Re: [PATCH bpf-next] bpf: add support to read cpu_entry in bpf program

[Yonghong Song <yonghong.song@linux.dev>]

On 4/27/24 8:18 AM, Florian Lehner wrote:
> Add new field "cpu_entry" to bpf_perf_event_data which could be read by
> bpf programs attached to perf events. The value contains the CPU value
> recorded by specifying sample_type with PERF_SAMPLE_CPU when calling
> perf_event_open().

You can use bpf_cast_to_kern_ctx kfunc which can cast 'struct bpf_perf_event_data'
ctx to 'struct bpf_perf_event_data_kern'.

struct bpf_perf_event_data_kern {
         bpf_user_pt_regs_t *regs;
         struct perf_sample_data *data;
         struct perf_event *event;
};

You can access bpf_perf_event_data_kern->data and then to access 'cpu_entry' field.

>
> Signed-off-by: Florian Lehner <dev@der-flo.net>
> ---
>   include/uapi/linux/bpf_perf_event.h       |  4 ++++
>   kernel/trace/bpf_trace.c                  | 13 +++++++++++++
>   tools/include/uapi/linux/bpf_perf_event.h |  4 ++++
>   3 files changed, 21 insertions(+)
>
> diff --git a/include/uapi/linux/bpf_perf_event.h b/include/uapi/linux/bpf_perf_event.h
> index eb1b9d21250c..4856b4396ece 100644
> --- a/include/uapi/linux/bpf_perf_event.h
> +++ b/include/uapi/linux/bpf_perf_event.h
> @@ -14,6 +14,10 @@ struct bpf_perf_event_data {
>   	bpf_user_pt_regs_t regs;
>   	__u64 sample_period;
>   	__u64 addr;
> +	struct {
> +		u32	cpu;
> +		u32	reserved;
> +	}			cpu_entry;
>   };
>   
>   #endif /* _UAPI__LINUX_BPF_PERF_EVENT_H__ */
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index afb232b1d7c2..2b303221af5c 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2176,6 +2176,11 @@ static bool pe_prog_is_valid_access(int off, int size, enum bpf_access_type type
>   		if (!bpf_ctx_narrow_access_ok(off, size, size_u64))
>   			return false;
>   		break;
> +	case bpf_ctx_range(struct bpf_perf_event_data, cpu_entry):
> +		bpf_ctx_record_field_size(info, size_u64);
> +		if (!bpf_ctx_narrow_access_ok(off, size, size_u64))
> +			return false;
> +		break;
>   	default:
>   		if (size != sizeof(long))
>   			return false;
> @@ -2208,6 +2213,14 @@ static u32 pe_prog_convert_ctx_access(enum bpf_access_type type,
>   				      bpf_target_off(struct perf_sample_data, addr, 8,
>   						     target_size));
>   		break;
> +	case offsetof(struct bpf_perf_event_data, cpu_entry):
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct bpf_perf_event_data_kern,
> +						       data), si->dst_reg, si->src_reg,
> +				      offsetof(struct bpf_perf_event_data_kern, data));
> +		*insn++ = BPF_LDX_MEM(BPF_DW, si->dst_reg, si->dst_reg,
> +				      bpf_target_off(struct perf_sample_data, cpu_entry, 8,
> +						     target_size));
> +		break;
>   	default:
>   		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct bpf_perf_event_data_kern,
>   						       regs), si->dst_reg, si->src_reg,
> diff --git a/tools/include/uapi/linux/bpf_perf_event.h b/tools/include/uapi/linux/bpf_perf_event.h
> index eb1b9d21250c..4856b4396ece 100644
> --- a/tools/include/uapi/linux/bpf_perf_event.h
> +++ b/tools/include/uapi/linux/bpf_perf_event.h
> @@ -14,6 +14,10 @@ struct bpf_perf_event_data {
>   	bpf_user_pt_regs_t regs;
>   	__u64 sample_period;
>   	__u64 addr;
> +	struct {
> +		u32	cpu;
> +		u32	reserved;
> +	}			cpu_entry;
>   };
>   
>   #endif /* _UAPI__LINUX_BPF_PERF_EVENT_H__ */