From: Dave Thaler <dthaler@xxxxxxxxxxxxx> Fix modulo zero, division by zero, overflow, and underflow. Also clarify how a negative immediate value is used in unsigned division Changes from last submission: addressed comments from Alexei. Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> --- Documentation/bpf/instruction-set.rst | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index e672d5ec6cc..2546630fcbd 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -99,19 +99,28 @@ code value description BPF_ADD 0x00 dst += src BPF_SUB 0x10 dst -= src BPF_MUL 0x20 dst \*= src -BPF_DIV 0x30 dst /= src +BPF_DIV 0x30 dst = (src != 0) ? (dst / src) : 0 BPF_OR 0x40 dst \|= src BPF_AND 0x50 dst &= src BPF_LSH 0x60 dst <<= src BPF_RSH 0x70 dst >>= src BPF_NEG 0x80 dst = ~src -BPF_MOD 0x90 dst %= src +BPF_MOD 0x90 dst = (src != 0) ? (dst % src) : dst BPF_XOR 0xa0 dst ^= src BPF_MOV 0xb0 dst = src BPF_ARSH 0xc0 sign extending shift right BPF_END 0xd0 byte swap operations (see `Byte swap instructions`_ below) ======== ===== ========================================================== +Underflow and overflow are allowed during arithmetic operations, +meaning the 64-bit or 32-bit value will wrap. If +eBPF program execution would result in division by zero, +the destination register is instead set to zero. +If execution would result in modulo by zero, +the destination register is instead set to the source register +as ``BPF_MOV`` would do, meaning that for ``BPF_ALU64`` the value +is unchanged whereas for ``BPF_ALU`` the upper 32 bits are zeroed. + ``BPF_ADD | BPF_X | BPF_ALU`` means:: dst_reg = (u32) dst_reg + (u32) src_reg; @@ -128,6 +137,11 @@ BPF_END 0xd0 byte swap operations (see `Byte swap instructions`_ below) dst_reg = dst_reg ^ imm32 +Also note that the division and modulo operations are unsigned. +Thus, for ``BPF_ALU``, 'imm' is first interpreted as an unsigned +32-bit value, whereas for ``BPF_ALU64``, 'imm' is first sign extended +to 64 bits and the result interpreted as an unsigned 64-bit value. +There are no instructions for signed division or modulo. Byte swap instructions ~~~~~~~~~~~~~~~~~~~~~~ -- 2.33.4
