bpf: Document EFAULT changes for sockopt

And add examples for how to correctly handle large optlens.
This is less relevant now when we don't EFAULT anymore, but
that's still the correct thing to do.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20230511170456.1759459-5-sdf@google.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Documentation/bpf/prog_cgroup_sockopt.rst

@@ -98,10 +98,65 @@ can access only the first ``PAGE_SIZE`` of that data. So it has to options:
   indicates that the kernel should use BPF's trimmed ``optval``.
 
 When the BPF program returns with the ``optlen`` greater than
-``PAGE_SIZE``, the userspace will receive ``EFAULT`` errno.
+``PAGE_SIZE``, the userspace will receive original kernel
+buffers without any modifications that the BPF program might have
+applied.
 
 Example
 =======
 
+Recommended way to handle BPF programs is as follows:
+
+.. code-block:: c
+
+	SEC("cgroup/getsockopt")
+	int getsockopt(struct bpf_sockopt *ctx)
+	{
+		/* Custom socket option. */
+		if (ctx->level == MY_SOL && ctx->optname == MY_OPTNAME) {
+			ctx->retval = 0;
+			optval[0] = ...;
+			ctx->optlen = 1;
+			return 1;
+		}
+
+		/* Modify kernel's socket option. */
+		if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
+			ctx->retval = 0;
+			optval[0] = ...;
+			ctx->optlen = 1;
+			return 1;
+		}
+
+		/* optval larger than PAGE_SIZE use kernel's buffer. */
+		if (ctx->optlen > PAGE_SIZE)
+			ctx->optlen = 0;
+
+		return 1;
+	}
+
+	SEC("cgroup/setsockopt")
+	int setsockopt(struct bpf_sockopt *ctx)
+	{
+		/* Custom socket option. */
+		if (ctx->level == MY_SOL && ctx->optname == MY_OPTNAME) {
+			/* do something */
+			ctx->optlen = -1;
+			return 1;
+		}
+
+		/* Modify kernel's socket option. */
+		if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
+			optval[0] = ...;
+			return 1;
+		}
+
+		/* optval larger than PAGE_SIZE use kernel's buffer. */
+		if (ctx->optlen > PAGE_SIZE)
+			ctx->optlen = 0;
+
+		return 1;
+	}
+
 See ``tools/testing/selftests/bpf/progs/sockopt_sk.c`` for an example
 of BPF program that handles socket options.