Re: [PATCHv3 bpf-next 5/6] bpf: Return value in kprobe get_func_ip only for entry address

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 09, 2022 at 12:12:44PM +0200, Jiri Olsa wrote:
> Changing return value of kprobe's version of bpf_get_func_ip
> to return zero if the attach address is not on the function's
> entry point.
> 
> For kprobes attached in the middle of the function we can't easily
> get to the function address especially now with the CONFIG_X86_KERNEL_IBT
> support.
> 
> If user cares about current IP for kprobes attached within the
> function body, they can get it with PT_REGS_IP(ctx).
> 
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/trace/bpf_trace.c                             | 11 ++++++++++-
>  tools/testing/selftests/bpf/progs/get_func_ip_test.c |  4 ++--
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index bcada91b0b3b..027abc38faab 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1029,8 +1029,17 @@ static const struct bpf_func_proto bpf_get_func_ip_proto_tracing = {
>  BPF_CALL_1(bpf_get_func_ip_kprobe, struct pt_regs *, regs)
>  {
>  	struct kprobe *kp = kprobe_running();
> +	uintptr_t addr;
>  
> -	return kp ? (uintptr_t)kp->addr : 0;
> +	if (!kp || !(kp->flags & KPROBE_FLAG_ON_FUNC_ENTRY))
> +		return 0;
> +
> +	addr = (uintptr_t)kp->addr;
> +#ifdef CONFIG_X86_KERNEL_IBT
> +	if (is_endbr(*((u32 *) addr - 1)))
> +		addr -= ENDBR_INSN_SIZE;
> +#endif

This has the same problem; -1 might not be a valid address. But since
this it not he multi stuff, I think you can more easily store state if
this correction is needed or not.