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_kptr_xchg helper, 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 ----- * C selftests require https://reviews.llvm.org/D119799 to pass. * Unlike BPF timers, kptr is not reset or freed on map_release_uref. * Referenced kptr storage is always treated as unsigned long * on kernel side, as BPF side cannot mutate it. The storage (8 bytes) is sufficient for both 32-bit and 64-bit platforms. * Use of WRITE_ONCE to reset unreferenced kptr on 32-bit systems is fine, as the actual pointer is always word sized, so the store tearing into two 32-bit stores won't be a problem as the other half is always zeroed out. Changelog: ---------- v1 -> v2 v1: https://lore.kernel.org/bpf/20220220134813.3411982-1-memxor@xxxxxxxxx * Address comments from Alexei * Rename bpf_btf_find_by_name_kind_all to bpf_find_btf_id * Reduce indentation level in that function * Always take reference regardless of module or vmlinux BTF * Also made it the same for btf_get_module_btf * Use kptr, kptr_ref, kptr_percpu, kptr_user type tags * Don't reserve tag namespace * Refactor btf_find_field to be side effect free, allocate and populate kptr_off_tab in caller * Move module reference to dtor patch * Remove support for BPF_XCHG, BPF_CMPXCHG insn * Introduce bpf_kptr_xchg helper * Embed offset array in struct bpf_map, populate and sort it once * Adjust copy_map_value to memcpy directly using this offset array * Removed size member from offset array to save space * Fix some problems pointed out by kernel test robot * Tidy selftests * Lots of other minor fixes 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 unreferenced kptr in map bpf: Allow storing referenced kptr in map bpf: Allow storing percpu kptr in map bpf: Allow storing user kptr in map bpf: Prevent escaping of kptr loaded from maps bpf: Adapt copy_map_value for multiple offset case bpf: Always raise reference in btf_get_module_btf bpf: Populate pairs of btf_id and destructor kfunc in btf bpf: Wire up freeing of referenced kptr bpf: Teach verifier about kptr_get kfunc helpers libbpf: Add kptr type tag macros to bpf_helpers.h selftests/bpf: Add C tests for kptr selftests/bpf: Add verifier tests for kptr include/linux/bpf.h | 109 ++- include/linux/btf.h | 23 + include/uapi/linux/bpf.h | 12 + kernel/bpf/arraymap.c | 14 +- kernel/bpf/btf.c | 623 ++++++++++++-- kernel/bpf/hashtab.c | 28 +- kernel/bpf/helpers.c | 21 + kernel/bpf/map_in_map.c | 5 +- kernel/bpf/syscall.c | 204 ++++- kernel/bpf/verifier.c | 412 ++++++++-- net/bpf/test_run.c | 39 +- tools/include/uapi/linux/bpf.h | 12 + tools/lib/bpf/bpf_helpers.h | 4 + .../selftests/bpf/prog_tests/map_kptr.c | 20 + tools/testing/selftests/bpf/progs/map_kptr.c | 236 ++++++ tools/testing/selftests/bpf/test_verifier.c | 60 +- .../testing/selftests/bpf/verifier/map_kptr.c | 763 ++++++++++++++++++ 17 files changed, 2394 insertions(+), 191 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/map_kptr.c create mode 100644 tools/testing/selftests/bpf/progs/map_kptr.c create mode 100644 tools/testing/selftests/bpf/verifier/map_kptr.c -- 2.35.1
