From: Dave Thaler <dthaler@xxxxxxxxxxxxx> [ Upstream commit 0eb9d19e2201068260e439a5c96dc85f9f3722a2 ] Fix modulo zero, division by zero, overflow, and underflow. Also clarify how a negative immediate value is used in unsigned division. Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx> Link: https://lore.kernel.org/bpf/20230124001218.827-1-dthaler1968@xxxxxxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- Documentation/bpf/instruction-set.rst | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index e672d5ec6cc7b..2d3fe59bd260f 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -99,19 +99,26 @@ 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, for ``BPF_ALU64`` the value of +the destination register is unchanged whereas for ``BPF_ALU`` the upper +32 bits of the destination register are zeroed. + ``BPF_ADD | BPF_X | BPF_ALU`` means:: dst_reg = (u32) dst_reg + (u32) src_reg; @@ -128,6 +135,11 @@ 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 ~~~~~~~~~~~~~~~~~~~~~~ -- 2.39.0
