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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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
>