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

[Josh Triplett]

On Mon, May 11, 2015 at 02:31:39PM +0000, Vineet Gupta wrote:
> On Tuesday 21 April 2015 11:17 PM, Josh Triplett 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.
> > 
> > Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Thiago Macieira <thiago.macieira@xxxxxxxxx>
> > Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
> > ---
> >  arch/Kconfig             |  7 ++++++
> >  include/linux/sched.h    | 14 ++++++++++++
> >  include/linux/syscalls.h |  6 +++---
> >  kernel/fork.c            | 55 +++++++++++++++++++++++++++++++-----------------
> >  4 files changed, 60 insertions(+), 22 deletions(-)
> > 
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 05d7a8a..4834a58 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -484,6 +484,13 @@ config HAVE_IRQ_EXIT_ON_IRQ_STACK
> >  	  This spares a stack switch and improves cache usage on softirq
> >  	  processing.
> >  
> > +config HAVE_COPY_THREAD_TLS
> > +	bool
> > +	help
> > +	  Architecture provides copy_thread_tls to accept tls argument via
> > +	  normal C parameter passing, rather than extracting the syscall
> > +	  argument from pt_regs.
> > +
> >  #
> >  # ABI hall of shame
> >  #
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index a419b65..2cc88c6 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -2480,8 +2480,22 @@ extern struct mm_struct *mm_access(struct task_struct *task, unsigned int mode);
> >  /* Remove the current tasks stale references to the old mm_struct */
> >  extern void mm_release(struct task_struct *, struct mm_struct *);
> >  
> > +#ifdef CONFIG_HAVE_COPY_THREAD_TLS
> > +extern int copy_thread_tls(unsigned long, unsigned long, unsigned long,
> > +			struct task_struct *, unsigned long);
> > +#else
> >  extern int copy_thread(unsigned long, unsigned long, unsigned long,
> >  			struct task_struct *);
> > +
> > +/* Architectures that haven't opted into copy_thread_tls get the tls argument
> > + * via pt_regs, so ignore the tls argument passed via C. */
> > +static inline int copy_thread_tls(
> > +		unsigned long clone_flags, unsigned long sp, unsigned long arg,
> > +		struct task_struct *p, unsigned long tls)
> > +{
> > +	return copy_thread(clone_flags, sp, arg, p);
> > +}
> > +#endif
> 
> Is this detour really needed. Can we not update copy_thread() of all arches in one
> go and add the tls arg, w/o using it.
> 
> And then arch maintainers can micro-optimize their code to use that arg vs.
> pt_regs->rxx version at their own leisure. The only downside I see with that is
> bigger churn (touches all arches), and a interim unused arg warning ?

In addition to the cleanup and simplification, the purpose of this patch
is specifically to make sure that any architecture opting into
HAVE_COPY_THREAD_TLS does *not* care how tls is passed in, and in
particular doesn't depend on it arriving in a specific syscall argument.
I have patches in flight (for CLONE_FD and clone4) that depend on that
assumption, by introducing additional syscalls (with tls passed
differently) that call down through these same code paths.

- Josh Triplett
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