Re: [PATCH 2/4] MIPS Kprobes: Deny probes on ll/sc instructions

[David Daney <david.daney@cavium.com>]

On 11/08/2011 09:05 AM, Maneesh Soni wrote:
>
> From: Maneesh Soni<manesoni@cisco.com>
>
> Deny probes on ll/sc instructions for MIPS kprobes
>
> As ll/sc instruction are for atomic read-modify-write operations, allowing
> probes on top of these insturctions is a bad idea.
>

s/insturctions/instructions/

Not only is it a bad idea, it will probably make them fail 100% of the time.

It is also an equally bad idea to place a probe between any LL and SC 
instructions.  How do you prevent that?

If you cannot prevent probes between LL and SC, why bother with this at all?

David Daney

> Signed-off-by: Victor Kamensky<kamensky@cisco.com>
> Signed-off-by: Maneesh Soni<manesoni@cisco.com>
> ---
>   arch/mips/kernel/kprobes.c |   31 +++++++++++++++++++++++++++++++
>   1 files changed, 31 insertions(+), 0 deletions(-)
>
> diff --git a/arch/mips/kernel/kprobes.c b/arch/mips/kernel/kprobes.c
> index 9fb1876..0ab1a5f 100644
> --- a/arch/mips/kernel/kprobes.c
> +++ b/arch/mips/kernel/kprobes.c
> @@ -113,6 +113,30 @@ insn_ok:
>   	return 0;
>   }
>
> +/*
> + * insn_has_ll_or_sc function checks whether instruction is ll or sc
> + * one; putting breakpoint on top of atomic ll/sc pair is bad idea;
> + * so we need to prevent it and refuse kprobes insertion for such
> + * instructions; cannot do much about breakpoint in the middle of
> + * ll/sc pair; it is upto user to avoid those places
> + */
> +static int __kprobes insn_has_ll_or_sc(union mips_instruction insn)
> +{
> +	int ret = 0;
> +
> +	switch (insn.i_format.opcode) {
> +	case ll_op:
> +	case lld_op:
> +	case sc_op:
> +	case scd_op:
> +		ret = 1;
> +		break;
> +	default:
> +		break;
> +	}
> +	return ret;
> +}
> +
>   int __kprobes arch_prepare_kprobe(struct kprobe *p)
>   {
>   	union mips_instruction insn;
> @@ -121,6 +145,13 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
>
>   	insn = p->addr[0];
>
> +	if (insn_has_ll_or_sc(insn)) {
> +		pr_notice("Kprobes for ll and sc instructions are not"
> +			  "supported\n");
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
>   	if (insn_has_delayslot(insn)) {
>   		pr_notice("Kprobes for branch and jump instructions are not"
>   			  "supported\n");