[PATCH] bpf: Fix mix-up of 4096 and page size.

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

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 commit updates the page size references to 4096 to ensure consistency
and prevent overflow issues in fragment size calculations.

Fixes: 1c1949982524 ("bpf: introduce frags support to bpf_prog_test_run_xdp()")
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
---
 net/bpf/test_run.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 501ec4249..eb5476184 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -124,7 +124,7 @@ struct xdp_test_data {
  * must be updated accordingly this gets changed, otherwise BPF selftests
  * will fail.
  */
-#define TEST_XDP_FRAME_SIZE (PAGE_SIZE - sizeof(struct xdp_page_head))
+#define TEST_XDP_FRAME_SIZE (4096 - sizeof(struct xdp_page_head))
 #define TEST_XDP_MAX_BATCH 256
 
 static void xdp_test_run_init_page(netmem_ref netmem, void *arg)
@@ -660,7 +660,7 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
 	void __user *data_in = u64_to_user_ptr(kattr->test.data_in);
 	void *data;
 
-	if (size < ETH_HLEN || size > PAGE_SIZE - headroom - tailroom)
+	if (size < ETH_HLEN || size > 4096 - headroom - tailroom)
 		return ERR_PTR(-EINVAL);
 
 	if (user_size > size)
@@ -1297,7 +1297,7 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
 			frag = &sinfo->frags[sinfo->nr_frags++];
 
 			data_len = min_t(u32, kattr->test.data_size_in - size,
-					 PAGE_SIZE);
+					 4096);
 			skb_frag_fill_page_desc(frag, page, 0, data_len);
 
 			if (copy_from_user(page_address(page), data_in + size,
-- 
2.43.5