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

[Saeed Mahameed <saeed@kernel.org>]

On Mon, 2020-12-07 at 22:48 -0800, Andrii Nakryiko wrote:
> On Mon, Dec 7, 2020 at 10:45 PM Saeed Mahameed <saeed@kernel.org>
> wrote:
> > On Mon, 2020-12-07 at 22:38 -0800, Andrii Nakryiko wrote:
> > > On Mon, Dec 7, 2020 at 10:26 PM Saeed Mahameed <saeed@kernel.org>
> > > wrote:
> > > > On Mon, 2020-12-07 at 19:14 -0800, Andrii Nakryiko wrote:
> > > > > On Sun, Dec 6, 2020 at 9:21 PM <saeed@kernel.org> wrote:
> > > > > > From: Saeed Mahameed <saeedm@nvidia.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..
> 
> Right, but bpftool is perfectly capable of doing that without adding
> APIs to libbpf. That's why I wrote above:
> 
>   > > 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.

Oh now i see, you want to scan for it in bpftool.. make sense.