Re: [PATCH 1/2] RISC-V: Align the shadow stack

[Palmer Dabbelt]

On Tue, 29 Nov 2022 18:56:48 PST (-0800), Khem Raj wrote:
> On Tue, Nov 29, 2022 at 6:50 PM Palmer Dabbelt <palmer@xxxxxxxxxxxx> wrote:
> > On Tue, 29 Nov 2022 18:47:55 PST (-0800), Khem Raj wrote:
> > > Hi Palmer
> > >
> > > On Tue, Nov 29, 2022 at 6:36 PM Palmer Dabbelt <palmer@xxxxxxxxxxxx> wrote:
> > >>
> > >> The standard RISC-V ABIs all require 16-byte stack alignment.  We're
> > >> only calling that one function on the shadow stack so I doubt it'd
> > >> result in a real issue, but might as well keep this lined up.
> > >
> > > Is 16-byte alignment required on rv32 as well ?
> >
> > For the standard ABIs that's the case, it's so the Q extension can spill
> > without aligning the stack.  There's also at least a proposed embedded
> > ABI that has just XLEN (32-bit on rv32) alignment, as the bigger stack
> > alignment has an impact on some use cases.
>
> Thanks, so in this case 16byte will be valid for both rv64/rv32 here.

Yes, though the long-alignment wouldn't break anything because we don't 
have Q support and we're just calling that one function -- it's not like 
the compiler is actively checking for 16-byte alignment or anything, 
it's just assuming it.

Still best to keep things to the spec where we can, though.

> > >> Fixes: 31da94c25aea ("riscv: add VMAP_STACK overflow detection")
> > >> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> > >> ---
> > >>  arch/riscv/kernel/traps.c | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >>
> > >> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > >> index be54ccea8c47..acdfcacd7e57 100644
> > >> --- a/arch/riscv/kernel/traps.c
> > >> +++ b/arch/riscv/kernel/traps.c
> > >> @@ -206,7 +206,7 @@ static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
> > >>   * shadow stack, handled_ kernel_ stack_ overflow(in kernel/entry.S) is used
> > >>   * to get per-cpu overflow stack(get_overflow_stack).
> > >>   */
> > >> -long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)];
> > >> +long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
> > >>  asmlinkage unsigned long get_overflow_stack(void)
> > >>  {
> > >>         return (unsigned long)this_cpu_ptr(overflow_stack) +
> > >> --
> > >> 2.38.1
> > >>
> > >>
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