On Tue, May 10, 2016 at 10:50:40PM -0400, Mike Frysinger wrote:

On 09 May 2016 22:40, Colin Ian King wrote:

quoted

On 09/05/16 22:31, Mike Frysinger wrote:

quoted

On 25 Apr 2016 20:42, Colin Ian King wrote:

quoted

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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