Re: [RFC PATCH bpf-next 7/8] bpf: Support ->fill_link_info for perf_event

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Yafang Shao <laoar.shao@gmail.com>
Cc: ast@kernel.org, daniel@iogearbox.net, john.fastabend@gmail.com,
	andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
	yhs@fb.com, kpsingh@kernel.org, sdf@google.com,
	haoluo@google.com, jolsa@kernel.org, quentin@isovalent.com,
	bpf@vger.kernel.org
Subject: Re: [RFC PATCH bpf-next 7/8] bpf: Support ->fill_link_info for perf_event
Date: Tue, 30 May 2023 17:37:13 -0700	[thread overview]
Message-ID: <20230531003713.ml3gb76q4zurue3a@MacBook-Pro-8.local> (raw)
In-Reply-To: <20230528142027.5585-8-laoar.shao@gmail.com>

On Sun, May 28, 2023 at 02:20:26PM +0000, Yafang Shao wrote:
> By adding support for ->fill_link_info to the perf_event link, users will
> be able to inspect it using `bpftool link show`. While users can currently
> access this information via `bpftool perf show`, consolidating the link
> information for all link types in one place would be more convenient.
> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  include/uapi/linux/bpf.h       |  6 ++++++
>  kernel/bpf/syscall.c           | 46 ++++++++++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  6 ++++++
>  3 files changed, 58 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 6be9b1d..1f2be1d 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -6438,6 +6438,12 @@ struct bpf_link_info {
>  			__aligned_u64 addrs;
>  			__u32 count;
>  		} kprobe_multi;
> +		struct {
> +			__aligned_u64 name;
> +			__aligned_u64 addr;

__aligned_u64 ? what is the reason?

> +			__u32 name_len;
> +			__u32 offset;
> +		} perf_event;
>  	};
>  } __attribute__((aligned(8)));
>  
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 33a72ec..b12707e 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3329,10 +3329,56 @@ static void bpf_perf_link_show_fdinfo(const struct bpf_link *link,
>  	seq_printf(seq, "offset:\t%llu\n", probe_offset);
>  }
>  
> +static int bpf_perf_link_fill_link_info(const struct bpf_link *link,
> +					struct bpf_link_info *info)
> +{
> +	struct bpf_perf_link *perf_link = container_of(link, struct bpf_perf_link, link);
> +	char __user *ubuf = u64_to_user_ptr(info->perf_event.name);
> +	u32 ulen = info->perf_event.name_len;
> +	const struct perf_event *event;
> +	u64 probe_offset, probe_addr;
> +	u32 prog_id, fd_type;
> +	const char *buf;
> +	size_t len;
> +	int err;
> +
> +	if (!ulen ^ !ubuf)
> +		return -EINVAL;
> +	if (!ubuf)
> +		return 0;
> +
> +	event = perf_get_event(perf_link->perf_file);
> +	if (IS_ERR(event))
> +		return PTR_ERR(event);
> +
> +	err = bpf_get_perf_event_info(event, &prog_id, &fd_type,
> +				      &buf, &probe_offset,
> +				      &probe_addr);
> +	if (err)
> +		return err;
> +
> +	len = strlen(buf);
> +	info->perf_event.name_len = len + 1;

this use of name_len contradicts with patch 8.
Is it 'in' or 'out' field?

> +	if (buf) {
> +		err = bpf_copy_to_user(ubuf, buf, ulen, len);
> +		if (err)
> +			return err;
> +	} else {
> +		char zero = '\0';
> +
> +		if (put_user(zero, ubuf))
> +			return -EFAULT;
> +	}
> +	info->perf_event.addr = probe_addr;
> +	info->perf_event.offset = probe_offset;
> +	return 0;
> +}
> +
>  static const struct bpf_link_ops bpf_perf_link_lops = {
>  	.release = bpf_perf_link_release,
>  	.dealloc = bpf_perf_link_dealloc,
>  	.show_fdinfo = bpf_perf_link_show_fdinfo,
> +	.fill_link_info = bpf_perf_link_fill_link_info,
>  };
>  
>  static int bpf_perf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 6be9b1d..1f2be1d 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -6438,6 +6438,12 @@ struct bpf_link_info {
>  			__aligned_u64 addrs;
>  			__u32 count;
>  		} kprobe_multi;
> +		struct {
> +			__aligned_u64 name;
> +			__aligned_u64 addr;
> +			__u32 name_len;
> +			__u32 offset;
> +		} perf_event;
>  	};
>  } __attribute__((aligned(8)));
>  
> -- 
> 1.8.3.1
> 