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

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Date:   Fri, 19 Jun 2020 14:35:27 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, 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>,
        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

On Thu, Jun 18, 2020 at 01:57:19PM -0700, John Fastabend wrote:
> 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.

ok, will add more details in next version

> 
> 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.

did not know there was bpf.rst ;-) will update

> 
> 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.

sure, thanks

jirka

> 
> Thanks!
> John
>