From: YiFei Zhu <zhuyifei@xxxxxxxxxx> To access the storage in a CGROUP_STORAGE map, one uses bpf_get_local_storage helper, which is extremely fast due to its use of per-CPU variables. However, its whole code is built on the assumption that one map can only be used by one program at any time, and this prohibits any sharing of data between multiple programs using these maps, eliminating a lot of use cases, such as some per-cgroup configuration storage, written to by a setsockopt program and read by a cg_sock_addr program. Why not use other map types? The great part of CGROUP_STORAGE map is that it is isolated by different cgroups its attached to. When one program uses bpf_get_local_storage, even on the same map, it gets different storages if it were run as a result of attaching to different cgroups. The kernel manages the storages, simplifying BPF program or userspace. In theory, one could probably use other maps like array or hash to do the same thing, but it would be a major overhead / complexity. Userspace needs to know when a cgroup is being freed in order to free up a space in the replacement map. This patch set introduces a significant change to the semantics of CGROUP_STORAGE map type. Instead of each storage being tied to one single attachment, it is shared across different attachments to the same cgroup, and persists until either the map or the cgroup attached to is being freed. User may use u64 as the key to the map, and the result would be that the attach type become ignored during key comparison, and programs of different attach types will share the same storage if the cgroups they are attached to are the same. How could this break existing users? * Users that uses detach & reattach / program replacement as a shortcut to zeroing the storage. Since we need sharing between programs, we cannot zero the storage. Users that expect this behavior should either attach a program with a new map, or explicitly zero the map with a syscall. This case is dependent on undocumented implementation details, so the impact should be very minimal. Patch 1 introduces a test on the old expected behavior of the map type. Patch 2 introduces a test showing how two programs cannot share one such map. Patch 3 implements the change of semantics to the map. Patch 4 amends the new test such that it yields the behavior we expect from the change. Patch 5 documents the map type. Changes since RFC: * Clarify commit message in patch 3 such that it says the lifetime of the storage is ended at the freeing of the cgroup_bpf, rather than the cgroup itself. * Restored an -ENOMEM check in __cgroup_bpf_attach. * Update selftests for recent change in network_helpers API. Changes since v1: * s/CHECK_FAIL/CHECK/ * s/bpf_prog_attach/bpf_program__attach_cgroup/ * Moved test__start_subtest to test_cg_storage_multi. * Removed some redundant CHECK_FAIL where they are already CHECK-ed. Changes since v2: * Lock cgroup_mutex during map_free. * Publish new storages only if attach is successful, by tracking exactly which storages are reused in an array of bools. * Mention bpftool map dump showing a value of zero for attach_type in patch 3 commit message. Changes since v3: * Use a much simpler lookup and allocate-if-not-exist from the fact that cgroup_mutex is locked during attach. * Removed an unnecessary spinlock hold. Changes since v4: * Changed semantics so that if the key type is struct bpf_cgroup_storage_key the map retains isolation between different attach types. Sharing between different attach types only occur when key type is u64. * Adapted tests and docs for the above change. Changes since v5: * Removed redundant NULL check before bpf_link__destroy. * Free BPF object explicitly, after asserting that object failed to load, in the event that the object did not fail to load. * Rename variable in bpf_cgroup_storage_key_cmp for clarity. * Added a lot of information to Documentation, more or less copied from what Martin KaFai Lau wrote. YiFei Zhu (5): selftests/bpf: Add test for CGROUP_STORAGE map on multiple attaches selftests/bpf: Test CGROUP_STORAGE map can't be used by multiple progs bpf: Make cgroup storages shared between programs on the same cgroup selftests/bpf: Test CGROUP_STORAGE behavior on shared egress + ingress Documentation/bpf: Document CGROUP_STORAGE map type Documentation/bpf/index.rst | 9 + Documentation/bpf/map_cgroup_storage.rst | 169 ++++++++ include/linux/bpf-cgroup.h | 12 +- kernel/bpf/cgroup.c | 67 +-- kernel/bpf/core.c | 12 - kernel/bpf/local_storage.c | 216 +++++----- .../bpf/prog_tests/cg_storage_multi.c | 403 ++++++++++++++++++ .../selftests/bpf/progs/cg_storage_multi.h | 13 + .../bpf/progs/cg_storage_multi_egress_only.c | 33 ++ .../bpf/progs/cg_storage_multi_isolated.c | 57 +++ .../bpf/progs/cg_storage_multi_shared.c | 57 +++ 11 files changed, 905 insertions(+), 143 deletions(-) create mode 100644 Documentation/bpf/map_cgroup_storage.rst create mode 100644 tools/testing/selftests/bpf/prog_tests/cg_storage_multi.c create mode 100644 tools/testing/selftests/bpf/progs/cg_storage_multi.h create mode 100644 tools/testing/selftests/bpf/progs/cg_storage_multi_egress_only.c create mode 100644 tools/testing/selftests/bpf/progs/cg_storage_multi_isolated.c create mode 100644 tools/testing/selftests/bpf/progs/cg_storage_multi_shared.c -- 2.27.0
