On Thu, Jan 7, 2021 at 3:32 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Thu, Jan 7, 2021 at 2:45 PM Vamsi Kodavanty <vamsi@xxxxxxxxxxxxxxxxxx> wrote: > > > > Andrii, > > Thank you so much for the quick response. You were right. I was > > trying to CO-RE the `tcpconnect.py` program. It had some `.rodata` > > which I removed. Now the program goes much further. It now finishes > > the relocations successfully. But, fails at the next step > > `bpf_object__load_progs`. > > It's discouraged to use top posting on the mailing list. So for the > future please reply inline. > [VAMSI-3] Apologies. Will be mindful next time on. > > > > > === > > libbpf: prog 'tcp_v6_connect_ret': relo #3: patched insn #50 > > (ALU/ALU64) imm 56 -> 56 > > libbpf: failed to open '/sys/bus/event_source/devices/kprobe/type': No > > such file or directory > > libbpf: failed to determine kprobe perf type: No such file or directory > > libbpf: prog 'tcp_v4_connect': failed to create kprobe > > 'tcp_v4_connect' perf event: No such file or directory > > libbpf: failed to auto-attach program 'tcp_v4_connect': -2 > > === > > > > This host has a 4.14 kernel AmazonLinux2 and does not have the above > > file. It instead has this > > > > === > > $ sudo cat /sys/kernel/debug/tracing/kprobe_events > > p:kprobes/p_sys_execve_bcc_2566 sys_execve > > === > > > > Right. Libbpf only supports a newer and safer way to attach to > kprobes. For your experiments, try to stick to tracepoints and you'll > have a better time. > > But it's another thing I've been meaning to add to libbpf for > supporting older kernels. I even have code written to do legacy kprobe > attachment, just need to find time to send a patch to add it as a > fallback for kernels that don't support new kprobe interface. > [VAMSI-3] I think that will be very helpful, as there are bound to be hosts with older kernels for the foreseeable future. Good to know it's being considered. > > > I am guessing this is a backward compatibility issue?. I will try to > > look at an earlier version of libbpf to see how this was handled. > > it was never supported by libbpf. BCC support both old and new APIs, > but old APIs is more dangerous, as it's easy to leave attached kprobe > BPF program active in the kernel unintentionally. E.g., if the process > crashes. But with bpf_link and its expected use of > bpf_link__destroy(), we can support the legacy API (it won't be any > safer, but still). > [VAMSI-3] Will use tracepoints to validate the process as you suggested. I will then try to see what I can do to use this on kprobe/kretprobe's. Will also check how BCC does this. I will start a new thread if needed. Again appreciate the inputs and direction very much. Best Regards Vamsi. > > Meanwhile, if you have further comments they are appreciated. > > > > And again I can't thank you enough for how helpful you have been and > > your time. Cheers!. > > > > > > > > > > On Thu, Jan 7, 2021 at 10:52 AM Andrii Nakryiko > > <andrii.nakryiko@xxxxxxxxx> wrote: > > > > > > On Thu, Jan 7, 2021 at 10:12 AM Vamsi Kodavanty > > > <vamsi@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > 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. > > > > > > Ah, right. We used to need vmlinux BTF only for CO-RE relocations, but > > > since then added a bunch more use cases. So some libbpf changes are > > > needed to make this work. But it should still work for CO-RE to have a > > > custom BTF. > > > > > > I'm not sure about making bpf_object__load_vmlinux_btf() load custom > > > BTF as the real kernel BTF, because that will never work for > > > fentry/fexit, struct_ops, etc. I think it is better to teach > > > bpf_object__load_vmlinux_btf() to not attempt to load real kernel BTF > > > if we need it only for CO-RE relocations *and* we have it overloaded > > > with target_btf_path > > > > > > > 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. > > > > > > Yeah, adding something like core_btf_path option to > > > bpf_object_open_opts would go nicely with this change. > > > > > > > > > > > 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 > > > > > > so here you successfully loaded custom BTF, which is good. > > > > > > > libbpf: Kernel doesn't support BTF, skipping uploading it. > > > > > > this just means that your BPF object's BTF won't be loaded into the > > > kernel. That's no big deal, ignore this. > > > > > > > libbpf: kernel doesn't support global data > > > > > > But this means that your BPF programs rely on global variables, which > > > are not supported by the kernel. So you need to change the code to not > > > use global variables to make this work on very old kernels. > > > > > > > > > > libbpf: failed to load object 'tcpconnect_bpf' > > > > libbpf: failed to load BPF skeleton 'tcpconnect_bpf': -95 > > > > failed to load BPF object: -95 > > > > > > This is probably OPNOTSUPP from the global data above > > > > > > > > > > > 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?. > > > > > > > > > > it will work with minimal libbpf logic changes. Nothing in principle > > > prevents this. > > > > > > > > > > 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.
