[PATCH AUTOSEL 6.1 20/49] bpf, docs: Fix modulo zero, division by zero, overflow, and underflow

Sasha Levin
2023-02-26T14:50:00Z

From: Dave Thaler <dthaler@microsoft.com>

[ 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@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230124001218.827-1-dthaler1968@googlemail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 5d798437dad47..3ba6475cfbfc7 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)

src_reg = src_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