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

Mark Rutland
2023-01-04T11:56:28Z

On Mon, Jan 02, 2023 at 10:35:27PM -0500, 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)

That's generally true, and makes sense to me, but ....

> 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)

... why do you need that for do_page_fault? That doesn't (currently) do any
entry/exit logic, so this seems unnecessary per the commit description.

Thanks,
Mark.

> {
> struct task_struct *tsk;
> struct vm_area_struct *vma;
> --
> 2.36.1
>