Re: [PATCHv2 1/2] clone: Support passing tls argument via C rather than pt_regs magic

Andrew Morton
2015-05-13T22:56:33Z

On Mon, 11 May 2015 12:29:19 -0700 Josh Triplett <josh@joshtriplett.org> wrote:

> clone with CLONE_SETTLS accepts an argument to set the thread-local
> storage area for the new thread. sys_clone declares an int argument
> tls_val in the appropriate point in the argument list (based on the
> various CLONE_BACKWARDS variants), but doesn't actually use or pass
> along that argument. Instead, sys_clone calls do_fork, which calls
> copy_process, which calls the arch-specific copy_thread, and copy_thread
> pulls the corresponding syscall argument out of the pt_regs captured at
> kernel entry (knowing what argument of clone that architecture passes
> tls in).
>
> Apart from being awful and inscrutable, that also only works because
> only one code path into copy_thread can pass the CLONE_SETTLS flag, and
> that code path comes from sys_clone with its architecture-specific
> argument-passing order. This prevents introducing a new version of the
> clone system call without propagating the same architecture-specific
> position of the tls argument.
>
> However, there's no reason to pull the argument out of pt_regs when
> sys_clone could just pass it down via C function call arguments.
>
> Introduce a new CONFIG_HAVE_COPY_THREAD_TLS for architectures to opt
> into, and a new copy_thread_tls that accepts the tls parameter as an
> additional unsigned long (syscall-argument-sized) argument.
> Change sys_clone's tls argument to an unsigned long (which does
> not change the ABI), and pass that down to copy_thread_tls.
>
> Architectures that don't opt into copy_thread_tls will continue to
> ignore the C argument to sys_clone in favor of the pt_regs captured at
> kernel entry, and thus will be unable to introduce new versions of the
> clone syscall.
>
> Patch co-authored by Josh Triplett and Thiago Macieira.
>
> ...
>
> @@ -1698,20 +1701,34 @@ long do_fork(unsigned long clone_flags,
> return nr;
> }
>
> +#ifndef CONFIG_HAVE_COPY_THREAD_TLS
> +/* For compatibility with architectures that call do_fork directly rather than
> + * using the syscall entry points below. */
> +long do_fork(unsigned long clone_flags,
> + unsigned long stack_start,
> + unsigned long stack_size,
> + int __user *parent_tidptr,
> + int __user *child_tidptr)
> +{
> + return _do_fork(clone_flags, stack_start, stack_size,
> + parent_tidptr, child_tidptr, 0);
> +}
> +#endif

drivers/misc/kgdbts.c:lookup_addr() has a reference to do_fork().
Doesn't link, with a basic `make allmodconfig'.