[PATCH] arm64: kprobe: fix an error in single stepping support

[Yeoreum Yun]

Hi,

>
> It is obvious a conflict between the code and the comment.
> The function aarch64_insn_is_steppable is used to check if a mrs
> instruction can be safe in single-stepping environment, in the
> comment it says only reading DAIF bits by mrs is safe in
> single-stepping environment, and other mrs instructions are not. So
> aarch64_insn_is_steppable should returen "TRUE" if the mrs instruction
> being single stepped is reading DAIF bits.
>
> And have verified using a kprobe kernel module which reads the DAIF bits by
> function arch_local_irq_save with offset setting to 0x4, confirmed that
> without this modification, it encounters
> "kprobe_init: register_kprobe failed, returned -22" error while inserting
> the kprobe kernel module. and with this modification, it can read the DAIF
> bits in single-stepping environment.
>
> Fixes: 2dd0e8d2d2a1 ("arm64: Kprobes with single stepping support")
> Cc: stable at vger.kernel.org
> Signed-off-by: Yiren Xie <1534428646 at qq.com>
> ---
>  arch/arm64/kernel/probes/decode-insn.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
> index 6438bf62e753..22383eb1c22c 100644
> --- a/arch/arm64/kernel/probes/decode-insn.c
> +++ b/arch/arm64/kernel/probes/decode-insn.c
> @@ -40,7 +40,7 @@ static bool __kprobes aarch64_insn_is_steppable(u32 insn)
>  		 */
>  		if (aarch64_insn_is_mrs(insn))
>  			return aarch64_insn_extract_system_reg(insn)
> -			     != AARCH64_INSN_SPCLREG_DAIF;
> +			     == AARCH64_INSN_SPCLREG_DAIF;
>
>  		/*
>  		 * The HINT instruction is steppable only if it is in whitelist
> --
> 2.34.1
>

Thanks to correct me. yes the comments seem conflict.

However, I couldn't agree to this change.
As I mention in last, when single-step runs, all DAIF bits set,
so, the result of reading DAIF is different between before install kprobe and after.
Also, I think reading some sys_reg in single-step seems ok (i.e. SYS_MIDR_EL1).

Therefore, allowing only install kprobe on DAIF reading doesn't seem
correct.

Thanks