First of all thank you very much for your quick response. And helpful pointers. It seems like you also think what I am attempting to do should work. Please see inline [VAMSI-2]. On Wed, Jan 6, 2021 at 3:55 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Wed, Jan 6, 2021 at 10:04 AM Vamsi Kodavanty > <vamsi@xxxxxxxxxxxxxxxxxx> wrote: > > > > Had a few questions on CO-RE dependencies and usage. From what I read > > CO-RE needs a supported kernel version and be compiled with > > `CONFIG_DEBUG_INFO_BTF=y`. > > > > I also understand there are three pieces to enable CO-RE > > functionality. (1) The BTF format. For efficient/compressed kernel > > symbol table. (2) clang changes to emit the BTF relocations. (3) > > BTF is not really a symbol table, rather a type information. Like > simpler and more compact DWARF. > > > `libbpf` changes to locate a BTF file and fix-up relocations. Once > > these 3 steps are done the resulting byte code is no different from > > non-CO-RE byte code. > > > > Given this I am hoping the knowledgeable folks on this mailer correct > > and guide me if I am stating something incorrectly. > > > > (1) Is the kernel support requirement ONLY for the purposes of > > generating and exposing the BTF file information on > > `/sys/kernel/btf/vmlinux`? So that the eBPF CO-RE applications > > `libbpf` can find the BTF information at a standard location?. > > /sys/kernel/btf/vmlinux is a standardized place, but libbpf will also > try to search for vmlinux image (and BTF info within it) in a few > standard locations, see [0]. Early versions of in-kernel BTF didn't > even expose /sys/kernel/btf/vmlinux. > > [0] https://github.com/libbpf/libbpf/blob/master/src/btf.c#L4580 > > > > > (2) If the answer to the above question is YES. Could the below > > mechanism be used so that it works on all kernels whether they support > > the `CONFIG_DEBUG_INFO_BTF` flag or not?. > > (a) Extract BTF generation process outside of the kernel build. > > Use this to generate the equivalent BTF file for it. > > Yes, CONFIG_DEBUG_INFO_BTF=y is the most convenient way to add BTF > info, but it's also possible to just embed BTF manually with a direct > invocation of pahole -J, see [1] on how it's done for > CONFIG_DEBUG_INFO_BTF. You can do that for *any* kernel image, no > matter the version, and it will work with CO-RE relocations. > > [1] https://github.com/torvalds/linux/blob/master/scripts/link-vmlinux.sh#L137-L170 > [VAMSI-2] Yes, this is exactly what I did. I extracted out the `gen_btf` from the `link-vmlinux.sh` (which uses pahole -J) and used it to generate a BTF file for the 4.14.0 kernel. > > (b) Make changes to `libbpf` to look for BTF not only at the > > standard locations but also at a user specified location. The BTF file > > generated in (a) can be presented here. > > You can already do that, actually, though it's not very obvious. You > can specify (or override) kernel BTF location by using > bpf_object__load_xattr() and passing target_btf_path pointing to your > BTF location (see [2]). I've been meaning to add it instead to a > bpf_object_open_opts, btw, to make its use possible with a BPF > skeleton. Also keep in mind that currently libbpf expects that custom > BTF to be an ELF file with .BTF section, not just a raw BTF data. But > we can improve that, of course. > > [2] https://github.com/libbpf/libbpf/blob/master/src/libbpf.h#L136-L141 [VAMSI-2] I took a look at this and what you suggested above does not work as is. Even if we used `bpf_object__load_xattr` with `target_btf_path`. It seems like `bpf_object__load_vmlinux_btf` is not yet modified to use the `target_btf_path` attribute. Only, the `bpf_object__relocate` looks at the `target_btf_path`. As you suggested enabling use from the BPF skeleton seems useful and I can possibly help with that. For now, just for proof of concept I modified the search options in `libbpf_find_kernel_btf` to include my custom path. And on a 4.14 AmazonLinux2 VM I observe these failures. libbpf: loading kernel BTF '/home/ec2-user/vmlinux.btf': 0 libbpf: Kernel doesn't support BTF, skipping uploading it. libbpf: kernel doesn't support global data libbpf: failed to load object 'tcpconnect_bpf' libbpf: failed to load BPF skeleton 'tcpconnect_bpf': -95 failed to load BPF object: -95 This is the reason I had posted on the mailer. If the CO-RE executable has relocations resolved by the time of the BPF load. Why do we need to check for kernel support?. Also, does this mean what I am attempting to do will not work?. Best Regards. And again thanks a lot for your precious time. - Vamsi. > > > > This should provide us a way to enable CO-RE functionality on older > > kernel versions as well. I tried to make the above changes and tried > > against a 4.14 kernel and it did not work. Either I am not doing > > something right or my assumptions are wrong. > > > > Thanks in advance for your time. And I hope someone here can guide me > > in the right direction. > > > > Regards > > Vamsi.
