Add verifier_var_off coverage for variable-offset stack reads from spilled
scalar constant zero values.
Cover single-slot and cross-slot spilled zero reads, a sub-8-byte spill
with neighbouring STACK_ZERO bytes, and a sub-8-byte spill with
neighbouring STACK_MISC bytes that must not be treated as zero.
Add verifier_spill_fill coverage for a fixed-offset stack read spanning
both STACK_ZERO bytes and scalar const-zero STACK_SPILL bytes.
Use verifier log assertions to check both the zero result and the precision
backtracking trail.
Assisted-by: opencode:gpt-5.5
Signed-off-by: Woojin Ji <random6.xyz@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260625-bpf-stack-var-off-zero-v1-v3-2-a068210a761b@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
e693da913d