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

Palmer Dabbelt
2022-11-30T03:00:44Z

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@rivosinc.com> 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@rivosinc.com> 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@rivosinc.com>
>> >> ---
>> >> 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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