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bpf, docs: Fix modulo zero, division by zero, overflow, and underflow 

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@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230124001218.827-1-dthaler1968@googlemail.com
This commit is contained in:
Dave Thaler 2023-01-24 00:12:18 +00:00 committed by Daniel Borkmann
parent ea403bb7b3
commit 0eb9d19e22
1 changed files with 14 additions and 2 deletions
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16
Documentation/bpf/instruction-set.rst
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@ -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
~~~~~~~~~~~~~~~~~~~~~~