Fix xdp_adjust_frags_tail_grow selftest on powerpc

For platforms on powerpc architecture with a default page size greater than 4096, there was an inconsistency in fragment size calculation. This caused the BPF selftest xdp_adjust_tail/xdp_adjust_frags_tail_grow to fail on powerpc.

The issue occurred because the fragment buffer size in bpf_prog_test_run_xdp() was set to 4096, while the actual data size in the fragment within the shared skb was checked against PAGE_SIZE (65536 on powerpc) in min_t, causing it to exceed 4096 and be set accordingly. This discrepancy led to an overflow when bpf_xdp_frags_increase_tail() checked for tailroom, as skb_frag_size(frag) could be greater than rxq->frag_size (when PAGE_SIZE > 4096).

This change fixes:

1. test_run by getting the correct arch dependent PAGE_SIZE.
2. selftest by caculating tailroom and getting correct PAGE_SIZE.

Changes:
v1 -> v2:
* Address comments from Alexander
* Use dynamic page size, cacheline size and size of struct skb_shared_info to calculate parameters.
* Fixed both test_run and selftest.

v1: https://lore.kernel.org/all/[email protected]/

Saket Kumar Bhaskar (2):
bpf, test_run: Replace hardcoded page size with dynamic PAGE_SIZE in
test_run
selftests/bpf: Refactor xdp_adjust_tail selftest with dynamic sizing

 .../bpf/prog_tests/xdp_adjust_tail.c          | 160 +++++++++++++-----
 .../bpf/progs/test_xdp_adjust_tail_grow.c     |  41 +++--
 2 files changed, 149 insertions(+), 52 deletions(-)

2.43.5