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
> 
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