Re: [PATCH v5 05/20] x86/decompressor: Use proper sequence to take the address of the GOT

[Ard Biesheuvel]

On Wed, 21 Jun 2023 at 13:09, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Wed, Jun 07, 2023 at 09:23:27AM +0200, Ard Biesheuvel wrote:
> > The 32-bit decompressor does not actually use a global offset table
> > (GOT), but as is common for 32-bit position independent code, it uses
> > the magic symbol _GLOBAL_OFFSET_TABLE_ as an anchor from which to derive
> > the actual runtime addresses of other symbols, using special @GOTOFF
> > symbol references that are resolved at link time, and populated with the
> > distance between the address of the magic _GLOBAL_OFFSET_TABLE_ anchor
> > and the address of the symbol in question.
> >
> > This means _GLOBAL_OFFSET_TABLE_ is the only symbol whose actual runtime
> > address needs to be determined explicitly, which is one of the first
> > things that happens in startup_32. However, it does so by taking the
> > absolute address via the immediate field of an ADD instruction (plus a
> > small offset), which seems to defeat the point.
> >
> > Fortunately, the assembler knows that _GLOBAL_OFFSET_TABLE_ is magic,
> > and emits a special relative relocation instead, and so the resulting
>
> Which special relocation do you mean?
>
> This guy:
>
> Relocation section '.rel.head.text' at offset 0x3a0 contains 12 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name
> 00000010  00000d0a R_386_GOTPC       00000000   _GLOBAL_OFFSET_TABLE_
>
> ?

Yep.

if you assemble this

movl $_GLOBAL_OFFSET_TABLE_, %eax
movl $_GLOBAL_OFFSET_TABLE, %eax

you'll end up with

   0: b8 01 00 00 00        mov    $0x1,%eax
1: R_386_GOTPC _GLOBAL_OFFSET_TABLE_
   5: b8 00 00 00 00        mov    $0x0,%eax
6: R_386_32 _GLOBAL_OFFSET_TABLE

So it is not possible to take the absolute address of
_GLOBAL_OFFSET_TABLE_ via an absolute relocation, you will always get
the relative offset instead.