bpf: Add value tracking for BPF_DIV #6544
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This patch introduces interval analysis (range tracking) and tnum
analysis (bitwise tracking) for both signed and unsigned division
operations in the BPF verifier.
The BPF verifier currently lacks support for value tracking on BPF_DIV
instructions, which can lead to false positives during verification of
BPF programs that utilize division instructions.
According to the BPF instruction set[1], the instruction's offset field
(`insn->off`) is used to distinguish between signed (`off == 1`) and
unsigned division (`off == 0`). Moreover, we also follow the BPF division
semantics to handle special cases, such as division by zero and signed
division overflow.
- UDIV: dst = (src != 0) ? (dst / src) : 0
- SDIV: dst = (src == 0) ? 0 : ((src == -1 && dst == LLONG_MIN) ? LLONG_MIN : (dst / src))
Here is the overview of the changes made in this patch:
1. For interval analysis:
- Added `scalar_min_max_udiv` and `scalar32_min_max_udiv` to update
umin/umax bounds, which is straightforward.
- Added `scalar_min_max_sdiv` and `scalar32_min_max_sdiv` to update
smin/smax bounds. It handles non-monotonic intervals by decomposing
the divisor range into negative, zero, and positive sub-ranges, and
computing the result range for each sub-range separately. Finally,
it combines the results to get the final smin/smax bounds.
2. For tnum analysis, we referred to LLVM's KnownBits implementation[2]
and the recent research on abstract interpretation of division[3]:
- Added `tnum_udiv` to compute the tnum for unsigned division. It
calculates the maximum possible result based on the maximum values
of the dividend tnum and the minimum values of the divisor tnum,
then constructs the resulting tnum accordingly. We have prove its
soundness using Rocq Prover[4].
- Added `tnum_sdiv` to compute the tnum for signed division. It splits
the operands into positive and negative components, then performs
calculation on absolute values using `tnum_udiv`, finally unions
the results to ensure soundness.
3. Also updated existing selftests based on the expected BPF_DIV behavior.
[1] https://www.kernel.org/doc/Documentation/bpf/standardization/instruction-set.rst
[2] https://llvm.org/doxygen/KnownBits_8cpp_source.html
[3] https://dl.acm.org/doi/10.1145/3728905
[4] https://github.com/shenghaoyuan/open-verified-artifacts/tree/main/tnum
Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Co-developed-by: Tianci Cao <ziye@zju.edu.cn>
Signed-off-by: Tianci Cao <ziye@zju.edu.cn>
Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
Now BPF_DIV has value tracking support via interval and tnum analysis. This patch adds selftests to cover various cases of signed and unsigned division operations, including edge cases like division by zero and signed division overflow. Specifically, these selftests are based on dead code elimination: If the BPF verifier can precisely analyze the result of a division operation, it can prune the path that leads to an error (here we use invalid memory access as the error case), allowing the program to pass verification. Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com> Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com> Co-developed-by: Tianci Cao <ziye@zju.edu.cn> Signed-off-by: Tianci Cao <ziye@zju.edu.cn> Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=1035457 expired. Closing PR. |
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Pull request for series with
subject: bpf: Add value tracking for BPF_DIV
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1035457