[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

Changes from last submission: addressed comments from Daniel.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
---
 Documentation/bpf/instruction-set.rst | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index e672d5ec6cc..a7f4574562c 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -99,19 +99,27 @@ 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 +136,11 @@ 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