On Tue, 19 May 2026 08:31:28 -0400
Steven Rostedt <[email protected]> wrote:
> >
> > To access @data, simple casting does not work. Thus we need a
> > new syntax:
> >
> > (STRUCT)(PTR,ASSIGN)->FIELD
> >
> > So the above case, we can do:
> >
> > data=(foo)(foo_list,list)->data
>
> Hmm, it may be better to make it one parenthesis?
>
> (STRUCT,PTR,ASSIGN)->FIELD
>
> data=(foo,foo_list,list)->data
OK, but I don't like this 3 parameters conversion. I want to
make it a kind of type casting with an option.
(STRUCT,ASSIGN)PTR->FIELD
data=(foo,list)foo_list->data
The second parenthesis will be eventually needed for nested casting,
for example, in above case, if the data is a pointer to another data
structure:
struct bar {
int value;
...
};
value=(bar)((foo,list)foo_list->data)->value
>
> That would make it easier to differentiate between a simple "typecast" and
> a container_of() by checking if the content between the parenthesis has a
> comma.
>
> Maybe even reorder it to:
>
> (PTR,STRUCT,ASSIGN)->FIELD
>
> data=(foo_list,foo,list)->data
>
> to match the order of container_of():
>
> data = container_of(foo_list, struct foo, list)->data;
>
> ?
This doesn't seem to conform to the rule here of using parentheses for
type casting, so I personally don't like it.
> > > diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> > > index e0d3a0da26af..b0829eb1cb52 100644
> > > --- a/kernel/trace/trace_probe.c
> > > +++ b/kernel/trace/trace_probe.c
> > > @@ -464,6 +464,26 @@ static const char *fetch_type_from_btf_type(struct
> > > btf *btf,
> > > return NULL;
> > > }
> > >
> > > +static int query_btf_struct(const char *sname, struct
> > > traceprobe_parse_context *ctx)
> > > +{
> > > + int id;
> > > +
> > > + if (!ctx->btf) {
> > > + struct btf *btf;
> >
> > This needs an empty line here.
>
> Sure.
>
> For conditional blocks, I don't always add a newline, but this is your code
> and I'll follow your suggestions.
Ah, this is just for fixing checkpatch.pl warning :-)
>
> >
> > > + id = bpf_find_btf_id(sname, BTF_KIND_STRUCT, &btf);
> > > + if (id < 0)
> > > + return -EINVAL;
> >
> > Why don't you return id (it has corresponding errno)?
>
> Because I forgot to ;-)
>
> >
> > > + ctx->btf = btf;
> > > + } else {
> > > + id = btf_find_by_name_kind(ctx->btf, sname, BTF_KIND_STRUCT);
> > > + if (id < 0)
> > > + return -EINVAL;
> >
> > Ditto.
> >
> > > + }
> > > +
> > > + ctx->last_struct = btf_type_by_id(ctx->btf, id);
> > > + return 0;
> > > +}
> > > +
> > > static int query_btf_context(struct traceprobe_parse_context *ctx)
> > > {
> > > const struct btf_param *param;
> > > @@ -471,12 +491,12 @@ static int query_btf_context(struct
> > > traceprobe_parse_context *ctx)
> > > struct btf *btf;
> > > s32 nr;
> > >
> > > - if (ctx->btf)
> > > - return 0;
> > > -
> > > if (!ctx->funcname)
> > > return -EINVAL;
> > >
> > > + if (ctx->btf)
> > > + return 0;
> > > +
> >
> > Could you tell me why this order is changed?
> > I think this type casting will allow us to skip checking funcname
> > because btf context is already specified.
>
> I wanted this to fail if btf was already set but funcname wasn't, because
> this should only be called for functions.
Hmm, OK. Then, can you make a separate patch for this?
>
> >
> > Ah, BTW, we may need to use a special struct btf* for type
> > casting. If the target function is in a module and the
> > casting type is defined in vmlinux, those are stored in
> > the different places...
>
> OK, I'll make a separate btf for it then. I'll have to make sure the btf
> used for parsing knows which one to use. Shouldn't be too hard if we check
> for the STRUCT flag in the ctx->flags.
Yeah, and personally, I think that flag should be the TYPECAST flag.
Thank you!
--
Masami Hiramatsu (Google) <[email protected]>
