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Date: Thu, 3 Nov 2022 12:45:47 -0700
From: Yonghong Song <yhs@...a.com>
To: Jiri Olsa <olsajiri@...il.com>, Namhyung Kim <namhyung@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Song Liu <songliubraving@...com>,
Peter Zijlstra <peterz@...radead.org>,
Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
Stanislav Fomichev <sdf@...gle.com>,
LKML <linux-kernel@...r.kernel.org>, bpf@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>,
Ingo Molnar <mingo@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper
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@...nel.org>
>> ---
>> 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);
...
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 */
The bpf_get_kern_ctx helper can be inlined as well.
>
>> + struct perf_sample_data *sd = ctx->data;
>> + void *data;
>> + u32 to_copy = sizeof(u64);
>> +
>> + /* only allow a single sample flag */
>> + if (!is_power_of_2(flags))
>> + return -EINVAL;
>> +
>> + /* support reading only already populated info */
>> + if (flags & ~sd->sample_flags)
>> + return -ENOENT;
>> +
>> + switch (flags) {
>> + case PERF_SAMPLE_IP:
>> + data = &sd->ip;
>> + break;
>> + case PERF_SAMPLE_ADDR:
>> + data = &sd->addr;
>> + break;
>
> AFAICS from pe_prog_convert_ctx_access you should be able to read addr
> directly from context right? same as sample_period.. so I think if this
> will be generic way to read sample data, should we add sample_period
> as well?
>
>
>> + default:
>> + return -ENOSYS;
>> + }
>> +
>> + if (!buf)
>> + return to_copy;
>> +
>> + if (size < to_copy)
>> + to_copy = size;
>
> should we fail in here instead? is there any point in returning
> not complete data?
>
> jirka
>
>
>> +
>> + memcpy(buf, data, to_copy);
>> + return to_copy;
>> +}
>> +
>> +static const struct bpf_func_proto bpf_perf_event_read_sample_proto = {
>> + .func = bpf_perf_event_read_sample,
>> + .gpl_only = true,
>> + .ret_type = RET_INTEGER,
>> + .arg1_type = ARG_PTR_TO_CTX,
>> + .arg2_type = ARG_PTR_TO_MEM_OR_NULL,
>> + .arg3_type = ARG_CONST_SIZE_OR_ZERO,
>> + .arg4_type = ARG_ANYTHING,
>> +};
>> +
>[...]
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