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.
2. We can simply not generate btf for these functions. This is similar to
--skip_encoding_btf_inconsistent_proto.
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.
Please share your comments on this. Is this something we want to handle?
If so, which of these options makes more sense?
Thanks,
Song
