On 09/09/2021 20.19, Andrii Nakryiko wrote:
Depending on what IDs we are talking about (sorry, I don't follow this
thread very closely, so if you are curious about some aspects of BTF
or libbpf APIs, it would be good to have a specific questions with
some context). BTF as kernel object has it's own ID allocated through
idr, so yes, they are unique. so vmlinux BTF object will have it's own
ID, while each module's BTF will have it's own.
But if we are talking about BTF type IDs, that's entirely different
thing. BTF type IDs start from 1 (0 is reserved for special 'VOID'
type) all the way to number of types in vmlinux BTF. Then each module
extends vmlinx BTF starting at N + 1 and going to N + M, where N is
number of BTF types in vmlinux BTF and M is number of added types in
module BTF.
So in that regard each module has BTF type IDs that are overlapping
with other modules, which is why for unique fetching of BTF types from
modules you also need BTF object FD or ID of a module BTF, and then
BTF type ID within that module. But as I said, I didn't follow along
closely, so not sure if I'm answering the right question, sorry.
Thanks for answering. This N vmlinux IDs + M module IDs was important
to know, thanks for correcting my understanding on this, as this does
affect our ideas for using BTF for XDP-hints.
This "just" means that the BTF ID will be per driver. I think we can
still make this work, as the AF_XDP userspace program will already need
to bind to a device. Thus, we can still send a simple btf_id in
metadata, and AF_XDP prog will just have device-map with expected
btf_id's from this device (to validate if it knows howto decode contents).
It is slightly more annoying for my xdp_frame + cpumap use-case, as it
can get XDP_REDIRECT'ed frames from many net_devices, but we do have
xdp_frame->dev_rx (net_device) avail, so I can resolve this.
--Jesper
Finding some random BTF ID in two module and notice they point to
different types.
# bpftool btf dump file /sys/kernel/btf/ixgbe | grep 95905
[95905] FUNC 'ixgbe_set_rx_mode' type_id=95829 linkage=static
# bpftool btf dump file /sys/kernel/btf/igc | grep 95905
[95905] FUNC 'igc_ethtool_get_link_ksettings' type_id=95904 linkage=static
