On 2024-09-25 17:50, Ard Biesheuvel wrote:
On Wed, 25 Sept 2024 at 23:11, Jason Andryuk <jason.andryuk@xxxxxxx> wrote:
Hi Ard,
On 2024-09-25 11:01, Ard Biesheuvel wrote:
From: Ard Biesheuvel <ardb@xxxxxxxxxx>
The .head.text section contains code that may execute from a different
address than it was linked at. This is fragile, given that the x86 ABI
can refer to global symbols via absolute or relative references, and the
toolchain assumes that these are interchangeable, which they are not in
this particular case.
In the case of the PVH code, there are some additional complications:
- the absolute references are in 32-bit code, which get emitted with
R_X86_64_32 relocations, and these are not permitted in PIE code;
- the code in question is not actually relocatable: it can only run
correctly from the physical load address specified in the ELF note.
So rewrite the code to only rely on relative symbol references: these
are always 32-bits wide, even in 64-bit code, and are resolved by the
linker at build time.
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Juergen queued up my patches to make the PVH entry point position
independent (5 commits):
https://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git/log/?h=linux-next
My commit that corresponds to this patch of yours is:
https://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git/commit/?h=linux-next&id=1db29f99edb056d8445876292f53a63459142309
(There are more changes to handle adjusting the page tables.)
Thanks for the head's up. Those changes look quite similar, so I guess
I should just rebase my stuff onto the xen tree.
The only thing that I would like to keep from my version is
+ lea (gdt - pvh_start_xen)(%ebp), %eax
If you rebase on top of the xen tree, using rva() would match the rest
of the code:
lea rva(gdt)(%ebp), %eax
+ add %eax, 2(%eax)
+ lgdt (%eax)
and
- .word gdt_end - gdt_start
- .long _pa(gdt_start)
+ .word gdt_end - gdt_start - 1
+ .long gdt_start - gdt
The first line is a bugfix, btw, so perhaps I should send that out
separately. But my series relies on all 32-bit absolute symbol
references being removed, since the linker rejects those when running
in PIE mode, and so the second line is needed to get rid of the _pa()
there.
Sounds good.
Regards,
Jason
