Re: [PATCH -next V12 3/7] riscv: entry: Add noinstr to prevent instrumentation inserted

Guo Ren
2023-01-04T01:41:09Z

On Tue, Jan 3, 2023 at 5:12 PM Alexandre Ghiti <alex@ghiti.fr> wrote:
>
> Hi Guo,
>
> On 1/3/23 04:35, guoren@kernel.org wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > 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@hirez.programming.kicks-ass.net/
> > Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
> > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > Tested-by: Jisheng Zhang <jszhang@kernel.org>
> > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@kernel.org>
> > ---
> > 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 549bde5c970a..96ec76c54ff2 100644
> > --- a/arch/riscv/kernel/traps.c
> > +++ b/arch/riscv/kernel/traps.c
> > @@ -95,9 +95,9 @@ 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")
> > #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 d86f7cebd4a7..b26f68eac61c 100644
> > --- a/arch/riscv/mm/fault.c
> > +++ b/arch/riscv/mm/fault.c
> > @@ -204,7 +204,7 @@ 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)
>
>
> (I dug the archive but can't find the series before v4, so sorry if it
> was already answered)
>
> I think we should not disable the instrumentation of those trap handlers
> as at least profiling them with ftrace would provide valuable
> information (and gcov would be nice too): why do we need to do that? A
> trap very early in the boot process is not recoverable anyway.
Everything that calls irqentry_enter() should be noinstr, and this
patch prepares for the next generic_entry convert.

eg:
asmlinkage void noinstr do_page_fault(struct pt_regs *regs)
{
irqentry_state_t state = irqentry_enter(regs);

__do_page_fault(regs);

local_irq_disable();

irqentry_exit(regs, state);
}
NOKPROBE_SYMBOL(do_page_fault);

You still could profile __do_page_fault.

>
> And I took a look at other architectures, none of them disables the
> instrumentation on do_page_fault.
That's not true, have a look at power & arm64. All of them have some
limitations at the entry of page_fault.

>
>
> > {
> > struct task_struct *tsk;
> > struct vm_area_struct *vma;

--
Best Regards
Guo Ren