Re: [PATCH v9 bpf-next 10/14] bpf: Add d_path helper

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

On Sat, Aug 1, 2020 at 10:04 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding d_path helper function that returns 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' and is zero terminated.
>
>   bpf_d_path(&file->f_path, buf, size);
>
> The helper calls directly d_path function, so there's only
> limited set of function it can be called from. Adding just
> very modest set for the start.
>
> 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       | 13 +++++++++
>  kernel/trace/bpf_trace.c       | 48 ++++++++++++++++++++++++++++++++++
>  scripts/bpf_helpers_doc.py     |  2 ++
>  tools/include/uapi/linux/bpf.h | 13 +++++++++
>  4 files changed, 76 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index eb5e0c38eb2c..a356ea1357bf 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3389,6 +3389,18 @@ union bpf_attr {
>   *             A non-negative value equal to or less than *size* on success,
>   *             or a negative error in case of failure.
>   *
> + * int bpf_d_path(struct path *path, char *buf, u32 sz)

nit: probably would be good to do `const struct path *` here, even if
we don't do const-ification properly in all helpers.

> + *     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' and

typo: in the provided buffer 'buf' of size ... ?

> + *             is zero terminated.
> + *
> + *     Return
> + *             On success, the strictly positive length of the string,
> + *             including the trailing NUL character. On error, a negative
> + *             value.
> + *
>   */
>  #define __BPF_FUNC_MAPPER(FN)          \
>         FN(unspec),                     \
> @@ -3533,6 +3545,7 @@ union bpf_attr {
>         FN(skc_to_tcp_request_sock),    \
>         FN(skc_to_udp6_sock),           \
>         FN(get_task_stack),             \
> +       FN(d_path),                     \
>         /* */
>
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index cb91ef902cc4..02a76e246545 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1098,6 +1098,52 @@ static const struct bpf_func_proto bpf_send_signal_thread_proto = {
>         .arg1_type      = ARG_ANYTHING,
>  };
>
> +BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> +{
> +       int len;
> +       char *p;
> +
> +       if (!sz)
> +               return -ENAMETOOLONG;

if we are modeling this after bpf_probe_read_str(), sz == 0 returns
success and just does nothing. I don't think anyone will ever handle
or expect this error. I'd just return 0.


> +
> +       p = d_path(path, buf, sz);
> +       if (IS_ERR(p)) {
> +               len = PTR_ERR(p);
> +       } else {
> +               len = buf + sz - p;
> +               memmove(buf, p, len);
> +       }
> +
> +       return len;
> +}
> +

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