selftests/bpf: Pass page size from userspace in sockopt_sk

Since there is no convenient way for bpf program to get PAGE_SIZE
from inside of the kernel, pass the value from userspace.

Zero-initialize the variable in bpf prog, otherwise it will cause
problems on some versions of Clang.
Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210408061310.95877-3-yauheni.kaliuta@redhat.com

tools/testing/selftests/bpf/prog_tests/sockopt_sk.c

@@ -200,6 +200,8 @@ static void run_test(int cgroup_fd)
 	if (!ASSERT_OK_PTR(skel, "skel_load"))
 		goto cleanup;
 
+	skel->bss->page_size = getpagesize();
+
 	skel->links._setsockopt =
 		bpf_program__attach_cgroup(skel->progs._setsockopt, cgroup_fd);
 	if (!ASSERT_OK_PTR(skel->links._setsockopt, "setsockopt_link"))

tools/testing/selftests/bpf/progs/sockopt_sk.c

@@ -7,9 +7,7 @@
 
 char _license[] SEC("license") = "GPL";
 
-#ifndef PAGE_SIZE
-#define PAGE_SIZE 4096
-#endif
+int page_size = 0; /* userspace should set it */
 
 #ifndef SOL_TCP
 #define SOL_TCP IPPROTO_TCP
@@ -89,7 +87,7 @@ int _getsockopt(struct bpf_sockopt *ctx)
 		 * program can only see the first PAGE_SIZE
 		 * bytes of data.
 		 */
-		if (optval_end - optval != PAGE_SIZE)
+		if (optval_end - optval != page_size)
 			return 0; /* EPERM, unexpected data size */
 
 		return 1;
@@ -160,7 +158,7 @@ int _setsockopt(struct bpf_sockopt *ctx)
 
 	if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
 		/* Original optlen is larger than PAGE_SIZE. */
-		if (ctx->optlen != PAGE_SIZE * 2)
+		if (ctx->optlen != page_size * 2)
 			return 0; /* EPERM, unexpected data size */
 
 		if (optval + 1 > optval_end)
@@ -174,7 +172,7 @@ int _setsockopt(struct bpf_sockopt *ctx)
 		 * program can only see the first PAGE_SIZE
 		 * bytes of data.
 		 */
-		if (optval_end - optval != PAGE_SIZE)
+		if (optval_end - optval != page_size)
 			return 0; /* EPERM, unexpected data size */
 
 		return 1;