On Thu, Jan 27, 2022 at 02:16:31PM +0100, Sven Schnelle wrote:
> Mark Rutland <mark.rutl...@arm.com> writes:
>
> > On Thu, Jan 27, 2022 at 07:46:01AM -0500, Steven Rostedt wrote:
> >> On Thu, 27 Jan 2022 12:27:04 +0000
> >> Mark Rutland <mark.rutl...@arm.com> wrote:
> >>
> >> > Ah, so those non-ELF relocations for the mcount_loc table just mean
> >> > "apply the
> >> > KASLR offset here", which is equivalent for all entries.
> >> >
> >> > That makes sense, thanks!
> >>
> >> And this is why we were having such a hard time understanding each other
> >> ;-)
> >
> > ;)
> >
> > With that in mind, I think that we understand that the build-time sort works
> > for:
> >
> > * arch/x86, becuase the non-ELF relocations for mcount_loc happen to be
> > equivalent.
> >
> > * arch/arm, because there's no dynamic relocaiton and the mcount_loc entries
> > have been finalized prior to sorting.
> >
> > ... but doesn't work for anyone else (including arm64) because the ELF
> > relocations are not equivalent, and need special care that is not yet
> > implemented.
>
> For s390 my idea is to just skip the addresses between __start_mcount_loc
> and __stop_mcount_loc, because for these addresses we know that they are
> 64 bits wide, so we just need to add the KASLR offset.
>
> I'm thinking about something like this:
>
> diff --git a/arch/s390/boot/compressed/decompressor.h
> b/arch/s390/boot/compressed/decompressor.h
> index f75cc31a77dd..015d7e2e94ef 100644
> --- a/arch/s390/boot/compressed/decompressor.h
> +++ b/arch/s390/boot/compressed/decompressor.h
> @@ -25,6 +25,8 @@ struct vmlinux_info {
> unsigned long rela_dyn_start;
> unsigned long rela_dyn_end;
> unsigned long amode31_size;
> + unsigned long start_mcount_loc;
> + unsigned long stop_mcount_loc;
> };
>
> /* Symbols defined by linker scripts */
> diff --git a/arch/s390/boot/startup.c b/arch/s390/boot/startup.c
> index 1aa11a8f57dd..7bb0d88db5c6 100644
> --- a/arch/s390/boot/startup.c
> +++ b/arch/s390/boot/startup.c
> @@ -88,6 +88,11 @@ static void handle_relocs(unsigned long offset)
> dynsym = (Elf64_Sym *) vmlinux.dynsym_start;
> for (rela = rela_start; rela < rela_end; rela++) {
> loc = rela->r_offset + offset;
> + if ((loc >= vmlinux.start_mcount_loc) &&
> + (loc < vmlinux.stop_mcount_loc)) {
> + (*(unsigned long *)loc) += offset;
> + continue;
> + }
> val = rela->r_addend;
> r_sym = ELF64_R_SYM(rela->r_info);
> if (r_sym) {
> @@ -232,6 +237,8 @@ static void offset_vmlinux_info(unsigned long offset)
> vmlinux.rela_dyn_start += offset;
> vmlinux.rela_dyn_end += offset;
> vmlinux.dynsym_start += offset;
> + vmlinux.start_mcount_loc += offset;
> + vmlinux.stop_mcount_loc += offset;
> }
>
> static unsigned long reserve_amode31(unsigned long safe_addr)
> diff --git a/arch/s390/kernel/vmlinux.lds.S b/arch/s390/kernel/vmlinux.lds.S
> index 42c43521878f..51c773405608 100644
> --- a/arch/s390/kernel/vmlinux.lds.S
> +++ b/arch/s390/kernel/vmlinux.lds.S
> @@ -213,6 +213,8 @@ SECTIONS
> QUAD(__rela_dyn_start) /*
> rela_dyn_start */
> QUAD(__rela_dyn_end) /* rela_dyn_end
> */
> QUAD(_eamode31 - _samode31) /* amode31_size
> */
> + QUAD(__start_mcount_loc)
> + QUAD(__stop_mcount_loc)
> } :NONE
>
> /* Debugging sections. */
>
> Not sure whether that would also work on power, and also not sure
> whether i missed something thinking about that. Maybe it doesn't even
> work. ;-)
I don't know enough about s390 or powerpc relocs to say whether that works, but
I can say that approach isn't going to work for arm64 without other signficant
changes.
I want to get the regression fixed ASAP, so can we take a simple patch for -rc2
which disables the build-time sort where it's currently broken (by limiting the
opt-in to arm and x86), then follow-up per-architecture to re-enable it if
desired/safe?
Thanks,
Mark.
