Re: [PATCH 1/9] bpf: Add d_path helper

Andrii Nakryiko
2020-05-14T22:06:14Z

On Wed, May 6, 2020 at 6:30 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding d_path helper function that returns full path
> for give 'struct path' object, which needs to be the
> kernel BTF 'path' object.
>
> The helper calls directly d_path function.
>
> Updating also bpf.h tools uapi header and adding
> 'path' to bpf_helpers_doc.py script.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> include/uapi/linux/bpf.h | 14 +++++++++++++-
> kernel/trace/bpf_trace.c | 31 +++++++++++++++++++++++++++++++
> scripts/bpf_helpers_doc.py | 2 ++
> tools/include/uapi/linux/bpf.h | 14 +++++++++++++-
> 4 files changed, 59 insertions(+), 2 deletions(-)
>

[...]

>
> +BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> +{
> + char *p = d_path(path, buf, sz - 1);
> + int len;
> +
> + if (IS_ERR(p)) {
> + len = PTR_ERR(p);
> + } else {
> + len = strlen(p);
> + if (len && p != buf) {
> + memmove(buf, p, len);
> + buf[len] = 0;
> + }
> + }
> +
> + return len;
> +}
> +
> +static u32 bpf_d_path_btf_ids[3];

Using shorter than 5 element array is "unconventional", but seems like
btf_distill_func_proto will never access elements that are not
ARG_PTR_TO_BTF_ID, so it's fine. But than again, if we are saving
space, why not just 1-element array? :)

> +static const struct bpf_func_proto bpf_d_path_proto = {
> + .func = bpf_d_path,
> + .gpl_only = true,
> + .ret_type = RET_INTEGER,
> + .arg1_type = ARG_PTR_TO_BTF_ID,
> + .arg2_type = ARG_PTR_TO_MEM,
> + .arg3_type = ARG_CONST_SIZE,
> + .btf_id = bpf_d_path_btf_ids,
> +};
> +

[...]

> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index b3643e27e264..bc13cad27872 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -3068,6 +3068,17 @@ union bpf_attr {
> * See: clock_gettime(CLOCK_BOOTTIME)
> * Return
> * Current *ktime*.
> + *
> + * int bpf_d_path(struct path *path, char *buf, u32 sz)
> + * Description
> + * Return full path for given 'struct path' object, which
> + * needs to be the kernel BTF 'path' object. The path is
> + * returned in buffer provided 'buf' of size 'sz'.
> + *

Please specify if it's always zero-terminated string (especially on truncation).

> + * Return
> + * length of returned string on success, or a negative
> + * error in case of failure
> + *
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -3195,7 +3206,8 @@ union bpf_attr {
> FN(get_netns_cookie), \
> FN(get_current_ancestor_cgroup_id), \
> FN(sk_assign), \
> - FN(ktime_get_boot_ns),
> + FN(ktime_get_boot_ns), \
> + FN(d_path),
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> * function eBPF program intends to call
> --
> 2.25.4
>