Martin KaFai Lau
a7ee927845
This patch adds necessary infrastructure to attach a struct_ops
map to a cgroup. The initial need was to support migrating
the legacy BPF_PROG_TYPE_SOCK_OPS to a struct_ops.
Recently, there are other struct_ops use cases that
need to attach struct_ops to a cgroup. For example,
the recent BPF OOM and memcg discussion in LSFMMBPF 2026.
The motivation is to create a consistent expectation
for attaching struct_ops to cgroup instead of each subsystem
creating its own infrastructure. This logic includes
hierarchy expectation, ordering expectation,
attachment API, and rcu gp.
There is already an existing implementation for attaching
multiple bpf progs to a cgroup. There are also tools
built around it for querying. Attaching a struct_ops map
(which is a group of bpf programs) could also adhere to
a similar API and potentially reuse most of the existing
implementation.
A couple of ideas have been tried. One of them
is to use mprog.c. In terms of the amount of changes,
I eventually came to the same conclusion as in
commit 120933984460 ("bpf: Implement mprog API on top of existing cgroup progs").
I then shifted the focus to reusing the current
{update,compute,activate,purge}_effective_progs() which has
the main logic that implements the mprog API.
Since then, I tried to add a 'struct cgroup *cgroup' member
to the existing 'struct bpf_struct_ops_link' and link_create
will create a 'struct bpf_struct_ops_link' object to be stored
in the pl->link. This turns out to have more changes on
both cgroup.c and bpf_struct_ops.c than I like.
This patch directly reuses the 'struct bpf_cgroup_link' which
cgroup.c already understands. Add 'struct bpf_map *map'
to 'struct bpf_cgroup_link'. In the future, as more subsystems
are extended by struct_ops, we may consider to make
'struct bpf_map *map' as a primary citizen of a link
like 'struct bpf_prog *prog' and directly add
'struct bpf_map *map' to the generic 'struct bpf_link'.
The pl->link could be the traditional 'prog' link or the
new 'map' link. The places that need to handle them differently
have already been refactored into the new prog_list_*() added in
the earlier patch. In those new prog_list_*(), this patch will
check "pl->link && pl->link->map", learn that it is a 'map' link
and handle it correctly.
The bpf_prog_array also needs to handle that its item can store
the traditional 'prog' or it can store a struct_ops map.
The places that need to handle them differently have also
been refactored into the new bpf_cgroup_array_*() added
in the earlier patch. The two differences are:
- different sentinel (dummy_bpf_prog in prog vs cfi_stub in struct_ops)
- the array for struct_ops may need to go through different
rcu gp.
The bpf_cgroup_array_*() functions use the cgroup_bpf_attach_type (ie atype)
to distinguish the array is storing prog or storing struct_ops map.
This patch also implements a separate struct bpf_link_ops
"cgroup_struct_ops_link_ops" to have a separate link_ops implementation
that only handles the cgroup's struct_ops link.
Questions:
- Although this patch did not change it, it is not obvious to me how
the replace_effective_progs() and purge_effective_progs() handle
cases when there are existing BPF_F_PREORDER progs attached
in the hlist.
Misc notes:
- CGROUP_TCP_SOCK_OPS is added to the 'enum cgroup_bpf_attach_type'.
The actual implementation of the tcp_bpf_ops (a struct_ops)
will be added in the next patch.
- free_after_mult_rcu_gp is added to 'struct bpf_struct_ops' such that
the bpf_prog_array can have a mix of sleepable and
non-sleepable prog in a struct_ops. This can tell
how the bpf_prog_array should be freed.
- For a struct_ops that supports cgroup attachment, it does not need to
implement its own reg/unreg function. reg/unreg to a cgroup is
done by the common infrastructure added in this patch.
- The cgroup's struct_ops link only supports BPF_F_ALLOW_MULTI.
This is enforced internally in cgroup_bpf_struct_ops_attach.
This should be consistent with the current prog's link
behavior in cgroup_bpf_link_attach.
In the future, we may allow each subsystem to choose differently.
- A cgroup_atype member is added to 'struct bpf_struct_ops'.
When a subsystem struct_ops needs to support cgroup attachment,
it needs to add a value to 'enum cgroup_bpf_attach_type'
and then assign it to the newly added cgroup_atype member
in the bpf_struct_ops.
- During LINK_CREATE in syscall, the patch uses the same
BPF_STRUCT_OPS (in attr->link_create.attach_type).
The bpf_struct_ops_link_create learns the map and
from the map it learns the st_ops. If the st_ops->cgroup_atype
is not 0, it will create a cgroup's link.
- When a subsystem registers a struct_ops that supports cgroup
attachment, the struct_ops infrastructure will also ask the
cgroup infrastructure to remember a few things. This is done
by calling cgroup_bpf_struct_ops_register().
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20260917200542.3689605-10-ameryhung@gmail.com
libbpf
This is the official home of the libbpf library.
Please use this Github repository for building and packaging libbpf and when using it in your projects through Git submodule.
Libbpf authoritative source code is developed as part of bpf-next Linux source
tree under
tools/lib/bpf subdirectory and is periodically synced to Github. As such, all the
libbpf changes should be sent to BPF mailing list,
please don't open PRs here unless you are changing Github-specific parts of libbpf
(e.g., Github-specific Makefile).
Libbpf and general BPF usage questions
Libbpf documentation can be found here. It's an ongoing effort and has ways to go, but please take a look and consider contributing as well.
Please check out libbpf-bootstrap and the companion blog post for the examples of building BPF applications with libbpf. libbpf-tools are also a good source of the real-world libbpf-based tracing tools.
See also "BPF CO-RE reference guide" for the coverage of practical aspects of building BPF CO-RE applications and "BPF CO-RE" for general introduction into BPF portability issues and BPF CO-RE origins.
All general BPF questions, including kernel functionality, libbpf APIs and their application, should be sent to bpf@vger.kernel.org mailing list. You can subscribe to it here and search its archive here. Please search the archive before asking new questions. It very well might be that this was already addressed or answered before.
bpf@vger.kernel.org is monitored by many more people and they will happily try to help you with whatever issue you have. This repository's PRs and issues should be opened only for dealing with issues pertaining to specific way this libbpf mirror repo is set up and organized.
Building libbpf
libelf is an internal dependency of libbpf and thus it is required to link
against and must be installed on the system for applications to work.
pkg-config is used by default to find libelf, and the program called can be
overridden with PKG_CONFIG.
If using pkg-config at build time is not desired, it can be disabled by
setting NO_PKG_CONFIG=1 when calling make.
To build both static libbpf.a and shared libbpf.so:
$ cd src
$ make
To build only static libbpf.a library in directory build/ and install them together with libbpf headers in a staging directory root/:
$ cd src
$ mkdir build root
$ BUILD_STATIC_ONLY=y OBJDIR=build DESTDIR=root make install
To build both static libbpf.a and shared libbpf.so against a custom libelf dependency installed in /build/root/ and install them together with libbpf headers in a build directory /build/root/:
$ cd src
$ PKG_CONFIG_PATH=/build/root/lib64/pkgconfig DESTDIR=/build/root make install
BPF CO-RE (Compile Once – Run Everywhere)
Libbpf supports building BPF CO-RE-enabled applications, which, in contrast to BCC, do not require Clang/LLVM runtime being deployed to target servers and doesn't rely on kernel-devel headers being available.
It does rely on kernel to be built with BTF type information, though. Some major Linux distributions come with kernel BTF already built in:
- Fedora 31+
- RHEL 8.2+
- OpenSUSE Tumbleweed (in the next release, as of 2020-06-04)
- Arch Linux (from kernel 5.7.1.arch1-1)
- Manjaro (from kernel 5.4 if compiled after 2021-06-18)
- Ubuntu 20.10
- Debian 11 (amd64/arm64)
If your kernel doesn't come with BTF built-in, you'll need to build custom kernel. You'll need:
pahole1.16+ tool (part ofdwarvespackage), which performs DWARF to BTF conversion;- kernel built with
CONFIG_DEBUG_INFO_BTF=yoption; - you can check if your kernel has BTF built-in by looking for
/sys/kernel/btf/vmlinuxfile:
$ ls -la /sys/kernel/btf/vmlinux
-r--r--r--. 1 root root 3541561 Jun 2 18:16 /sys/kernel/btf/vmlinux
To develop and build BPF programs, you'll need Clang/LLVM 10+. The following distributions have Clang/LLVM 10+ packaged by default:
- Fedora 32+
- Ubuntu 20.04+
- Arch Linux
- Ubuntu 20.10 (LLVM 11)
- Debian 11 (LLVM 11)
- Alpine 3.13+
Otherwise, please make sure to update it on your system.
The following resources are useful to understand what BPF CO-RE is and how to use it:
- BPF CO-RE reference guide
- BPF Portability and CO-RE
- HOWTO: BCC to libbpf conversion
- libbpf-tools in BCC repo contain lots of real-world tools converted from BCC to BPF CO-RE. Consider converting some more to both contribute to the BPF community and gain some more experience with it.
Distributions
Distributions packaging libbpf from this mirror:
Benefits of packaging from the mirror over packaging from kernel sources:
- Consistent versioning across distributions.
- No ties to any specific kernel, transparent handling of older kernels. Libbpf is designed to be kernel-agnostic and work across multitude of kernel versions. It has built-in mechanisms to gracefully handle older kernels, that are missing some of the features, by working around or gracefully degrading functionality. Thus libbpf is not tied to a specific kernel version and can/should be packaged and versioned independently.
- Continuous integration testing via GitHub Actions.
- Static code analysis via LGTM and Coverity.
Package dependencies of libbpf, package names may vary across distros:
- zlib
- libelf
bpf-next to Github sync
All the gory details of syncing can be found in scripts/sync-kernel.sh
script. See SYNC.md for instruction.
Some header files in this repo (include/linux/*.h) are reduced versions of
their counterpart files at
bpf-next's
tools/include/linux/*.h to make compilation successful.
License
This work is dual-licensed under BSD 2-clause license and GNU LGPL v2.1 license. You can choose between one of them if you use this work.
SPDX-License-Identifier: BSD-2-Clause OR LGPL-2.1