[PATCH v9 2/4] arm64: mte: change ASYNC and SYNC TCF settings into bitfields

[Peter Collingbourne]

On Wed, Jul 7, 2021 at 4:11 AM Will Deacon <will at kernel.org> wrote:
>
> On Fri, Jul 02, 2021 at 12:41:08PM -0700, Peter Collingbourne wrote:
> > Allow the user program to specify both ASYNC and SYNC TCF modes by
> > repurposing the existing constants as bitfields. This will allow the
> > kernel to select one of the modes on behalf of the user program. With
> > this patch the kernel will always select async mode, but a subsequent
> > patch will make this configurable.
> >
> > Link: https://linux-review.googlesource.com/id/Icc5923c85a8ea284588cc399ae74fd19ec291230
> > Signed-off-by: Peter Collingbourne <pcc at google.com>
> > Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> > ---
> > v9:
> > - make mte_update_sctlr_user static
> >
> >  arch/arm64/include/asm/processor.h |  3 ++
> >  arch/arm64/kernel/mte.c            | 70 ++++++++++++------------------
> >  include/uapi/linux/prctl.h         | 11 ++---
> >  3 files changed, 37 insertions(+), 47 deletions(-)
>
> ...
>
> >  long set_mte_ctrl(struct task_struct *task, unsigned long arg)
> >  {
> > -     u64 sctlr = task->thread.sctlr_user & ~SCTLR_EL1_TCF0_MASK;
> >       u64 mte_ctrl = (~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
> >                       SYS_GCR_EL1_EXCL_MASK) << MTE_CTRL_GCR_USER_EXCL_SHIFT;
> >
> >       if (!system_supports_mte())
> >               return 0;
> >
> > -     switch (arg & PR_MTE_TCF_MASK) {
> > -     case PR_MTE_TCF_NONE:
> > -             sctlr |= SCTLR_EL1_TCF0_NONE;
> > -             break;
> > -     case PR_MTE_TCF_SYNC:
> > -             sctlr |= SCTLR_EL1_TCF0_SYNC;
> > -             break;
> > -     case PR_MTE_TCF_ASYNC:
> > -             sctlr |= SCTLR_EL1_TCF0_ASYNC;
> > -             break;
> > -     default:
> > -             return -EINVAL;
> > -     }
> > +     if (arg & PR_MTE_TCF_ASYNC)
> > +             mte_ctrl |= MTE_CTRL_TCF_ASYNC;
> > +     if (arg & PR_MTE_TCF_SYNC)
> > +             mte_ctrl |= MTE_CTRL_TCF_SYNC;
> >
> > -     if (task != current) {
> > -             task->thread.sctlr_user = sctlr;
> > -             task->thread.mte_ctrl = mte_ctrl;
> > -     } else {
> > -             set_task_sctlr_el1(sctlr);
> > -             set_gcr_el1_excl(mte_ctrl);
> > +     task->thread.mte_ctrl = mte_ctrl;
> > +     if (task == current) {
> > +             mte_update_sctlr_user(task);
>
> In conjunction with the next patch, what happens if we migrate at this
> point? I worry that we can install a stale sctlr_user value.
>
> > +             set_task_sctlr_el1(task->thread.sctlr_user);

In this case, we will call mte_update_sctlr_user when scheduled onto
the new CPU as a result of the change to mte_thread_switch, and both
the scheduler and prctl will set SCTLR_EL1 to the new (correct) value
for the current CPU.

Peter