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Message-ID: <20230504081858.GV15906@kitsune.suse.cz> Date: Thu, 4 May 2023 10:18:58 +0200 From: Michal Suchánek <msuchanek@...e.de> To: Quentin Monnet <quentin@...valent.com> Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>, Alexander Lobakin <aleksander.lobakin@...el.com>, Alexander Lobakin <alobakin@...lbox.org>, Alexei Starovoitov <ast@...nel.org>, Shung-Hsi Yu <shung-hsi.yu@...e.com>, Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Song Liu <songliubraving@...com>, Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2 bpf 02/11] bpftool: define a local bpf_perf_link to fix accessing its fields Hello, On Thu, May 04, 2023 at 12:43:52AM +0100, Quentin Monnet wrote: > On Fri, 21 Apr 2023 at 08:39, Michal Suchánek <msuchanek@...e.de> wrote: > > > > On Thu, Apr 20, 2023 at 04:07:38PM -0700, Andrii Nakryiko wrote: > > > On Fri, Apr 14, 2023 at 9:28 AM Michal Suchánek <msuchanek@...e.de> wrote: > > > > > > > > On Fri, Apr 14, 2023 at 05:18:27PM +0200, Alexander Lobakin wrote: > > > > > From: Michal Suchánek <msuchanek@...e.de> > > > > > Date: Fri, 14 Apr 2023 11:54:57 +0200 > > > > > > > > > > > Hello, > > > > > > > > > > Hey-hey, > > > > > > > > > > > > > > > > > On Thu, Apr 21, 2022 at 12:38:58AM +0000, Alexander Lobakin wrote: > > > > > >> When building bpftool with !CONFIG_PERF_EVENTS: > > > > > >> > > > > > >> skeleton/pid_iter.bpf.c:47:14: error: incomplete definition of type 'struct bpf_perf_link' > > > > > >> perf_link = container_of(link, struct bpf_perf_link, link); > > > > > >> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > > >> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:74:22: note: expanded from macro 'container_of' > > > > > >> ((type *)(__mptr - offsetof(type, member))); \ > > > > > >> ^~~~~~~~~~~~~~~~~~~~~~ > > > > > >> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:68:60: note: expanded from macro 'offsetof' > > > > > >> #define offsetof(TYPE, MEMBER) ((unsigned long)&((TYPE *)0)->MEMBER) > > > > > >> ~~~~~~~~~~~^ > > > > > >> skeleton/pid_iter.bpf.c:44:9: note: forward declaration of 'struct bpf_perf_link' > > > > > >> struct bpf_perf_link *perf_link; > > > > > >> ^ > > > > > >> > > > > > >> &bpf_perf_link is being defined and used only under the ifdef. > > > > > >> Define struct bpf_perf_link___local with the `preserve_access_index` > > > > > >> attribute inside the pid_iter BPF prog to allow compiling on any > > > > > >> configs. CO-RE will substitute it with the real struct bpf_perf_link > > > > > >> accesses later on. > > > > > >> container_of() is not CO-REd, but it is a noop for > > > > > >> bpf_perf_link <-> bpf_link and the local copy is a full mirror of > > > > > >> the original structure. > > > > > >> > > > > > >> Fixes: cbdaf71f7e65 ("bpftool: Add bpf_cookie to link output") > > > > > > > > > > > > This does not solve the problem completely. Kernels that don't have > > > > > > CONFIG_PERF_EVENTS in the first place are also missing the enum value > > > > > > BPF_LINK_TYPE_PERF_EVENT which is used as the condition for handling the > > > > > > cookie. > > > > > > > > > > Sorry, I haven't been working with my home/private stuff for more than a > > > > > year already. I may get back to it some day when I'm tired of Lua (curse > > > > > words, sorry :D), but for now the series is "a bit" abandoned. > > > > > > > > This part still appllies and works for me with the caveat that > > > > BPF_LINK_TYPE_PERF_EVENT also needs to be defined. > > > > > > > > > I think there was alternative solution proposed there, which promised to > > > > > be more flexible. But IIRC it also doesn't touch the enum (was it added > > > > > recently? Because it was building just fine a year ago on config without > > > > > perf events). > > > > > > > > It was added in 5.15. Not sure there is a kernel.org LTS kernel usable > > > > for CO-RE that does not have it, technically 5.4 would work if it was > > > > built monolithic, it does not have module BTF, only kernel IIRC. > > > > > > > > Nonetheless, the approach to handling features completely missing in the > > > > running kernel should be figured out one way or another. I would be > > > > surprised if this was the last feature to be added that bpftool needs to > > > > know about. > > > > > > Are we talking about bpftool built from kernel sources or from Github? > > > Kernel source version should have access to latest UAPI headers and so > > > BPF_LINK_TYPE_PERF_EVENT should be available. Github version, if it > > > doesn't do that already, can use UAPI headers distributed (and used > > > for building) with libbpf through submodule. > > > > It does have a copy of the uapi headers but apparently does not use > > them. Using them directly might cause conflict with vmlinux.h, though. > > Indeed, using the UAPI header here conflicts with vmlinux.h. > > Looking again at some code I started last year but never finalised, I > used the following approach, redefining BPF_LINK_TYPE_PERF_EVENT with > CO-RE: > > enum bpf_link_type___local { > BPF_LINK_TYPE_PERF_EVENT___local = 7, > }; That's the same as I did except I used simple define instead of this fake enum. The enum only has value when it is complete and the compiler can check that a switch uses only known values, and can confuse things when values are missing. Thanks Michal
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