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

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Message-ID: <Y7VpN/fFjqGJbxPu@FVFF77S0Q05N>
Date:   Wed, 4 Jan 2023 11:55:35 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     guoren@...nel.org
Cc:     arnd@...db.de, palmer@...osinc.com, tglx@...utronix.de,
        peterz@...radead.org, luto@...nel.org, conor.dooley@...rochip.com,
        heiko@...ech.de, jszhang@...nel.org, lazyparser@...il.com,
        falcon@...ylab.org, chenhuacai@...nel.org, apatel@...tanamicro.com,
        atishp@...shpatra.org, ben@...adent.org.uk, bjorn@...nel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Guo Ren <guoren@...ux.alibaba.com>,
        Björn Töpel <bjorn@...osinc.com>
Subject: Re: [PATCH -next V12 3/7] riscv: entry: Add noinstr to prevent
 instrumentation inserted

On Mon, Jan 02, 2023 at 10:35:27PM -0500, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.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@...osinc.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Tested-by: Jisheng Zhang <jszhang@...nel.org>
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.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
>