Add text about underflow, overflow, and division by zero

Signed-off-by: Dave Thaler <[email protected]>

Author: dthaler

isa/README.md

@@ -7,6 +7,7 @@ Some documentation links include:
 * https://docs.cilium.io/en/stable/bpf/
 * https://www.kernel.org/doc/Documentation/networking/filter.txt
 * https://pchaigno.github.io/bpf/2021/10/20/ebpf-instruction-sets.html
+* https://www.kernel.org/doc/html/latest/bpf/bpf_design_QA.html#instruction-level-questions
 
 Some implementation links include:
 

isa/kernel.org/instruction-set.rst

@@ -185,6 +185,13 @@ The 4-bit 'code' field encodes the operation as follows:
   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.
+
+``BPF_DIV`` has an implicit program exit condition as well. If
+eBPF program execution would result in division by zero,
+program execution must be gracefully aborted.
+
 Examples:
 
 ``BPF_ADD | BPF_X | BPF_ALU`` (0x0c) means::