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SubjectRe: [PATCH] arm64: do not enforce strict 16 byte alignment to stack pointer
On Wed, May 11, 2016 at 05:56:54PM +0100, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
>
> copy_thread should not be enforcing 16 byte aligment and returning
> -EINVAL. Other architectures trap misaligned stack access with SIGBUS
> so arm64 should follow this convention, so remove the strict enforcement
> check.
>
> For example, currently clone(2) fails with -EINVAL when passing
> a misaligned stack and this gives little clue to what is wrong. Instead,
> it is arguable that a SIGBUS on the fist access to a misaligned stack
> allows one to figure out that it is a misaligned stack issue rather
> than trying to figure out why an unconventional (and undocumented)
> -EINVAL is being returned.
>
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
> arch/arm64/kernel/process.c | 3 ---
> 1 file changed, 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 5655f756..8414971 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -258,9 +258,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
> 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;
> }

As we discussed on the linux-man list, I don't expect this change to
break existing working user apps since they pass an aligned stack
already. I really doubt anyone relies on the -EINVAL here.

That said, I don't think we should add a cc stable (which you haven't
anyway), at least we have a point in time where this change was made. As
the patch stands:

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

(but let's wait for Will's opinion as well)

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