Magnus Karlsson [off-list ref] writes:

When the need_wakeup flag was added to AF_XDP, the format of the
XDP_MMAP_OFFSETS getsockopt was extended. Code was added to the
kernel to take care of compatibility issues arrising from running
applications using any of the two formats. However, libbpf was
not extended to take care of the case when the application/libbpf
uses the new format but the kernel only supports the old
format. This patch adds support in libbpf for parsing the old
format, before the need_wakeup flag was added, and emulating a
set of static need_wakeup flags that will always work for the
application.

Hi Magnus

While you're looking at backwards compatibility issues with xsk: libbpf
currently fails to compile on a system that has old kernel headers
installed (this is with kernel-headers 5.3):

$ echo "#include <bpf/xsk.h>" | gcc -x c - 
In file included from <stdin>:1:
/usr/include/bpf/xsk.h: In function ‘xsk_ring_prod__needs_wakeup’:
/usr/include/bpf/xsk.h:82:21: error: ‘XDP_RING_NEED_WAKEUP’ undeclared (first use in this function)
   82 |  return *r->flags & XDP_RING_NEED_WAKEUP;
      |                     ^~~~~~~~~~~~~~~~~~~~
/usr/include/bpf/xsk.h:82:21: note: each undeclared identifier is reported only once for each function it appears in
/usr/include/bpf/xsk.h: In function ‘xsk_umem__extract_addr’:
/usr/include/bpf/xsk.h:173:16: error: ‘XSK_UNALIGNED_BUF_ADDR_MASK’ undeclared (first use in this function)
  173 |  return addr & XSK_UNALIGNED_BUF_ADDR_MASK;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/bpf/xsk.h: In function ‘xsk_umem__extract_offset’:
/usr/include/bpf/xsk.h:178:17: error: ‘XSK_UNALIGNED_BUF_OFFSET_SHIFT’ undeclared (first use in this function)
  178 |  return addr >> XSK_UNALIGNED_BUF_OFFSET_SHIFT;
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



How would you prefer to handle this? A patch like the one below will fix
the compile errors, but I'm not sure it makes sense semantically?

-Toke

diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
index 584f6820a639..954d66e85208 100644
--- a/tools/lib/bpf/xsk.h
+++ b/tools/lib/bpf/xsk.h

@@ -79,7 +79,11 @@ xsk_ring_cons__rx_desc(const struct xsk_ring_cons *rx, __u32 idx)
 
 static inline int xsk_ring_prod__needs_wakeup(const struct xsk_ring_prod *r)
 {
+#ifdef XDP_RING_NEED_WAKEUP
        return *r->flags & XDP_RING_NEED_WAKEUP;
+#else
+       return 0;
+#endif
 }
 
 static inline __u32 xsk_prod_nb_free(struct xsk_ring_prod *r, __u32 nb)

@@ -170,12 +174,20 @@ static inline void *xsk_umem__get_data(void *umem_area, __u64 addr)
 
 static inline __u64 xsk_umem__extract_addr(__u64 addr)
 {
+#ifdef XSK_UNALIGNED_BUF_ADDR_MASK
        return addr & XSK_UNALIGNED_BUF_ADDR_MASK;
+#else
+       return addr;
+#endif
 }
 
 static inline __u64 xsk_umem__extract_offset(__u64 addr)
 {
+#ifdef XSK_UNALIGNED_BUF_OFFSET_SHIFT
        return addr >> XSK_UNALIGNED_BUF_OFFSET_SHIFT;
+#else
+       return 0;
+#endif
 }
 
 static inline __u64 xsk_umem__add_offset_to_addr(__u64 addr)