libbpf: Fix null dereference in xsk_socket__delete

Fix a possible null pointer dereference in xsk_socket__delete that
will occur if a null pointer is fed into the function.

Fixes: 2f6324a ("libbpf: Support shared umems between queues and devices")
Reported-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Magnus Karlsson <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: magnus-karlsson

tools/lib/bpf/xsk.c

@@ -891,13 +891,14 @@ int xsk_umem__delete(struct xsk_umem *umem)
 void xsk_socket__delete(struct xsk_socket *xsk)
 {
 	size_t desc_sz = sizeof(struct xdp_desc);
-	struct xsk_ctx *ctx = xsk->ctx;
 	struct xdp_mmap_offsets off;
+	struct xsk_ctx *ctx;
 	int err;
 
 	if (!xsk)
 		return;
 
+	ctx = xsk->ctx;
 	if (ctx->prog_fd != -1) {
 		xsk_delete_bpf_maps(xsk);
 		close(ctx->prog_fd);