Hi Song,
On Thu, Jan 11, 2024 at 09:51:05PM +0000, Song Liu 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.
Could you clarify what offline means?
I wonder if libbpf should just give a way for applications to query
inline status. Seems most flexible. And maybe also a special SEC() def?
Although I suspect end users might want a flag to "attach anyways; I'm
aware", so a more generic api (eg `btf__is_inlined(..)`) results in a
smaller and maximally flexible result.
> 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.
Seems reasonable. Decl tag is another option but probably too heavy.
> 2. We can simply not generate btf for these functions. This is similar to
> --skip_encoding_btf_inconsistent_proto.
This option seems a bit confusing. Basically you're suggesting that:
func_in_avilable_filter_functions && !func_in_btf
implies partially inlined function, right? Seems a bit non-obvious.
[...]
Thanks,
Daniel
