Re: aarch64 clone() man page omission

[Colin Ian King <colin.king-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>]

On 11/05/16 14:18, Catalin Marinas wrote:
> On Tue, May 10, 2016 at 10:50:40PM -0400, Mike Frysinger wrote:
>> On 09 May 2016 22:40, Colin Ian King wrote:
>>> On 09/05/16 22:31, Mike Frysinger wrote:
>>>> On 25 Apr 2016 20:42, Colin Ian King wrote:
>>>>> currently, the aarch64 clone() system call requires the stack to be
>>>>> aligned at a 16 byte boundary, see arch/arm64/kernel/process.c,
>>>>> copy_thread():
>>>>>
>>>>>                 if (stack_start) {
>>>>>                         if (is_compat_thread(task_thread_info(p)))
>>>>>                                 childregs->compat_sp = stack_start;
>>>>>                         /* 16-byte aligned stack mandatory on AArch64 */
>>>>>                         else if (stack_start & 15)
>>>>>                                 return -EINVAL;
>>>>>                         else
>>>>>                                 childregs->sp = stack_start;
>>>>>                 }
>>>>>
>>>>>
>>>>> ..and returns -EINVAL if not aligned correctly.  This should be added to
>>>>> the manual page clone(2) as it took me a while to figure out why clone()
>>>>> was failing with -EINVAL for aarch64 but not on x86.
>>>>
>>>> seems weird for the kernel to be enforcing this.  is it just because of
>>>> the stated ABI ?  or is there some weird requirement in the kernel itself
>>>> that requires this ?  it's not like other arches have this check, and
>>>> there are def ABI requirements about stack alignments in C.
>>>
>>> The article here indicates it is an aarch64 convention:
>>>
>>> https://community.arm.com/groups/processors/blog/2015/11/19/using-the-stack-in-aarch32-and-aarch64
>>
>> that checks my point about the ABI having alignment requirements, but
>> that doesn't mean it needs to be checked/enforced in the kernel.  all
>> the limitations i see there can be seen in other arches, but we don't
>> have those arches do any stack alignment checking.  so should we be
>> dropping it from aarch64 ?  why does it need to be special here ?
> 
> It is not just a software ABI requirement but a hardware one. If you try
> to access the stack with an unaligned SP value, you get a fault followed
> by a SIGBUS delivered to the user application. We decided to enforce
> this at the copy_thread() level, it is easier to catch such issue early
> than debugging SIGBUS delivered to a thread.
> 
Rather than returning -EINVAL would it be more useful re-align
stack_start to the 16 byte boundary in copy_thread as a silent but
useful fixup?  It took me a while to debug the -EINVAL on the clone()
system call to figure out what was wrong because I didn't realize
aarch64 has this constraint.

Colin
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