Re: lsm_cgroup.c selftest fails to compile when CONFIG_PACKET!=y

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

On Thu, 2024-01-18 at 19:58 +0800, Shung-Hsi Yu wrote:
> Hi,
> 
> Compilation of lsm_cgroup.c will fail if the vmlinux.h comes from a
> kernel that does _not_ have CONFIG_PACKET=y. The reason is that the
> definition of struct sockaddr_ll is not present in vmlinux.h and the
> compiler will complain that is has an incomplete type.
> 
>     CLNG-BPF [test_maps] lsm_cgroup.bpf.o
>   progs/lsm_cgroup.c:105:21: error: variable has incomplete type 'struct sockaddr_ll'
>     105 |         struct sockaddr_ll sa = {};
>         |                            ^
>   progs/lsm_cgroup.c:105:9: note: forward declaration of 'struct sockaddr_ll'
>     105 |         struct sockaddr_ll sa = {};
>         |                ^
>   1 error generated.
> 
> While including linux/if_packet.h somehow made the compilation works for
> me, IIUC this isn't a proper solution because vmlinux.h and kernel
> headers should not be used at the same time (and would lead to
> redefinition error when the kernel is built with CONFIG_PACKET=y, e.g.
> on BPF CI).
> 
> What would be the suggested way to work around this?
> 
> Thanks,
> Shung-Hsi

Hi Shung-Hsi,

One option is to use CO-RE, e.g. as at the bottom of this email
(not sure if people would agree with me).
But that would not produce usable test anyways,
as load would fail with unresolved CO-RE relocation.

But what is your final goal?
As far as I understand, selftests are supposed to be built and run
using specific configuration, here is how config for x86 CI is prepared:

./scripts/kconfig/merge_config.sh \
         ./tools/testing/selftests/bpf/config \
         ./tools/testing/selftests/bpf/config.vm \
         ./tools/testing/selftests/bpf/config.x86_64

(root is kernel source).
I'm not sure if other configurations are supposed to be supported.

Thanks,
Eduard

---

diff --git a/tools/testing/selftests/bpf/progs/lsm_cgroup.c b/tools/testing/selftests/bpf/progs/lsm_cgroup.c
index 02c11d16b692..8381e5a202c8 100644
--- a/tools/testing/selftests/bpf/progs/lsm_cgroup.c
+++ b/tools/testing/selftests/bpf/progs/lsm_cgroup.c
@@ -2,6 +2,7 @@
 
 #include "vmlinux.h"
 #include "bpf_tracing_net.h"
+#include <bpf/bpf_core_read.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
@@ -98,11 +99,15 @@ int BPF_PROG(socket_post_create2, struct socket *sock, int family,
        return real_create(sock, family, protocol);
 }
 
+struct sockaddr_ll___local {
+       __be16 sll_protocol;
+} __attribute__((preserve_access_index));
+
 static __always_inline int real_bind(struct socket *sock,
                                     struct sockaddr *address,
                                     int addrlen)
 {
-       struct sockaddr_ll sa = {};
+       __u16 sll_protocol = 0;
 
        if (sock->sk->__sk_common.skc_family != AF_PACKET)
                return 1;
@@ -110,8 +115,10 @@ static __always_inline int real_bind(struct socket *sock,
        if (sock->sk->sk_kern_sock)
                return 1;
 
-       bpf_probe_read_kernel(&sa, sizeof(sa), address);
-       if (sa.sll_protocol)
+       bpf_probe_read_kernel(
+               &sll_protocol, sizeof(sll_protocol),
+               (__u8*)address + bpf_core_field_offset(struct sockaddr_ll___local, sll_protocol));
+       if (sll_protocol)
                return 0; /* EPERM */
 
        /* Can access cgroup local storage. */