Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with code-patching selftests

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 27 Jan 2022 at 15:55, Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Thu, Jan 27, 2022 at 02:59:31PM +0100, Ard Biesheuvel wrote:
> > On Thu, 27 Jan 2022 at 14:24, Mark Rutland <mark.rutland@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?
>

No you're right. We'd have to use different sequences here depending
on whether the relocation target is populated or not, which currently
we don't care about.