Tests: Pin 32-bit arithmetic on xdp_md.data as a verifier-level xfail

xdp_md.data is u32 in C, so ctx.data - k lowers to a 32-bit add on the
loaded value, exactly as C ranks it, and the verifier rejects it: it
tracks data as a packet pointer and prohibits 32-bit pointer arithmetic
(observed in CI). IR and llc succeed, so the entry is at the verifier
level. It flips to passing when packet-pointer fields get 64-bit
pointer rank (TODO in expr_pass._descriptor).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: r41k0u

tests/failing_tests/vmlinux/ctx_data_arith.py

@@ -0,0 +1,27 @@
+# xdp_md.data is declared u32, so C's rules make ctx.data - k a 32-bit
+# subtraction, and that is what the compiler emits (w0 += -1). The verifier
+# tracks data as a packet pointer and rejects it: "R0 32-bit pointer
+# arithmetic prohibited". C hits the same and casts through (void *)(long).
+# The planned fix gives the packet-pointer fields 64-bit pointer rank so no
+# cast is needed; see the TODO in expr_pass._descriptor. The same u32 rank on
+# a scalar field is pinned as passing in passing_tests/vmlinux/ctx_field_rank.py.
+from ctypes import c_int32, c_int64
+from pythonbpf import bpf, section, bpfglobal, compile
+from vmlinux import struct_xdp_md
+
+
+@bpf
+@section("xdp")
+def prog(ctx: struct_xdp_md) -> c_int64:
+    k = c_int32(1)
+    d = ctx.data - k
+    return c_int64(d)
+
+
+@bpf
+@bpfglobal
+def LICENSE() -> str:
+    return "GPL"
+
+
+compile()

tests/test_config.toml

@@ -60,3 +60,7 @@
 "failing_tests/loops/for_continue.py" = {reason = "for/while loops not implemented: continue is not supported", level = "ir"}
 
 "failing_tests/loops/for_map_items.py" = {reason = "for/while loops not implemented: no sugar over bpf_for_each_map_elem()-style map iteration exists yet", level = "ir"}
+
+# 32-bit arithmetic on a packet-pointer context field. IR and llc succeed; the
+# verifier rejects it with "R0 32-bit pointer arithmetic prohibited".
+"failing_tests/vmlinux/ctx_data_arith.py" = {reason = "xdp_md.data is u32 in C but a packet pointer to the verifier; 32-bit arithmetic on it is prohibited until packet-pointer fields get 64-bit pointer rank", level = "verifier"}