netdev - Re: [PATCH bpf] tools/bpftool: Add/Fix support for modules btf dump

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Date:   Mon, 07 Dec 2020 22:45:34 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH bpf] tools/bpftool: Add/Fix support for modules btf dump

On Mon, 2020-12-07 at 22:38 -0800, Andrii Nakryiko wrote:
> On Mon, Dec 7, 2020 at 10:26 PM Saeed Mahameed <saeed@...nel.org>
> wrote:
> > On Mon, 2020-12-07 at 19:14 -0800, Andrii Nakryiko wrote:
> > > On Sun, Dec 6, 2020 at 9:21 PM <saeed@...nel.org> wrote:
> > > > From: Saeed Mahameed <saeedm@...dia.com>
> > > > 
[...]
> > > > 
> > > > I am not sure why this hasn't been added by the original
> > > > patchset
> > > 
> > > because I never though of dumping module BTF by id, given there
> > > is
> > > nicely named /sys/kernel/btf/<module> :)
> > > 
> > 
> > What if i didn't compile my kernel with SYSFS ? a user experience
> > is a
> > user experience, there is no reason to not support dump a module
> > btf by
> > id or to have different behavior for different BTF sources.
> 
> Hm... I didn't claim otherwise and didn't oppose the feature, why the
> lecture about user experience?
> 

Sorry wasn't a lecture, just wanted to emphasize the motivation.

> Not having sysfs is a valid point. In such cases, if BTF dumping is
> from ID and we see that it's a module BTF, finding vmlinux BTF from
> ID
> makes sense.
> 
> > I can revise this patch to support -B option and lookup vmlinux
> > file if
> > not provided for module btf dump by ids.
> 
> yep
> 
> > but we  still need to pass base_btf to btf__get_from_id() in order
> > to
> > support that, as was done for btf__parse_split() ... :/
> 
> btf__get_from_id_split() might be needed, yes.
> 
> > Are you sure you don't like the current patch/libbpf API ? it is
> > pretty
> > straight forward and correct.
> 
> I definitely don't like adding btf_get_kernel_id() API to libbpf.
> There is nothing special about it to warrant adding it as a public
> API. Everything we discussed can be done by bpftool.
> 

What about the case where sysfs isn't available ?
we still need to find vmlinux's btf id..