Re: aarch64 clone() man page omission

[Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org>]

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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 ?
-mike

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