Re: [PATCH v8 bpf-next 09/13] bpf: Add d_path helper

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

On Wed, Jul 22, 2020 at 2:14 PM 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(+)
>

[...]

>
> +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);

not sure if it's worth it, but if len == sz - 1 then memmove is not
necessary. Again, don't know if worth it, as it's probably not going
to be a common case.

> +               buf[len] = 0;
> +               /* Include the trailing NUL. */
> +               len++;
> +       }
> +
> +       return len;
> +}
> +
> +BTF_SET_START(btf_whitelist_d_path)
> +BTF_ID(func, vfs_truncate)
> +BTF_ID(func, vfs_fallocate)
> +BTF_ID(func, dentry_open)
> +BTF_ID(func, vfs_getattr)
> +BTF_ID(func, filp_close)
> +BTF_SET_END(btf_whitelist_d_path)


We should probably comply with an updated coding style ([0]) and use
an allowlist name for this?

  [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=49decddd39e5f6132ccd7d9fdc3d7c470b0061bb

> +
> +static bool bpf_d_path_allowed(const struct bpf_prog *prog)
> +{
> +       return btf_id_set_contains(&btf_whitelist_d_path, prog->aux->attach_btf_id);
> +}
> +
> +BTF_ID_LIST(bpf_d_path_btf_ids)
> +BTF_ID(struct, path)
> +
> +static const struct bpf_func_proto bpf_d_path_proto = {
> +       .func           = bpf_d_path,
> +       .gpl_only       = false,
> +       .ret_type       = RET_INTEGER,
> +       .arg1_type      = ARG_PTR_TO_BTF_ID,
> +       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg3_type      = ARG_CONST_SIZE,

I feel like we had a discussion about ARG_CONST_SIZE vs
ARG_CONST_SIZE_OR_ZERO before, maybe on some different thread.
Basically, this >0 restriction was a major nuisance for
bpf_perf_event_output() cases, so much that we changed it to _OR_ZERO.
In practice, while it might never be the case that we have sz == 0
passed into the function, having to prove this to the verifier is a
PITA. Unless there is a very strong reason not to, let's mark this as
ARG_CONST_SIZE_OR_ZERO and handle sz == 0 case as a noop?

> +       .btf_id         = bpf_d_path_btf_ids,
> +       .allowed        = bpf_d_path_allowed,
> +};
> +

[...]