Re: [RFC PATCH bpf-next v2 1/4] tracing/kprobe: bpf: Check error injectable event is on function entry

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Dec 26, 2017 at 04:46:59PM +0900, Masami Hiramatsu wrote:
> Check whether error injectable event is on function entry or not.
> Currently it checks the event is ftrace-based kprobes or not,
> but that is wrong. It should check if the event is on the entry
> of target function. Since error injection will override a function
> to just return with modified return value, that operation must
> be done before the target function starts making stackframe.
> 
> As a side effect, bpf error injection is no need to depend on
> function-tracer. It can work with sw-breakpoint based kprobe
> events too.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>  kernel/trace/Kconfig        |    2 --
>  kernel/trace/bpf_trace.c    |    6 +++---
>  kernel/trace/trace_kprobe.c |    8 +++++---
>  kernel/trace/trace_probe.h  |   12 ++++++------
>  4 files changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index ae3a2d519e50..6400e1bf97c5 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -533,9 +533,7 @@ config FUNCTION_PROFILER
>  config BPF_KPROBE_OVERRIDE
>  	bool "Enable BPF programs to override a kprobed function"
>  	depends on BPF_EVENTS
> -	depends on KPROBES_ON_FTRACE
>  	depends on HAVE_KPROBE_OVERRIDE
> -	depends on DYNAMIC_FTRACE_WITH_REGS
>  	default n
>  	help
>  	 Allows BPF to override the execution of a probed function and
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index f6d2327ecb59..d663660f8392 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -800,11 +800,11 @@ int perf_event_attach_bpf_prog(struct perf_event *event,
>  	int ret = -EEXIST;
>  
>  	/*
> -	 * Kprobe override only works for ftrace based kprobes, and only if they
> -	 * are on the opt-in list.
> +	 * Kprobe override only works if they are on the function entry,
> +	 * and only if they are on the opt-in list.
>  	 */
>  	if (prog->kprobe_override &&
> -	    (!trace_kprobe_ftrace(event->tp_event) ||
> +	    (!trace_kprobe_on_func_entry(event->tp_event) ||
>  	     !trace_kprobe_error_injectable(event->tp_event)))
>  		return -EINVAL;
>  
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 91f4b57dab82..265e3e27e8dc 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -88,13 +88,15 @@ static nokprobe_inline unsigned long trace_kprobe_nhit(struct trace_kprobe *tk)
>  	return nhit;
>  }
>  
> -int trace_kprobe_ftrace(struct trace_event_call *call)
> +bool trace_kprobe_on_func_entry(struct trace_event_call *call)
>  {
>  	struct trace_kprobe *tk = (struct trace_kprobe *)call->data;
> -	return kprobe_ftrace(&tk->rp.kp);
> +
> +	return kprobe_on_func_entry(tk->rp.kp.addr, tk->rp.kp.symbol_name,
> +				    tk->rp.kp.offset);

That would be nice, but did you test this?
My understanding that kprobe will restore all regs and
here we need to override return ip _and_ value.
Could you add a patch with the test the way Josef did
or describe the steps to test this new mode?