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

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Date:   Thu, 27 Jan 2022 14:54:47 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Yinan Liu <yinan@...ux.alibaba.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        Sachin Sant <sachinp@...ux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with
 code-patching selftests

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@....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.