netdev - RE: [PATCHv3 0/9] bpf: Add d

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Message-ID: <5eebd52fc68ee_6d292ad5e7a285b816@john-XPS-13-9370.notmuch>
Date:   Thu, 18 Jun 2020 13:57:19 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andrii Nakryiko <andriin@...com>,
        Brendan Gregg <bgregg@...flix.com>,
        Florent Revest <revest@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: RE: [PATCHv3 0/9] bpf: Add d_path helper

Jiri Olsa wrote:
> hi,
> adding d_path helper to return full path for 'path' object.
> 
> I originally added and used 'file_path' helper, which did the same,
> but used 'struct file' object. Then realized that file_path is just
> a wrapper for d_path, so we'd cover more calling sites if we add
> d_path helper and allowed resolving BTF object within another object,
> so we could call d_path also with file pointer, like:
> 
>   bpf_d_path(&file->f_path, buf, size);
> 
> This feature is mainly to be able to add dpath (filepath originally)
> function to bpftrace:
> 
>   # bpftrace -e 'kfunc:vfs_open { printf("%s\n", dpath(args->path)); }'
> 
> v3 changes:
>   - changed tests to use seleton and vmlinux.h [Andrii]
>   - refactored to define ID lists in C object [Andrii]
>   - changed btf_struct_access for nested ID check,
>     instead of adding new function for that [Andrii]
>   - fail build with CONFIG_DEBUG_INFO_BTF if libelf is not detected [Andrii]
> 
> Also available at:
>   https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git
>   bpf/d_path
> 
> thanks,
> jirka

Hi Jira, Apologize for waiting until v3 to look at this series, but a
couple general requests as I review this.

In the cover letter can we get some more details. The above is really
terse/cryptic in my opinion. The bpftrace example gives good motiviation,
but nothing above mentions a new .BTF_ids section and the flow to create
and use this section.

Also if we add a BTF_ids  section adding documentation in btf.rst should
happen as well. I would like to see something in the ELF File Format
Interface section and BTF Generation sections.

I'm not going to nitpick if its in this series or a stand-alone patch
but do want to see it. So far the Documentation on BTF is fairly
good and I want to avoid these kind of gaps.

Thanks!
John