On Thu, Jan 27, 2022 at 02:59:31PM +0100, Ard Biesheuvel wrote:
> On Thu, 27 Jan 2022 at 14:24, Mark Rutland <mark.rutl...@arm.com> wrote:
> >
> > On Thu, Jan 27, 2022 at 02:07:03PM +0100, Ard Biesheuvel wrote:
> > > I suppose that on arm64, we can work around this by passing
> > > --apply-dynamic-relocs to the linker, so that all R_AARCH64_RELATIVE
> > > targets are prepopulated with the link time value of the respective
> > > addresses. It does cause some bloat, which is why we disable that
> > > today, but we could make that dependent on ftrace being enabled.
> >
> > We'd also need to teach the build-time sort to update the relocations,
> > unless
> > you mean to also change the boot-time reloc code to RMW with the offset?
>
> Why would that be necessary? Every RELA entry has the same effect on
> its target address, as it just adds a fixed offset.
Currently in relocate_kernel() we generate the absolute address from the
relocation alone, with the core of the relocation logic being as follows, with
x9 being the pointer to a RELA entry, and x23 being the offset relative to the
default load address:
ldp x12, x13, [x9], #24
ldr x14, [x9, #-8]
add x14, x14, x23 // relocate
str x14, [x12, x23]
... and (as per another reply), a sample RELA entry currently contains:
0xffff8000090b1ab0 // default load VA of pointer to update
0x0000000000000403 // R_AARCH64_RELATIVE
0xffff8000090b6000 // default load VA of addr to write
So either:
* That code stays as-is, and we must update the relocs to correspond to their
new sorted locations, or we'll blat the sorted values with the original
relocs as we do today.
* The code needs to change to RMW: read the existing value, add the offset
(ignoring the content of the RELA entry's addend field), and write it back.
This is what I meant when I said "change the boot-time reloc code to RMW with
the offset".
Does that make sense, or have I misunderstood?
Thanks,
Mark.
