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>
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@ -85,8 +85,33 @@ Q: Can loops be supported in a safe way?
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A: It's not clear yet.
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BPF developers are trying to find a way to
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support bounded loops where the verifier can guarantee that
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the program terminates in less than 4096 instructions.
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support bounded loops.
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Q: What are the verifier limits?
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--------------------------------
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A: The only limit known to the user space is BPF_MAXINSNS (4096).
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It's the maximum number of instructions that the unprivileged bpf
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program can have. The verifier has various internal limits.
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Like the maximum number of instructions that can be explored during
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program analysis. Currently, that limit is set to 1 million.
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Which essentially means that the largest program can consist
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of 1 million NOP instructions. There is a limit to the maximum number
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of subsequent branches, a limit to the number of nested bpf-to-bpf
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calls, a limit to the number of the verifier states per instruction,
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a limit to the number of maps used by the program.
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All these limits can be hit with a sufficiently complex program.
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There are also non-numerical limits that can cause the program
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to be rejected. The verifier used to recognize only pointer + constant
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expressions. Now it can recognize pointer + bounded_register.
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bpf_lookup_map_elem(key) had a requirement that 'key' must be
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a pointer to the stack. Now, 'key' can be a pointer to map value.
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The verifier is steadily getting 'smarter'. The limits are
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being removed. The only way to know that the program is going to
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be accepted by the verifier is to try to load it.
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The bpf development process guarantees that the future kernel
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versions will accept all bpf programs that were accepted by
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the earlier versions.
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Instruction level questions
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---------------------------
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