Re: aarch64 clone() man page omission

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

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On 11 May 2016 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.

as i said, that same behavior can be observed on other arches.  i know of
at least one for sure that if the stack is unaligned, then push/pop ops
will also trigger SIGBUS.  x86 tends to be more forgiving, but if it isn't
16bytes, then it is known that SSE optimized code will often fault.

so the question is still: why is aarch64 enforcing in the kernel what all
other arches have left alone even when they behave the same in hardware ?
-mike

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