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.
--
Catalin
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