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Date:   Thu, 3 Nov 2022 12:45:47 -0700
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Subject: Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper
Content-Language: en-US
To:     Jiri Olsa <olsajiri@gmail.com>, Namhyung Kim <namhyung@kernel.org>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Martin KaFai Lau <kafai@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
        Stanislav Fomichev <sdf@google.com>,
        LKML <linux-kernel@vger.kernel.org>, bpf@vger.kernel.org,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
References: <20221101052340.1210239-1-namhyung@kernel.org>
 <20221101052340.1210239-3-namhyung@kernel.org> <Y2DuzmnUm6NIh25a@krava>
From:   Yonghong Song <yhs@meta.com>
In-Reply-To: <Y2DuzmnUm6NIh25a@krava>
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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@kernel.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,
>> +};
>> +
>[...]