Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > 22.10.2020 02:40, Kees Cook пишет:
> > > On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > >> The vfp_kmode_exception() function now is unreachable using relative
> > >> branching in THUMB2 kernel configuration, resulting in a "relocation
> > >> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > >> linker error. Let's use long jump in order to fix the issue.
> > >
> > > Eek. Is this with gcc or clang?
> >
> > GCC 9.3.0
> >
> > >> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > >
> > > Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > That commit may have implicitly moved the location of .vfp11_veneer,
> > > though I thought I had chosen the correct position.
> >
> > I re-checked that the fixes tag is correct.
> >
> > >> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
> > >> ---
> > >>  arch/arm/vfp/vfphw.S | 3 ++-
> > >>  1 file changed, 2 insertions(+), 1 deletion(-)
> > >>
> > >> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > >> index 4fcff9f59947..6e2b29f0c48d 100644
> > >> --- a/arch/arm/vfp/vfphw.S
> > >> +++ b/arch/arm/vfp/vfphw.S
> > >> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > >>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > >>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > >>    teq     r3, #USR_MODE
> > >> -  bne     vfp_kmode_exception     @ Returns through lr
> > >> +  ldr     r1, =vfp_kmode_exception
> > >> +  bxne    r1                      @ Returns through lr
> > >>
> > >>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > >>    DBGSTR1 "fpexc %08x", r1
> > >
> > > This seems like a workaround though? I suspect the vfp11_veneer needs
> > > moving?
> > >
> >
> > I don't know where it needs to be moved. Please feel free to make a
> > patch if you have a better idea, I'll be glad to test it.
>
> I might have just been distracted by the common "vfp" prefix. It's
> possible that the text section shuffling just ended up being very large,
> so probably this patch is right then!
>

I already sent a fix for this issue:

https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1