selftests/bpf: Cover negative buffer pointer offsets

selftests/bpf: Cover negative buffer pointer offsets

Add verifier coverage for constant negative offsets on PTR_TO_TP_BUFFER
and PTR_TO_BUF pointers. Both programs adjust the buffer pointer by -8
and access it at offset zero, so the negative effective start must be
rejected at load time.

Switch the raw tracepoint writable attach checks from nbd_send_request
to bpf_testmod_test_writable_bare_tp, avoiding a dependency on the NBD
tracepoint. Keep the existing past-end case and add a case with a
negative var_off compensated by a positive instruction offset. The
effective start remains non-negative, so the program loads, but its
access end exceeds the writable context size and
bpf_raw_tracepoint_open() must return -EINVAL.

Cc: stable@vger.kernel.org # 5.2.0
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://patch.msgid.link/20260714093846.18159-3-sun.jian.kdev@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>