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

Alexandre Ghiti
2023-01-03T09:12:14Z

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.

And I took a look at other architectures, none of them disables the
instrumentation on do_page_fault.

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