Re: [PATCH V6 05/11] riscv: traps: Add noinstr to prevent instrumentation inserted

[Guo Ren]

On Mon, Oct 3, 2022 at 7:41 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Sat, Oct 01, 2022 at 09:24:45PM -0400, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > Without noinstr the compiler is free to insert instrumentation (think
> > all the k*SAN, KCov, GCov, ftrace etc..) which can call code we're not
> > yet ready to run this early in the entry path, for instance it could
> > rely on RCU which isn't on yet, or expect lockdep state. (by peterz)
> >
> > Link: https://lore.kernel.org/linux-riscv/YxcQ6NoPf3AH0EXe@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/raw
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > ---
> >  arch/riscv/kernel/traps.c | 4 ++--
> >  arch/riscv/mm/fault.c     | 2 +-
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > index 635e6ec26938..588e17c386c6 100644
> > --- a/arch/riscv/kernel/traps.c
> > +++ b/arch/riscv/kernel/traps.c
> > @@ -92,9 +92,9 @@ static void do_trap_error(struct pt_regs *regs, int signo, int code,
> >  }
> >
> >  #if defined(CONFIG_XIP_KERNEL) && defined(CONFIG_RISCV_ALTERNATIVE)
> > -#define __trap_section               __section(".xip.traps")
> > +#define __trap_section __noinstr_section(".xip.traps")
>
> I assume that for CONFIG_XIP_KERNEL, KPROBES is not possible, and so functions
> marked with __trap_section don't need to be excluded from kprobes.
>
> Is that assumption correct, or does something need to be done to inhibit that?
Correct!

In riscv, "we select HAVE_KPROBES if !XIP_KERNEL", so don't worry
about that. I don't think we could enable kprobe for XIP_KERNEL in the
future.

>
> Thanks,
> Mark.
>
> >  #else
> > -#define __trap_section
> > +#define __trap_section noinstr
> >  #endif
> >  #define DO_ERROR_INFO(name, signo, code, str)                                \
> >  asmlinkage __visible __trap_section void name(struct pt_regs *regs)  \
> > diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> > index f2fbd1400b7c..c7829289e806 100644
> > --- a/arch/riscv/mm/fault.c
> > +++ b/arch/riscv/mm/fault.c
> > @@ -203,7 +203,7 @@ static inline bool access_error(unsigned long cause, struct vm_area_struct *vma)
> >   * This routine handles page faults.  It determines the address and the
> >   * problem, and then passes it off to one of the appropriate routines.
> >   */
> > -asmlinkage void do_page_fault(struct pt_regs *regs)
> > +asmlinkage void noinstr do_page_fault(struct pt_regs *regs)
> >  {
> >       struct task_struct *tsk;
> >       struct vm_area_struct *vma;
> > --
> > 2.36.1
> >



-- 
Best Regards
 Guo Ren