On Mon, 2020-12-07 at 19:14 -0800, Andrii Nakryiko wrote: > On Sun, Dec 6, 2020 at 9:21 PM <saeed@xxxxxxxxxx> wrote: > > From: Saeed Mahameed <saeedm@xxxxxxxxxx> > > > > While playing with BTF for modules, i noticed that executing the > > command: > > $ bpftool btf dump id <module's btf id> > > > > Fails due to lack of information in the BTF data. > > > > Maybe I am missing a step but actually adding the support for this > > is > > very simple. > > yes, bpftool takes -B <path> argument for specifying base BTF. So if > you added -B /sys/kernel/btf/vmlinux, it should have worked. I've > added auto-detection logic for the case of `btf dump file > /sys/kernel/btf/<module>` (see [0]), and we can also add it for when > ID corresponds to a module BTF. But I think it's simplest to re-use > the logic and just open /sys/kernel/btf/vmlinux, instead of adding > narrowly-focused libbpf API for that. > When dumping with a file it works even without the -B since you lookup the vmlinux file, but the issue is not dumping from a file source, we need it by id.. > > To completely parse modules BTF data, we need the vmlinux BTF as > > their > > "base btf", which can be easily found by iterating through the btf > > ids and > > looking for btf.name == "vmlinux". > > > > 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. I can revise this patch to support -B option and lookup vmlinux file if not provided for module btf dump by ids. 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() ... :/ Are you sure you don't like the current patch/libbpf API ? it is pretty straight forward and correct. > > "Integrate kernel module BTF support", as adding the support for > > this is very trivial. Unless i am missing something, CCing Andrii.. > > > > Signed-off-by: Saeed Mahameed <saeedm@xxxxxxxxxx> > > CC: Andrii Nakryiko <andrii@xxxxxxxxxx> > > --- > > tools/lib/bpf/btf.c | 57 > > ++++++++++++++++++++++++++++++++++++++++ > > tools/lib/bpf/btf.h | 2 ++ > > tools/lib/bpf/libbpf.map | 1 + > > 3 files changed, 60 insertions(+) > > > > [...]
