| Message ID | 20230625204907.57291-7-julien@xen.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | xen/arm: Enable UBSAN support | expand |
On 25/06/2023 22:49, Julien Grall wrote: > > > From: Julien Grall <jgrall@amazon.com> > > The instruction CBNZ can only jump to a pc-relative that is in the > range +/- 1MB. > > Alternative instructions replacement are living in a separate > subsection of the init section. This is usually placed towards > the end of the linker. Whereas text is towards the beginning. > > While today Xen is quite small (~1MB), it could grow up to > 2MB in the current setup. So there is no guarantee that the > target address in the text section will be within the range +/- > 1MB of the CBNZ in alternative section. > > The easiest solution is to have the target address within the > same section of the alternative. This means that we need to > duplicate a couple of instructions. > > Signed-off-by: Julien Grall <jgrall@amazon.com> > > ---- > > I couldn't come up with a solution that would not change the number > of instructions executed in the entry path. It looks like the max offset is indeed 1MB for conditional branches and I cannot think of any better way of doing this, so: Reviewed-by: Michal Orzel <michal.orzel@amd.com> > --- > xen/arch/arm/arm64/entry.S | 21 ++++++++++++++++----- > 1 file changed, 16 insertions(+), 5 deletions(-) > > diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S > index 95f1a9268419..492591fdef54 100644 > --- a/xen/arch/arm/arm64/entry.S > +++ b/xen/arch/arm/arm64/entry.S > @@ -242,13 +242,24 @@ > msr daifclr, \iflags > bl enter_hypervisor_from_guest > > + /* > + * CBNZ can only address an offset of +/- 1MB. This means, it is > + * not possible to jump outside of an alternative because > + * the .text section and .altinstr_replacement may be further > + * appart. The easiest way is to duplicate the few instructions s/appart/apart ~Michal
diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S index 95f1a9268419..492591fdef54 100644 --- a/xen/arch/arm/arm64/entry.S +++ b/xen/arch/arm/arm64/entry.S @@ -242,13 +242,24 @@ msr daifclr, \iflags bl enter_hypervisor_from_guest + /* + * CBNZ can only address an offset of +/- 1MB. This means, it is + * not possible to jump outside of an alternative because + * the .text section and .altinstr_replacement may be further + * appart. The easiest way is to duplicate the few instructions + * that need to be skipped. + */ alternative_if SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT - cbnz x19, 1f - alternative_else_nop_endif - - mov x0, sp - bl do_trap_\trap + cbnz x19, 1f + mov x0, sp + bl do_trap_\trap 1: + alternative_else + nop + mov x0, sp + bl do_trap_\trap + alternative_endif + exit hyp=0, compat=\compat .endm