Introduction ------------ This set enables storing pointers of a certain type in BPF map, and extends the verifier to enforce type safety and lifetime correctness properties. The infrastructure being added is generic enough for allowing storing any kind of pointers whose type is available using BTF (user or kernel) in the future (e.g. strongly typed memory allocation in BPF program), which are internally tracked in the verifier as PTR_TO_BTF_ID, but for now the series limits them to four kinds of pointers obtained from the kernel. Obviously, use of this feature depends on map BTF. 1. Unreferenced kernel pointer In this case, there are very few restrictions. The pointer type being stored must match the type declared in the map value. However, such a pointer when loaded from the map can only be dereferenced, but not passed to any in-kernel helpers or kernel functions available to the program. This is because while the verifier's exception handling mechanism coverts BPF_LDX to PROBE_MEM loads, which are then handled specially by the JIT implementation, the same liberty is not available to accesses inside the kernel. The pointer by the time it is passed into a helper has no lifetime related guarantees about the object it is pointing to, and may well be referencing invalid memory. 2. Referenced kernel pointer This case imposes a lot of restrictions on the programmer, to ensure safety. To transfer the ownership of a reference in the BPF program to the map, the user must use the BPF_XCHG instruction, which returns the old pointer contained in the map, as an acquired reference, and releases verifier state for the referenced pointer being exchanged, as it moves into the map. This a normal PTR_TO_BTF_ID that can be used with in-kernel helpers and kernel functions callable by the program. However, if BPF_LDX is used to load a referenced pointer from the map, it is still not permitted to pass it to in-kernel helpers or kernel functions. To obtain a reference usable with helpers, the user must invoke a kfunc helper which returns a usable reference (which also must be eventually released before BPF_EXIT, or moved into a map). Since the load of the pointer (preserving data dependency ordering) must happen inside the RCU read section, the kfunc helper will take a pointer to the map value, which must point to the actual pointer of the object whose reference is to be raised. The type will be verified from the BTF information of the kfunc, as the prototype must be: T *func(T **, ... /* other arguments */); Then, the verifier checks whether pointer at offset of the map value points to the type T, and permits the call. This convention is followed so that such helpers may also be called from sleepable BPF programs, where RCU read lock is not necessarily held in the BPF program context, hence necessiating the need to pass in a pointer to the actual pointer to perform the load inside the RCU read section. 3. per-CPU kernel pointer These have very little restrictions. The user can store a PTR_TO_PERCPU_BTF_ID into the map, and when loading from the map, they must NULL check it before use, because while a non-zero value stored into the map should always be valid, it can still be reset to zero on updates. After checking it to be non-NULL, it can be passed to bpf_per_cpu_ptr and bpf_this_cpu_ptr helpers to obtain a PTR_TO_BTF_ID to underlying per-CPU object. It is also permitted to write 0 and reset the value. 4. Userspace pointer The verifier recently gained support for annotating BTF with __user type tag. This indicates pointers pointing to memory which must be read using the bpf_probe_read_user helper to ensure correct results. The set also permits storing them into the BPF map, and ensures user pointer cannot be stored into other kinds of pointers mentioned above. When loaded from the map, the only thing that can be done is to pass this pointer to bpf_probe_read_user. No dereference is allowed. Notes ----- This set requires the following LLVM fix to pass the BPF CI: https://reviews.llvm.org/D119799 Also, I applied Alexei's suggestion of removing callback for btf_find_field, but that 'ugly' is still required, since bad offset alignment etc. can return an error, and we don't want to leave a partial ptr_off_tab around in that case. The other option is freeing inside btf_find_field, but that would be more code conditional on BTF_FIELD_KPTR, when the caller can do it based on ret < 0. TODO ---- Needs a lot more testing, especially for stuff apart from verifier correctness. Will work on that in parallel during v1 review. The idea was to get a little more feedback (esp. for kptr_get stuff) before moving forward with adding more tests. Posting it now to just get discussion started. The verifier tests fairly comprehensively test many edge cases I could think of. Kumar Kartikeya Dwivedi (15): bpf: Factor out fd returning from bpf_btf_find_by_name_kind bpf: Make btf_find_field more generic bpf: Allow storing PTR_TO_BTF_ID in map bpf: Allow storing referenced PTR_TO_BTF_ID in map bpf: Allow storing PTR_TO_PERCPU_BTF_ID in map bpf: Allow storing __user PTR_TO_BTF_ID in map bpf: Prevent escaping of pointers loaded from maps bpf: Adapt copy_map_value for multiple offset case bpf: Populate pairs of btf_id and destructor kfunc in btf bpf: Wire up freeing of referenced PTR_TO_BTF_ID in map bpf: Teach verifier about kptr_get style kfunc helpers net/netfilter: Add bpf_ct_kptr_get helper libbpf: Add __kptr* macros to bpf_helpers.h selftests/bpf: Add C tests for PTR_TO_BTF_ID in map selftests/bpf: Add verifier tests for PTR_TO_BTF_ID in map include/linux/bpf.h | 90 ++- include/linux/btf.h | 24 + include/net/netfilter/nf_conntrack_core.h | 17 + kernel/bpf/arraymap.c | 13 +- kernel/bpf/btf.c | 565 ++++++++++++++-- kernel/bpf/hashtab.c | 27 +- kernel/bpf/map_in_map.c | 5 +- kernel/bpf/syscall.c | 227 ++++++- kernel/bpf/verifier.c | 311 ++++++++- net/bpf/test_run.c | 17 +- net/netfilter/nf_conntrack_bpf.c | 132 +++- net/netfilter/nf_conntrack_core.c | 17 - tools/lib/bpf/bpf_helpers.h | 4 + .../selftests/bpf/prog_tests/map_btf_ptr.c | 13 + .../testing/selftests/bpf/progs/map_btf_ptr.c | 105 +++ .../testing/selftests/bpf/progs/test_bpf_nf.c | 31 + tools/testing/selftests/bpf/test_verifier.c | 57 +- .../selftests/bpf/verifier/map_btf_ptr.c | 624 ++++++++++++++++++ 18 files changed, 2144 insertions(+), 135 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/map_btf_ptr.c create mode 100644 tools/testing/selftests/bpf/progs/map_btf_ptr.c create mode 100644 tools/testing/selftests/bpf/verifier/map_btf_ptr.c -- 2.35.1
