Re: [PATCH bpf-next] bpf: add new helper fd2path for mapping a file descriptor to a pathname

[Yonghong Song <yhs@fb.com>]

On 10/17/19 2:26 AM, Zwb wrote:
> When people want to identify which file system files are being opened,
> read, and written to, they can use this helper with file descriptor as
> input to achieve this goal. Other pseudo filesystems are also supported.

For the description, a link to related bcc issue may provide more 
background why this is needed.

Please send to bpf mailing list bpf@vger.kernel.org. This way, more 
people can catch and look at this patch.

Please provide a test case in tools/testing/selftests/bpf directory.

> 
> Signed-off-by: Zwb <ethercflow@gmail.com>
> ---
>   include/linux/bpf.h            |  1 +
>   include/uapi/linux/bpf.h       |  1 +
>   kernel/bpf/core.c              |  1 +
>   kernel/bpf/helpers.c           | 39 ++++++++++++++++++++++++++++++++++
>   kernel/trace/bpf_trace.c       |  2 ++
>   tools/include/uapi/linux/bpf.h |  1 +
>   6 files changed, 45 insertions(+)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 282e28bf41ec..c0a710cf2c88 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1055,6 +1055,7 @@ extern const struct bpf_func_proto bpf_get_local_storage_proto;
>   extern const struct bpf_func_proto bpf_strtol_proto;
>   extern const struct bpf_func_proto bpf_strtoul_proto;
>   extern const struct bpf_func_proto bpf_tcp_sock_proto;
> +extern const struct bpf_func_proto bpf_fd2path_proto;
>   
>   /* Shared helpers among cBPF and eBPF. */
>   void bpf_user_rnd_init_once(void);
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index a65c3b0c6935..a4a5d432e572 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2769,6 +2769,7 @@ union bpf_attr {
>   	FN(get_current_pid_tgid),	\
>   	FN(get_current_uid_gid),	\
>   	FN(get_current_comm),		\
> +	FN(fd2path),			\

As Daniel mentioned, please put this at the end of enum to
avoid breaking backward compatibility. Also this needs proper
documentation.

>   	FN(get_cgroup_classid),		\
>   	FN(skb_vlan_push),		\
>   	FN(skb_vlan_pop),		\
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 66088a9e9b9e..349a8b1be232 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2042,6 +2042,7 @@ const struct bpf_func_proto bpf_get_current_uid_gid_proto __weak;
>   const struct bpf_func_proto bpf_get_current_comm_proto __weak;
>   const struct bpf_func_proto bpf_get_current_cgroup_id_proto __weak;
>   const struct bpf_func_proto bpf_get_local_storage_proto __weak;
> +const struct bpf_func_proto bpf_fd2path_proto __weak;
>   
>   const struct bpf_func_proto * __weak bpf_get_trace_printk_proto(void)
>   {
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 5e28718928ca..0832536c7ddb 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -487,3 +487,42 @@ const struct bpf_func_proto bpf_strtoul_proto = {
>   	.arg4_type	= ARG_PTR_TO_LONG,
>   };
>   #endif
> +
> +BPF_CALL_3(bpf_fd2path, char *, dst, u32, size, int, fd)
> +{
> +	struct fd f;
> +	int ret;
> +	char *p;
> +
> +	ret = security_locked_down(LOCKDOWN_BPF_READ);
> +	if (ret < 0)
> +		goto out;

The LOCKDOWN_BPF_READ is used when kernel internal data is exposed.
That is why currently only bpf_probe_read() and bpf_probe_read_str()
are checked.
Not 100% sure whether this helper needs check of LOCKDOWN_BPF_READ
or not.

> +
> +	f = fdget_raw(fd);
> +	if (!f.file)
> +		goto out;
> +
> +	p = d_path(&f.file->f_path, dst, size);

inside d_path, spin_lock() could be called. I guess it is okay.

> +	if (IS_ERR_OR_NULL(p))
> +		ret = PTR_ERR(p);
> +	else {
> +		ret = strlen(p);
> +		memmove(dst, p, ret);
> +		dst[ret] = 0;
> +	}
> +
> +	if (unlikely(ret < 0))
> +out:
> +		memset(dst, '0', size);

The coding style here is not great. Maybe you can do
	if (IS_ERR_OR_NULL(p)) {
		ret = PTRERR(p);
		goto error;
	}

	ret = strlen(p);
	memmove(dst, p, ret);
	dst[ret] = '\0';
	return ret;

    error:
	memset(dst, '0', size);
	return ret;

> +
> +	return ret;
> +}
> +
> +const struct bpf_func_proto bpf_fd2path_proto = {
> +	.func       = bpf_fd2path,
> +	.gpl_only   = true,
> +	.ret_type   = RET_INTEGER,
> +	.arg1_type  = ARG_PTR_TO_UNINIT_MEM,
> +	.arg2_type  = ARG_CONST_SIZE,
> +	.arg3_type  = ARG_ANYTHING,
> +};
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 44bd08f2443b..0ca7fdefb8e5 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -735,6 +735,8 @@ tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   #endif
>   	case BPF_FUNC_send_signal:
>   		return &bpf_send_signal_proto;
> +	case BPF_FUNC_fd2path:
> +		return &bpf_fd2path_proto;
>   	default:
>   		return NULL;
>   	}
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index a65c3b0c6935..a4a5d432e572 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -2769,6 +2769,7 @@ union bpf_attr {
>   	FN(get_current_pid_tgid),	\
>   	FN(get_current_uid_gid),	\
>   	FN(get_current_comm),		\
> +	FN(fd2path),			\
>   	FN(get_cgroup_classid),		\
>   	FN(skb_vlan_push),		\
>   	FN(skb_vlan_pop),		\
>