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| author | Alexei Starovoitov <ast@kernel.org> | 2019-04-17 18:27:01 -0700 |
|---|---|---|
| committer | Daniel Borkmann <daniel@iogearbox.net> | 2019-04-23 00:39:12 +0200 |
| commit | 3b8802446d27522cd6d32178ba975cc492611f31 (patch) | |
| tree | 13c3e3e2e01ef1aa55ba08d489383d337619806d /Documentation/bpf | |
| parent | 4519efa6f8ea343e43ade21b0189b0b295439202 (diff) | |
| download | linux-rt-3b8802446d27522cd6d32178ba975cc492611f31.tar.gz | |
bpf: document the verifier limits
Document the verifier limits.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Diffstat (limited to 'Documentation/bpf')
| -rw-r--r-- | Documentation/bpf/bpf_design_QA.rst | 29 |
1 files changed, 27 insertions, 2 deletions
diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst index 10453c627135..cb402c59eca5 100644 --- a/Documentation/bpf/bpf_design_QA.rst +++ b/Documentation/bpf/bpf_design_QA.rst @@ -85,8 +85,33 @@ Q: Can loops be supported in a safe way? A: It's not clear yet. BPF developers are trying to find a way to -support bounded loops where the verifier can guarantee that -the program terminates in less than 4096 instructions. +support bounded loops. + +Q: What are the verifier limits? +-------------------------------- +A: The only limit known to the user space is BPF_MAXINSNS (4096). +It's the maximum number of instructions that the unprivileged bpf +program can have. The verifier has various internal limits. +Like the maximum number of instructions that can be explored during +program analysis. Currently, that limit is set to 1 million. +Which essentially means that the largest program can consist +of 1 million NOP instructions. There is a limit to the maximum number +of subsequent branches, a limit to the number of nested bpf-to-bpf +calls, a limit to the number of the verifier states per instruction, +a limit to the number of maps used by the program. +All these limits can be hit with a sufficiently complex program. +There are also non-numerical limits that can cause the program +to be rejected. The verifier used to recognize only pointer + constant +expressions. Now it can recognize pointer + bounded_register. +bpf_lookup_map_elem(key) had a requirement that 'key' must be +a pointer to the stack. Now, 'key' can be a pointer to map value. +The verifier is steadily getting 'smarter'. The limits are +being removed. The only way to know that the program is going to +be accepted by the verifier is to try to load it. +The bpf development process guarantees that the future kernel +versions will accept all bpf programs that were accepted by +the earlier versions. + Instruction level questions --------------------------- |