On Thu, Nov 3, 2022 at 2:21 PM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> On 11/3/22 1:55 PM, Song Liu wrote:
> >
> >
> >> On Nov 3, 2022, at 12:45 PM, Yonghong Song <yhs@xxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 11/1/22 3:02 AM, Jiri Olsa wrote:
> >>> On Mon, Oct 31, 2022 at 10:23:39PM -0700, Namhyung Kim wrote:
> >>>> The bpf_perf_event_read_sample() helper is to get the specified sample
> >>>> data (by using PERF_SAMPLE_* flag in the argument) from BPF to make a
> >>>> decision for filtering on samples. Currently PERF_SAMPLE_IP and
> >>>> PERF_SAMPLE_DATA flags are supported only.
> >>>>
> >>>> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> >>>> ---
> >>>> include/uapi/linux/bpf.h | 23 ++++++++++++++++
> >>>> kernel/trace/bpf_trace.c | 49 ++++++++++++++++++++++++++++++++++
> >>>> tools/include/uapi/linux/bpf.h | 23 ++++++++++++++++
> >>>> 3 files changed, 95 insertions(+)
> >>>>
> >>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >>>> index 94659f6b3395..cba501de9373 100644
> >>>> --- a/include/uapi/linux/bpf.h
> >>>> +++ b/include/uapi/linux/bpf.h
> >>>> @@ -5481,6 +5481,28 @@ union bpf_attr {
> >>>> * 0 on success.
> >>>> *
> >>>> * **-ENOENT** if the bpf_local_storage cannot be found.
> >>>> + *
> >>>> + * long bpf_perf_event_read_sample(struct bpf_perf_event_data *ctx, void *buf, u32 size, u64 sample_flags)
> >>>> + * Description
> >>>> + * For an eBPF program attached to a perf event, retrieve the
> >>>> + * sample data associated to *ctx* and store it in the buffer
> >>>> + * pointed by *buf* up to size *size* bytes.
> >>>> + *
> >>>> + * The *sample_flags* should contain a single value in the
> >>>> + * **enum perf_event_sample_format**.
> >>>> + * Return
> >>>> + * On success, number of bytes written to *buf*. On error, a
> >>>> + * negative value.
> >>>> + *
> >>>> + * The *buf* can be set to **NULL** to return the number of bytes
> >>>> + * required to store the requested sample data.
> >>>> + *
> >>>> + * **-EINVAL** if *sample_flags* is not a PERF_SAMPLE_* flag.
> >>>> + *
> >>>> + * **-ENOENT** if the associated perf event doesn't have the data.
> >>>> + *
> >>>> + * **-ENOSYS** if system doesn't support the sample data to be
> >>>> + * retrieved.
> >>>> */
> >>>> #define ___BPF_FUNC_MAPPER(FN, ctx...) \
> >>>> FN(unspec, 0, ##ctx) \
> >>>> @@ -5695,6 +5717,7 @@ union bpf_attr {
> >>>> FN(user_ringbuf_drain, 209, ##ctx) \
> >>>> FN(cgrp_storage_get, 210, ##ctx) \
> >>>> FN(cgrp_storage_delete, 211, ##ctx) \
> >>>> + FN(perf_event_read_sample, 212, ##ctx) \
> >>>> /* */
> >>>> /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
> >>>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> >>>> index ce0228c72a93..befd937afa3c 100644
> >>>> --- a/kernel/trace/bpf_trace.c
> >>>> +++ b/kernel/trace/bpf_trace.c
> >>>> @@ -28,6 +28,7 @@
> >>>> #include <uapi/linux/bpf.h>
> >>>> #include <uapi/linux/btf.h>
> >>>> +#include <uapi/linux/perf_event.h>
> >>>> #include <asm/tlb.h>
> >>>> @@ -1743,6 +1744,52 @@ static const struct bpf_func_proto bpf_read_branch_records_proto = {
> >>>> .arg4_type = ARG_ANYTHING,
> >>>> };
> >>>> +BPF_CALL_4(bpf_perf_event_read_sample, struct bpf_perf_event_data_kern *, ctx,
> >>>> + void *, buf, u32, size, u64, flags)
> >>>> +{
> >>> I wonder we could add perf_btf (like we have tp_btf) program type that
> >>> could access ctx->data directly without helpers
> >>
> >> Martin and I have discussed an idea to introduce a generic helper like
> >> bpf_get_kern_ctx(void *ctx)
> >> Given a context, the helper will return a PTR_TO_BTF_ID representing the
> >> corresponding kernel ctx. So in the above example, user could call
> >>
> >> struct bpf_perf_event_data_kern *kctx = bpf_get_kern_ctx(ctx);
> >> ...
> >
> > This is an interesting idea!
> >
> >> To implement bpf_get_kern_ctx helper, the verifier can find the type
> >> of the context and provide a hidden btf_id as the second parameter of
> >> the actual kernel helper function like
> >> bpf_get_kern_ctx(ctx) {
> >> return ctx;
> >> }
> >> /* based on ctx_btf_id, find kctx_btf_id and return it to verifier */
> >
> > I think we will need a map of ctx_btf_id => kctx_btf_id. Shall we somehow
> > expose this to the user?
>
> Yes, inside the kernel we need ctx_btf_id -> kctx_btf_id mapping.
> Good question. We might not want to this mapping as a stable API.
> So using kfunc might be more appropriate.
Ok, now I don't think I'm following well.. ;-)
So currently perf event type BPF programs can have perf_event
data context directly as an argument, but we want to disallow it?
I guess the context id mapping can be done implicitly based on
the prog type and/or attach type, but probably I'm missing
something here. :)
Thanks,
Namhyung
