On Tue, Aug 9, 2022 at 10:38 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 8/8/22 5:02 PM, Andrii Nakryiko wrote:
> > On Tue, Jul 26, 2022 at 10:11 AM Yonghong Song <yhs@xxxxxx> wrote:
> >>
> >> Add struct argument information in btf_func_model and such information
> >> will be used in arch specific function arch_prepare_bpf_trampoline()
> >> to prepare argument access properly in trampoline.
> >>
> >> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> >> ---
> >> include/linux/bpf.h | 9 +++++++++
> >> 1 file changed, 9 insertions(+)
> >>
> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> >> index 20c26aed7896..173b42cf3940 100644
> >> --- a/include/linux/bpf.h
> >> +++ b/include/linux/bpf.h
> >> @@ -726,10 +726,19 @@ enum bpf_cgroup_storage_type {
> >> */
> >> #define MAX_BPF_FUNC_REG_ARGS 5
> >>
> >> +/* The maximum number of struct arguments a single function may have. */
> >> +#define MAX_BPF_FUNC_STRUCT_ARGS 2
> >> +
> >> struct btf_func_model {
> >> u8 ret_size;
> >> u8 nr_args;
> >> u8 arg_size[MAX_BPF_FUNC_ARGS];
> >> + /* The struct_arg_idx should be in increasing order like (0, 2, ...).
> >> + * The struct_arg_bsize encodes the struct field byte size
> >> + * for the corresponding struct argument index.
> >> + */
> >> + u8 struct_arg_idx[MAX_BPF_FUNC_STRUCT_ARGS];
> >> + u8 struct_arg_bsize[MAX_BPF_FUNC_STRUCT_ARGS];
> >
> > Few questions here. It might be a bad idea, but I thought I'd bring it
> > up anyway.
> >
> > So, is there any benefit into having these separate struct_arg_idx and
> > struct_arg_bsize fields and then processing arg_size completely
> > separate from struct_arg_idx/struct_arg_bsize in patch #4? Reading
> > patch #4 it felt like it would be much easier to keep track of things
> > if we had a single loop going over all the arguments, and then if some
> > argument is a struct -- do some extra step to copy up to 16 bytes onto
> > stack and store the pointer there (and skip up to one extra argument).
> > And if it's not a struct arg -- do what we do right now.
> >
> > What if instead we keep btf_func_mode definition as is, but for struct
> > argument we add extra flag to arg_size[struct_arg_idx] value to mark
> > that it is a struct argument. This limits arg_size to 128 bytes, but I
> > think it's more than enough for both struct and non-struct cases,
> > right? Distill function would make sure that nr_args matches number of
> > logical arguments and not number of registers.
> >
> > Would that work? Would that make anything harder to implement in
> > arch-specific code? I don't see what, but I haven't grokked all the
> > details of patch #4, so I'm sorry if I missed something obvious. The
> > way I see it, it will make overall logic for saving/restoring
> > registers more uniform, roughly:
> >
> > for (int arg_idx = 0; arg_idx < model->arg_size; arg_idx++) {
> > if (arg & BTF_FMODEL_STRUCT_ARG) {
> > /* handle struct, calc extra registers "consumed" from
> > arg_size[arg_idx] ~BTF_FMODEL_STRUCT_ARG */
> > } else {
> > /* just a normal register */
> > }
> > }
>
> Your suggestion sounds good to me. Yes, we already have
> arg_size array. We can add a
> bool is_struct_arg[MAX_BPF_FUNC_ARGS];
> to indicate whether the argument is a struct or not.
> In this case, we can avoid duplication between
> arg_size and struct_arg_bsize.
>
I was imagining that we'll just use the existing arg_size and define
that the upper bit is a struct/non-struct bit. But if that's too
confusing and cryptic, I wonder if it's better to have
u8 arg_flags[MAX_BPF_FUNC_ARGS];
instead and define the BPF_FNARG_STRUCT flag.
For what you did in your other patch set (u8/u16 handling for func
result), we can then define ret_flags and have a flag whether the
argument is integer and whether it's signed in such flags (instead of
bit fields).
This way we have a unified and more extendable "size+flags" approach
both for input arguments and return result.
WDYT?
> >
> >
> > If we do stick to current approach, though, let's please
> > s/struct_arg_bsize/struct_arg_size/. Isn't arg_size also and already
> > in bytes? It will keep naming and meaning consistent across struct and
> > non-struct args.
> >
> > BTW, by not having btf_func_model encode register indices in
> > struct_arg_idx we keep btf_func_model pretty architecture-agnostic,
> > right? It will be per each architecture specific implementation to
> > perform mapping this *model* into actual registers used?
> >
> >
> >
> >
> >> };
> >>
> >> /* Restore arguments before returning from trampoline to let original function
> >> --
> >> 2.30.2
> >>
