Subject: Re: [PATCH bpf-next 4/5] bpf: Add bpf_get_func_ip helper for kprobe programs

From: Masami Hiramatsu <[email-masked]>
Date: Thu, 1 Jul 2021 11:01:18 +0900

On Wed, 30 Jun 2021 18:45:46 -0700
Yonghong Song <[email-masked]> wrote:

> 
> 
> On 6/30/21 4:58 PM, Masami Hiramatsu wrote:
> > On Wed, 30 Jun 2021 10:47:01 -0700
> > Yonghong Song <[email-masked]> wrote:
> > 
> >>
> >>
> >> On 6/29/21 12:29 PM, Jiri Olsa wrote:
> >>> Adding bpf_get_func_ip helper for BPF_PROG_TYPE_KPROBE programs,
> >>> so it's now possible to call bpf_get_func_ip from both kprobe and
> >>> kretprobe programs.
> >>>
> >>> Taking the caller's address from 'struct kprobe::addr', which is
> >>> defined for both kprobe and kretprobe.
> >>>
> >>> Signed-off-by: Jiri Olsa <[email-masked]>
> >>> ---
> >>>    include/uapi/linux/bpf.h       |  2 +-
> >>>    kernel/bpf/verifier.c          |  2 ++
> >>>    kernel/trace/bpf_trace.c       | 14 ++++++++++++++
> >>>    kernel/trace/trace_kprobe.c    | 20 ++++++++++++++++++--
> >>>    kernel/trace/trace_probe.h     |  5 +++++
> >>>    tools/include/uapi/linux/bpf.h |  2 +-
> >>>    6 files changed, 41 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >>> index 83e87ffdbb6e..4894f99a1993 100644
> >>> --- a/include/uapi/linux/bpf.h
> >>> +++ b/include/uapi/linux/bpf.h
> >>> @@ -4783,7 +4783,7 @@ union bpf_attr {
> >>>     *
> >>>     * u64 bpf_get_func_ip(void *ctx)
> >>>     * 	Description
> >>> - * 		Get address of the traced function (for tracing programs).
> >>> + * 		Get address of the traced function (for tracing and kprobe programs).
> >>>     * 	Return
> >>>     * 		Address of the traced function.
> >>>     */
> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>> index 701ff7384fa7..b66e0a7104f8 100644
> >>> --- a/kernel/bpf/verifier.c
> >>> +++ b/kernel/bpf/verifier.c
> >>> @@ -5979,6 +5979,8 @@ static bool has_get_func_ip(struct bpf_verifier_env *env)
> >>>    			return -ENOTSUPP;
> >>>    		}
> >>>    		return 0;
> >>> +	} else if (type == BPF_PROG_TYPE_KPROBE) {
> >>> +		return 0;
> >>>    	}
> >>>    
> >>>    	verbose(env, "func %s#%d not supported for program type %d\n",
> >>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> >>> index 9edd3b1a00ad..1a5bddce9abd 100644
> >>> --- a/kernel/trace/bpf_trace.c
> >>> +++ b/kernel/trace/bpf_trace.c
> >>> @@ -961,6 +961,18 @@ static const struct bpf_func_proto bpf_get_func_ip_proto_tracing = {
> >>>    	.arg1_type	= ARG_PTR_TO_CTX,
> >>>    };
> >>>    
> >>> +BPF_CALL_1(bpf_get_func_ip_kprobe, struct pt_regs *, regs)
> >>> +{
> >>> +	return trace_current_kprobe_addr();
> >>> +}
> >>> +
> >>> +static const struct bpf_func_proto bpf_get_func_ip_proto_kprobe = {
> >>> +	.func		= bpf_get_func_ip_kprobe,
> >>> +	.gpl_only	= true,
> >>> +	.ret_type	= RET_INTEGER,
> >>> +	.arg1_type	= ARG_PTR_TO_CTX,
> >>> +};
> >>> +
> >>>    const struct bpf_func_proto *
> >>>    bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >>>    {
> >>> @@ -1092,6 +1104,8 @@ kprobe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >>>    	case BPF_FUNC_override_return:
> >>>    		return &bpf_override_return_proto;
> >>>    #endif
> >>> +	case BPF_FUNC_get_func_ip:
> >>> +		return &bpf_get_func_ip_proto_kprobe;
> >>>    	default:
> >>>    		return bpf_tracing_func_proto(func_id, prog);
> >>>    	}
> >>> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> >>> index ea6178cb5e33..b07d5888db14 100644
> >>> --- a/kernel/trace/trace_kprobe.c
> >>> +++ b/kernel/trace/trace_kprobe.c
> >>> @@ -1570,6 +1570,18 @@ static int kretprobe_event_define_fields(struct trace_event_call *event_call)
> >>>    }
> >>>    
> >>>    #ifdef CONFIG_PERF_EVENTS
> >>> +/* Used by bpf get_func_ip helper */
> >>> +DEFINE_PER_CPU(u64, current_kprobe_addr) = 0;
> >>
> >> Didn't check other architectures. But this should work
> >> for x86 where if nested kprobe happens, the second
> >> kprobe will not call kprobe handlers.
> > 
> > No problem, other architecture also does not call nested kprobes handlers.
> > However, you don't need this because you can use kprobe_running()
> > in kprobe context.
> > 
> > kp = kprobe_running();
> > if (kp)
> > 	return kp->addr;
> > 
> > BTW, I'm not sure why don't you use instruction_pointer(regs)?
> 
> How about kretprobe? I guess kp->addr should still point to
> function address but instruction_pointer(regs) does not?

It is now under review (waiting for merge), please check this series.

https://lore.kernel.org/bpf/162400000592.506599.4695807810528866713.stgit@devnote2/

Then you can use instruction_pointer(regs) in kretprobes too.

Thank you,

> 
> > 
> > Thank you,
> > 
> >>
> >> This essentially is to provide an additional parameter to
> >> bpf program. Andrii is developing a mechanism to
> >> save arbitrary data in *current task_struct*, which
> >> might be used here to save current_kprobe_addr, we can
> >> save one per cpu variable.
> >>
> >>> +
> >>> +u64 trace_current_kprobe_addr(void)
> >>> +{
> >>> +	return *this_cpu_ptr(&current_kprobe_addr);
> >>> +}
> >>> +
> >>> +static void trace_current_kprobe_set(struct trace_kprobe *tk)
> >>> +{
> >>> +	__this_cpu_write(current_kprobe_addr, (u64) tk->rp.kp.addr);
> >>> +}
> >>>    
> >>>    /* Kprobe profile handler */
> >>>    static int
> >>> @@ -1585,6 +1597,7 @@ kprobe_perf_func(struct trace_kprobe *tk, struct pt_regs *regs)
> >>>    		unsigned long orig_ip = instruction_pointer(regs);
> >>>    		int ret;
> >>>    
> >>> +		trace_current_kprobe_set(tk);
> >>>    		ret = trace_call_bpf(call, regs);
> >>>    
> >>>    		/*
> >>> @@ -1631,8 +1644,11 @@ kretprobe_perf_func(struct trace_kprobe *tk, struct kretprobe_instance *ri,
> >>>    	int size, __size, dsize;
> >>>    	int rctx;
> >>>    
> >>> -	if (bpf_prog_array_valid(call) && !trace_call_bpf(call, regs))
> >>> -		return;
> >>> +	if (bpf_prog_array_valid(call)) {
> >>> +		trace_current_kprobe_set(tk);
> >>> +		if (!trace_call_bpf(call, regs))
> >>> +			return;
> >>> +	}
> >>>    
> >>>    	head = this_cpu_ptr(call->perf_events);
> >>>    	if (hlist_empty(head))
> >> [...]
> > 
> > 

-- 
Masami Hiramatsu <[email-masked]>