On Mon, Dec 7, 2020 at 10:26 PM Saeed Mahameed <saeed@xxxxxxxxxx> wrote: > > 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. Hm... I didn't claim otherwise and didn't oppose the feature, why the lecture about user experience? 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. > > > > "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(+) > > > > > > > [...] >
