear to be loaded on a 4 byte alignment
>>> not
>>> an 8 byte alignment.  You can read the 4 byte members of the ELF
>>> header structure fine, but not the 8 byte members as they are
>>> aligned
>>> to 4 bytes.
>>>
>>> So, my question is, should grub load the ELF headers to an 8 byte
>>> alignment, or should the OS be prepared for them to be aligned to 4
>>> byte?
>> The 64bit part of the ELF spec appears to specifically made such that
>> 64bit values are aligned to 64bit boundaries (I have taken a look at
>> https://en.wikipedia.org/wiki/Executable_and_Linkable_Format for an
>> overview).
>>
>> Therefore, loading an ELF64 image with its many 64bit values to an
>> address not aligned to 8 byte boundaries sounds to me like a probable
>> oversight going back to ELF32 days and thus a bug for ELF64, unless
>> someone has a very good argument to the contrary.
> On this basis, is the logical change to fix the definition of the "ELF-
> Symbols" boot information tag, which is currently:
>
> struct multiboot_tag_elf_sections
> {
>    multiboot_uint32_t type;
>    multiboot_uint32_t size;
>    multiboot_uint32_t num;
>    multiboot_uint32_t entsize;
>    multiboot_uint32_t shndx;
>    char sections[0];
> };
>
> grub aligns this to an 8 byte boundary, but then the odd number of
> uint32_t mean that sections is on a 4 byte boundary and is designed
> within it's ELF defintion to be on 8 byte boundary.
>
> So I would propose adding a 4 byte pad between shndx and sections, so
> that sections on an 8 byte boundary.  This might break things until
> they're recompiled but won't require any changes to code elsewhere and
> I can't see how else to fix it without a new information defintion.
This WILL break things until they are recompiled in every MB2 kernel,
in addition to grub, every other MB2-capable bootloader and documentation.
Until then, the kernel would have to somewhat guess whether it was given
the old (not aligned) tag or the new one. While it may be possible as long
as the padding is filled with a value that cannot appear in the first bytes
of sections, the users would have to add the discovery code into kernels.
MB2 is one of the most commonly used (if not the most common) boot protocols
for hobby OSes.

IMHO the only way out of it would be to create a new tag type and leave the
old one as it is right now. This won't break current implementations, though
it will pass redundant information. After some transition period we may drop
the old format.
> I can include this in my patch (first one, taking time and will be
> terrible) to enable multiboot2 on ARM64 if this is the approach
> required/desired?
>
> Chris
>
>> Uli
>>
>> _______________________________________________
>> Grub-devel mailing list
>> Grub-devel@gnu.org
>> https://lists.gnu.org/mailman/listinfo/grub-devel
>
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel


-- 
Krystian Hebel
Firmware Engineer
https://3mdeb.com | @3mdeb_com

Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Chris Plant]

On Tue, 2020-09-22 at 08:33 +0200, Krystian Hebel wrote:
> On 21.09.2020 21:14, Chris Plant wrote:
> > On Sat, 2020-05-23 at 14:33 +0200, Hans Ulrich Niedermann wrote:
> > > On Sat, 23 May 2020 12:21:27 +0100
> > > Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> > > 
> > > > On Sat, 2020-05-23 at 12:43 +0200, Hans Ulrich Niedermann
> > > > wrote:
> > > > > On Fri, 22 May 2020 17:23:35 +0100
> > > > > Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> > > > >    
> > > > > > I'm continuing to work on Multiboot2 support on aarch64,
> > > > > > and
> > > > > > I'm
> > > > > > looking at the alignment of the ELF headers which are
> > > > > > passed
> > > > > > through
> > > > > > MB2.
> > > > > At the risk of me sounding stupid...
> > > > > 
> > > > > Having read the MB2 specs quite thoroughly in the past few
> > > > > months, I
> > > > > still have no idea what "ELF headers" being "passed through
> > > > > MB2"
> > > > > could
> > > > > be about. The MB2 spec defines a MB2 header consisting of a
> > > > > four
> > > > > MB2
> > > > > header magic fields and a set of MB2 header tags.
> > > > > 
> > > > > The only thing ELF related I could find there is the MB2
> > > > > header
> > > > > address
> > > > > tag which duplicates some information from ELF (if OS image
> > > > > is in
> > > > > ELF format) or contains address information (if OS image is
> > > > > in
> > > > > non-ELF format). Is that what you are talking about?
> > > > Yes, this is what I'm talking about.  I'm using the MB2 tag
> > > > (type
> > > > 9)
> > > > to see where the ELF section headers are.
> > > Ah. So the type 9 you are referring to is the "ELF-Symbols" boot
> > > information tag (i.e. information the bootloader communicates to
> > > the
> > > OS
> > > image), not an MB2 header tag (information the OS image
> > > communicates
> > > to
> > > the bootloader).
> > > 
> > > I am catching up with you, finally.
> > > 
> > > > Apologies, I've not been clear enough and rambled too
> > > > much.  The
> > > > issue
> > > > I see is nothing to do with grub's MB2 implementation, but
> > > > instead
> > > > in
> > > > the way grub is loading the ELF section headers.
> > > > 
> > > > The ELF section headers appear to be loaded on a 4 byte
> > > > alignment
> > > > not
> > > > an 8 byte alignment.  You can read the 4 byte members of the
> > > > ELF
> > > > header structure fine, but not the 8 byte members as they are
> > > > aligned
> > > > to 4 bytes.
> > > > 
> > > > So, my question is, should grub load the ELF headers to an 8
> > > > byte
> > > > alignment, or should the OS be prepared for them to be aligned
> > > > to 4
> > > > byte?
> > > The 64bit part of the ELF spec appears to specifically made such
> > > that
> > > 64bit values are aligned to 64bit boundaries (I have taken a look
> > > at
> > > https://en.wikipedia.org/wiki/Executable_and_Linkable_Format for
> > > an
> > > overview).
> > > 
> > > Therefore, loading an ELF64 image with its many 64bit values to
> > > an
> > > address not aligned to 8 byte boundaries sounds to me like a
> > > probable
> > > oversight going back to ELF32 days and thus a bug for ELF64,
> > > unless
> > > someone has a very good argument to the contrary.
> > On this basis, is the logical change to fix the definition of the
> > "ELF-
> > Symbols" boot information tag, which is currently:
> > 
> > struct multiboot_tag_elf_sections
> > {
> >    multiboot_uint32_t type;
> >    multiboot_uint32_t size;
> >    multiboot_uint32_t num;
> >    multiboot_uint32_t entsize;
> >    multiboot_uint32_t shndx;
> >    char sections[0];
> > };
> > 
> > grub aligns this to an 8 byte boundary, but then the odd number of
> > uint32_t mean that sections is on a 4 byte boundary and is designed
> > within it's ELF defintion to be on 8 byte boundary.
> > 
> > So I would propose adding a 4 byte pad between shndx and sections,
> > so
> > that sections on an 8 byte boundary.  This might break things until
> > they're recompiled but won't require any changes to code elsewhere
> > and
> > I can't see how else to fix it without a new information defintion.
> This WILL break things until they are recompiled in every MB2 kernel,
> in addition to grub, every other MB2-capable bootloader and
> documentation.
> Until then, the kernel would have to somewhat guess whether it was
> given
> the old (not aligned) tag or the new one. While it may be possible as
> long
> as the padding is filled with a value that cannot appear in the first
> bytes
> of sections, the users would have to add the discovery code into
> kernels.
> MB2 is one of the most commonly used (if not the most common) boot
> protocols
> for hobby OSes.

Understood.


> 
> IMHO the only way out of it would be to create a new tag type and
> leave the
> old one as it is right now. This won't break current implementations,
> though
> it will pass redundant information. After some transition period we
> may drop
> the old format.

I'll create a new tag, struct multiboot_tag_elf64_sections which is
correctly aligned.

> > I can include this in my patch (first one, taking time and will be
> > terrible) to enable multiboot2 on ARM64 if this is the approach
> > required/desired?
> > 
> > Chris
> > 
> > > Uli
> > > 
> > > _______________________________________________
> > > Grub-devel mailing list
> > > Grub-devel@gnu.org
> > > https://lists.gnu.org/mailman/listinfo/grub-devel
> > 
> > _______________________________________________
> > Grub-devel mailing list
> > Grub-devel@gnu.org
> > https://lists.gnu.org/mailman/listinfo/grub-devel
> 
> 

Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Chris Plant]

On Tue, 2020-09-22 at 07:53 +0100, Chris Plant wrote:
> On Tue, 2020-09-22 at 08:33 +0200, Krystian Hebel wrote:
> > On 21.09.2020 21:14, Chris Plant wrote:
> > > On Sat, 2020-05-23 at 14:33 +0200, Hans Ulrich Niedermann wrote:
> > > > On Sat, 23 May 2020 12:21:27 +0100
> > > > Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> > > > 
> > > > > On Sat, 2020-05-23 at 12:43 +0200, Hans Ulrich Niedermann
> > > > > wrote:
> > > > > > On Fri, 22 May 2020 17:23:35 +0100
> > > > > > Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> > > > > >    
> > > > > > > I'm continuing to work on Multiboot2 support on aarch64,
> > > > > > > and
> > > > > > > I'm
> > > > > > > looking at the alignment of the ELF headers which are
> > > > > > > passed
> > > > > > > through
> > > > > > > MB2.
> > > > > > At the risk of me sounding stupid...
> > > > > > 
> > > > > > Having read the MB2 specs quite thoroughly in the past few
> > > > > > months, I
> > > > > > still have no idea what "ELF headers" being "passed through
> > > > > > MB2"
> > > > > > could
> > > > > > be about. The MB2 spec defines a MB2 header consisting of a
> > > > > > four
> > > > > > MB2
> > > > > > header magic fields and a set of MB2 header tags.
> > > > > > 
> > > > > > The only thing ELF related I could find there is the MB2
> > > > > > header
> > > > > > address
> > > > > > tag which duplicates some information from ELF (if OS image
> > > > > > is in
> > > > > > ELF format) or contains address information (if OS image is
> > > > > > in
> > > > > > non-ELF format). Is that what you are talking about?
> > > > > Yes, this is what I'm talking about.  I'm using the MB2 tag
> > > > > (type
> > > > > 9)
> > > > > to see where the ELF section headers are.
> > > > Ah. So the type 9 you are referring to is the "ELF-Symbols"
> > > > boot
> > > > information tag (i.e. information the bootloader communicates
> > > > to
> > > > the
> > > > OS
> > > > image), not an MB2 header tag (information the OS image
> > > > communicates
> > > > to
> > > > the bootloader).
> > > > 
> > > > I am catching up with you, finally.
> > > > 
> > > > > Apologies, I've not been clear enough and rambled too
> > > > > much.  The
> > > > > issue
> > > > > I see is nothing to do with grub's MB2 implementation, but
> > > > > instead
> > > > > in
> > > > > the way grub is loading the ELF section headers.
> > > > > 
> > > > > The ELF section headers appear to be loaded on a 4 byte
> > > > > alignment
> > > > > not
> > > > > an 8 byte alignment.  You can read the 4 byte members of the
> > > > > ELF
> > > > > header structure fine, but not the 8 byte members as they are
> > > > > aligned
> > > > > to 4 bytes.
> > > > > 
> > > > > So, my question is, should grub load the ELF headers to an 8
> > > > > byte
> > > > > alignment, or should the OS be prepared for them to be
> > > > > aligned
> > > > > to 4
> > > > > byte?
> > > > The 64bit part of the ELF spec appears to specifically made
> > > > such
> > > > that
> > > > 64bit values are aligned to 64bit boundaries (I have taken a
> > > > look
> > > > at
> > > > https://en.wikipedia.org/wiki/Executable_and_Linkable_Format
> > > > for
> > > > an
> > > > overview).
> > > > 
> > > > Therefore, loading an ELF64 image with its many 64bit values to
> > > > an
> > > > address not aligned to 8 byte boundaries sounds to me like a
> > > > probable
> > > > oversight going back to ELF32 days and thus a bug for ELF64,
> > > > unless
> > > > someone has a very good argument to the contrary.
> > > On this basis, is the logical change to fix the definition of the
> > > "ELF-
> > > Symbols" boot information tag, which is currently:
> > > 
> > > struct multiboot_tag_elf_sections
> > > {
> > >    multiboot_uint32_t type;
> > >    multiboot_uint32_t size;
> > >    multiboot_uint32_t num;
> > >    multiboot_uint32_t entsize;
> > >    multiboot_uint32_t shndx;
> > >    char sections[0];
> > > };
> > > 
> > > grub aligns this to an 8 byte boundary, but then the odd number
> > > of
> > > uint32_t mean that sections is on a 4 byte boundary and is
> > > designed
> > > within it's ELF defintion to be on 8 byte boundary.
> > > 
> > > So I would propose adding a 4 byte pad between shndx and
> > > sections,
> > > so
> > > that sections on an 8 byte boundary.  This might break things
> > > until
> > > they're recompiled but won't require any changes to code
> > > elsewhere
> > > and
> > > I can't see how else to fix it without a new information
> > > defintion.
> > This WILL break things until they are recompiled in every MB2
> > kernel,
> > in addition to grub, every other MB2-capable bootloader and
> > documentation.
> > Until then, the kernel would have to somewhat guess whether it was
> > given
> > the old (not aligned) tag or the new one. While it may be possible
> > as
> > long
> > as the padding is filled with a value that cannot appear in the
> > first
> > bytes
> > of sections, the users would have to add the discovery code into
> > kernels.
> > MB2 is one of the most commonly used (if not the most common) boot
> > protocols
> > for hobby OSes.
> 
> Understood.

Looking at the multiboot2 documentation, the documentation defines
struct multiboot_tag_elf_sections as:

u32 type
u32 size
u16 num
u16 entsize
u16 shndx
u16 reserved
varies section headers

Which is 8 byte aligned, so reads from section headers would be OK on
ARM64.  

However, the header file defines as documented previously, with num,
entsize and shndx as u32 with no reserved field.

Is there a reason for the change from u16 to u32?  Should I keep the
new one as u32?

I will update the documentation for the "original" elf section tag when
I add documentation for the elf64 section tag.

> 
> 
> > IMHO the only way out of it would be to create a new tag type and
> > leave the
> > old one as it is right now. This won't break current
> > implementations,
> > though
> > it will pass redundant information. After some transition period we
> > may drop
> > the old format.
> 
> I'll create a new tag, struct multiboot_tag_elf64_sections which is
> correctly aligned.
> 
> > > I can include this in my patch (first one, taking time and will
> > > be
> > > terrible) to enable multiboot2 on ARM64 if this is the approach
> > > required/desired?
> > > 
> > > Chris
> > > 
> > > > Uli
> > > > 
> > > > _______________________________________________
> > > > Grub-devel mailing list
> > > > Grub-devel@gnu.org
> > > > https://lists.gnu.org/mailman/listinfo/grub-devel
> > > 
> > > _______________________________________________
> > > Grub-devel mailing list
> > > Grub-devel@gnu.org
> > > https://lists.gnu.org/mailman/listinfo/grub-devel
> 
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel