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

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Jan 27, 2022 at 01:03:34PM +0100, Ard Biesheuvel wrote:
> On Thu, 27 Jan 2022 at 12:47, Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > [adding LKML so this is easier for others to find]
> >
> > If anyone wants to follow the thread from the start, it's at:
> >
> >   https://lore.kernel.org/linuxppc-dev/944D10DA-8200-4BA9-8D0A-3BED9AA99F82@linux.ibm.com/
> >
> > Ard, I was under the impression that the 32-bit arm kernel was (virtually)
> > relocatable, but I couldn't spot where, and suspect I'm mistaken. Do you know
> > whether it currently does any boot-time dynamic relocation?
> 
> No, it does not.

Thanks for comfirming!

So 32-bit arm should be able to opt into the build-time sort as-is.

> > Steve asked for a bit more detail on IRC, so the below is an attempt to explain
> > what's actually going on here.
> >
> > The short answer is that relocatable kernels (e.g. those with KASLR support)
> > need to handle the kernel being loaded at (somewhat) arbitrary virtual
> > addresses. Even where code can be position-independent, any pointers in the
> > kernel image (e.g. the contents of the mcount_loc table) need to be updated to
> > account for the specific VA the kernel was loaded at -- arch code does this
> > early at boot time by applying dynamic (ELF) relocations.
> 
> These architectures use place-relative extables for the same reason:
> place relative references are resolved at build time rather than at
> runtime during relocation, making a build time sort feasible.
> 
> arch/alpha/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE
> arch/arm64/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE
> arch/ia64/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE
> arch/parisc/include/asm/uaccess.h:#define ARCH_HAS_RELATIVE_EXTABLE
> arch/powerpc/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE
> arch/riscv/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE
> arch/s390/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE
> arch/x86/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE
> 
> Note that the swap routine becomes something like the below, given
> that the relative references need to be fixed up after the entry
> changes place in the sorted list.
> 
> static void swap_ex(void *a, void *b, int size)
> {
>         struct exception_table_entry *x = a, *y = b, tmp;
>         int delta = b - a;
> 
>         tmp = *x;
>         x->insn = y->insn + delta;
>         y->insn = tmp.insn - delta;
>         ...
> }
> 
> As a bonus, the resulting footprint of the table in the image is
> reduced by 8x, given that every 8 byte pointer has an accompanying 24
> byte RELA record, so we go from 32 bytes to 4 bytes for every call to
> __gnu_mcount_mc.

Absolutely -- it'd be great if we could do that for the callsite locations; the
difficulty is that the entries are generated by the compiler itself, so we'd
either need some build/link time processing to convert each absolute 64-bit
value to a relative 32-bit offset, or new compiler options to generate those as
relative offsets from the outset.

Thanks,
Mark.