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.