On Mon, May 9, 2022 at 2:54 PM Andrii Nakryiko
[off-list ref] wrote:

On Fri, Apr 29, 2022 at 2:16 PM Stanislav Fomichev [off-list ref] wrote:

quoted

sk_priority & sk_mark are writable, the rest is readonly.

Add new ldx_offset fixups to lookup the offset of struct field.
Allow using test.kfunc regardless of prog_type.

One interesting thing here is that the verifier doesn't
really force me to add NULL checks anywhere :-/

Signed-off-by: Stanislav Fomichev <redacted>
---
 tools/testing/selftests/bpf/test_verifier.c   | 54 ++++++++++++++++++-
 .../selftests/bpf/verifier/lsm_cgroup.c       | 34 ++++++++++++
 2 files changed, 87 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/verifier/lsm_cgroup.c

[...]

quoted

diff --git a/tools/testing/selftests/bpf/verifier/lsm_cgroup.c b/tools/testing/selftests/bpf/verifier/lsm_cgroup.c
new file mode 100644
index 000000000000..af0efe783511
--- /dev/null
+++ b/tools/testing/selftests/bpf/verifier/lsm_cgroup.c

@@ -0,0 +1,34 @@
+#define SK_WRITABLE_FIELD(tp, field, size, res) \
+{ \
+       .descr = field, \
+       .insns = { \
+               /* r1 = *(u64 *)(r1 + 0) */ \
+               BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, 0), \
+               /* r1 = *(u64 *)(r1 + offsetof(struct socket, sk)) */ \
+               BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, 0), \
+               /* r2 = *(u64 *)(r1 + offsetof(struct sock, <field>)) */ \
+               BPF_LDX_MEM(size, BPF_REG_2, BPF_REG_1, 0), \
+               /* *(u64 *)(r1 + offsetof(struct sock, <field>)) = r2 */ \
+               BPF_STX_MEM(size, BPF_REG_1, BPF_REG_2, 0), \
+               BPF_MOV64_IMM(BPF_REG_0, 1), \
+               BPF_EXIT_INSN(), \
+       }, \
+       .result = res, \
+       .errstr = res ? "no write support to 'struct sock' at off" : "", \
+       .prog_type = BPF_PROG_TYPE_LSM, \
+       .expected_attach_type = BPF_LSM_CGROUP, \
+       .kfunc = "socket_post_create", \
+       .fixup_ldx = { \
+               { "socket", "sk", 1 }, \
+               { tp, field, 2 }, \
+               { tp, field, 3 }, \
+       }, \
+}
+
+SK_WRITABLE_FIELD("sock_common", "skc_family", BPF_H, REJECT),
+SK_WRITABLE_FIELD("sock", "sk_sndtimeo", BPF_DW, REJECT),
+SK_WRITABLE_FIELD("sock", "sk_priority", BPF_W, ACCEPT),
+SK_WRITABLE_FIELD("sock", "sk_mark", BPF_W, ACCEPT),
+SK_WRITABLE_FIELD("sock", "sk_pacing_rate", BPF_DW, REJECT),
+

have you tried writing it as C program and adding the test to
test_progs? Does something not work there?

Seems like it should work, I don't see any issues with writing 5
programs to test each field.
But test_verified still feels like a better fit? Any reason in
particular you'd prefer test_progs over test_verifier?