On Thu, Jan 11, 2024 at 1:51 PM Song Liu <songliubraving@xxxxxxxx> wrote:
>
> The problem
>
> Inlining can cause surprises to tracing users, especially when the tool
> appears to be working. For example, with
>
> [root@ ~]# bpftrace -e 'kprobe:switch_mm {}'
> Attaching 1 probe...
>
> The user may not realize switch_mm() is inlined by leave_mm(), and we are
> not tracing the code path leave_mm => switch_mm. (This is x86_64, and both
> functions are in arch/x86/mm/tlb.c.)
>
> We have folks working on ideas to create offline tools to detect such
> issues for critical use cases at compile time. However, I think it is
> necessary to handle it at program load/attach time.
>
>
> Detect "inlined by some callers" functions
>
> This appears to be straightforward in pahole. Something like the following
> should do the work:
>
> diff --git i/btf_encoder.c w/btf_encoder.c
> index fd040086827e..e546a059eb4b 100644
> --- i/btf_encoder.c
> +++ w/btf_encoder.c
> @@ -885,6 +885,15 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder, struct functio
> struct llvm_annotation *annot;
> const char *name;
>
> + if (function__inlined(fn)) {
> + /* This function is inlined by some callers. */
> + }
> +
> btf_fnproto_id = btf_encoder__add_func_proto(encoder, &fn->proto);
> name = function__name(fn);
> btf_fn_id = btf_encoder__add_ref_type(encoder, BTF_KIND_FUNC, btf_fnproto_id, name, false);
>
>
> Mark "inlined by some callers" functions
>
> We have a few options to mark these functions.
>
> 1. We can set struct btf_type.info.kind_flag for inlined function. Or we
> can use a bit from info.vlen.
It doesn't feel right to use kflag or vlen for this particular
property. I think decl_tag is the generic way to have extra
information/annotation.
>
> 2. We can simply not generate btf for these functions. This is similar to
> --skip_encoding_btf_inconsistent_proto.
This is too drastic, IMO. Even if some function was inlined somewhere,
it still might be important to trace non-inlined calls. So just
removing BTF is a regression in behavior.
>
>
> Handle tracing inlined functions
>
> If we go with option 1 above, we have a few options to handle program
> load/attach to "inlined by some callers" functions:
>
> a) We can reject the load/attach;
> b) We can rejuct the load/attach, unless the user set a new flag;
> c) We can simply print a warning, and let the load/attach work.
>
I'd start with c), probably. Everything else is a regression in
behavior compared to what we have today.
>
> Please share your comments on this. Is this something we want to handle?
> If so, which of these options makes more sense?
>
> Thanks,
> Song
>
>
