Patch [1] fixed possible kernel crash due to specific sdiv/smod operations in bpf program. The following are related operations and the expected results of those operations: - LLONG_MIN/-1 = LLONG_MIN - INT_MIN/-1 = INT_MIN - LLONG_MIN%-1 = 0 - INT_MIN%-1 = 0 Those operations are replaced with codes which won't cause kernel crash. This patch documents what operations may cause exception and what replacement operations are. [1] https://lore.kernel.org/all/20240913150326.1187788-1-yonghong.song@xxxxxxxxx/ Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx> --- .../bpf/standardization/instruction-set.rst | 25 +++++++++++++++---- 1 file changed, 20 insertions(+), 5 deletions(-) diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst index ab820d565052..d150c1d7ad3b 100644 --- a/Documentation/bpf/standardization/instruction-set.rst +++ b/Documentation/bpf/standardization/instruction-set.rst @@ -347,11 +347,26 @@ register. ===== ===== ======= ========================================================== Underflow and overflow are allowed during arithmetic operations, meaning -the 64-bit or 32-bit value will wrap. If BPF program execution would -result in division by zero, the destination register is instead set to zero. -If execution would result in modulo by zero, for ``ALU64`` the value of -the destination register is unchanged whereas for ``ALU`` the upper -32 bits of the destination register are zeroed. +the 64-bit or 32-bit value will wrap. There are also a few arithmetic operations +which may cause exception for certain architectures. Since crashing the kernel +is not an option, those operations are replaced with alternative operations. + +.. table:: Arithmetic operations with possible exceptions + + ===== ========== ============================= ========================== + name class original replacement + ===== ========== ============================= ========================== + DIV ALU64/ALU dst /= 0 dst = 0 + SDIV ALU64/ALU dst s/= 0 dst = 0 + MOD ALU64 dst %= 0 dst = dst (no replacement) + MOD ALU dst %= 0 dst = (u32)dst + SMOD ALU64 dst s%= 0 dst = dst (no replacement) + SMOD ALU dst s%= 0 dst = (u32)dst + SDIV ALU64 dst s/= -1 (dst = LLONG_MIN) dst = LLONG_MIN + SDIV ALU dst s/= -1 (dst = INT_MIN) dst = (u32)INT_MIN + SMOD ALU64 dst s%= -1 (dst = LLONG_MIN) dst = 0 + SMOD ALU dst s%= -1 (dst = INT_MIN) dst = 0 + ===== ========== ============================= =========================== ``{ADD, X, ALU}``, where 'code' = ``ADD``, 'source' = ``X``, and 'class' = ``ALU``, means:: -- 2.43.5
