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Date:   Tue, 29 Nov 2022 18:50:56 -0800 (PST)
Subject:     Re: [PATCH 1/2] RISC-V: Align the shadow stack
In-Reply-To: <CAMKF1srMj5aH4_+iwbFuu+AEgyyz85y+UPEszUGeQ7U=u19R3Q@mail.gmail.com>
CC:     jszhang@kernel.org, guoren@kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
From:   Palmer Dabbelt <palmer@rivosinc.com>
To:     Khem Raj <raj.khem@gmail.com>
Message-ID: <mhng-0a99c7e2-7e15-4a5f-a5fa-1e4c844a5ecd@palmer-ri-x1c9a>
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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.

>> 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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