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bpf: document the verifier limits

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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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Alexei Starovoitov 2019-04-17 18:27:01 -07:00 committed by Daniel Borkmann
parent 4519efa6f8
commit 3b8802446d
1 changed files with 27 additions and 2 deletions
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29
Documentation/bpf/bpf_design_QA.rst
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@ -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
---------------------------