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

[dthaler1968@googlemail.com]

From: Dave Thaler <dthaler@microsoft.com>

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>
---
 Documentation/bpf/instruction-set.rst | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index e672d5ec6cc..2ba7c618f33 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,
+the destination register is instead left unchanged.
+
 ``BPF_ADD | BPF_X | BPF_ALU`` means::
 
   dst_reg = (u32) dst_reg + (u32) src_reg;
@@ -128,6 +135,10 @@ 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,
+where 'imm' is first sign extended to 64 bits and then converted
+to an unsigned 64-bit value.  There are no instructions for
+signed division or modulo.
 
 Byte swap instructions
 ~~~~~~~~~~~~~~~~~~~~~~
-- 
2.33.4