On Thu, Mar 26, 2020 at 7:29 AM KP Singh [off-list ref] wrote:

From: KP Singh <redacted>

Document how eBPF programs (BPF_PROG_TYPE_LSM) can be loaded and
attached (BPF_LSM_MAC) to the LSM hooks.

Signed-off-by: KP Singh <redacted>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Florent Revest <redacted>
Reviewed-by: Thomas Garnier <redacted>
---

This needs another pass and re-reading, has a bunch of outdated info :)

type should match/be compatible as well?

+
+This can be further simplified (if one has access to the BTF information at
+build time) by generating the ``vmlinux.h`` with:
+
+.. code-block:: console
+
+        # bpftool dump file <path-to-btf-vmlinux> format c > vmlinux.h
+

bpftool btf *dump* file

+.. note:: ``path-to-btf-vmlinux`` can be ``/sys/kernel/btf/vmlinux`` if the
+         build environment matches the environment the BPF programs are
+         deployed in.
+
+The ``vmlinux.h`` can then simply be included in the BPF programs without
+requiring the definition of the types.
+
+The eBPF programs can be declared using the``BPF_PROG``
+macros defined in `tools/lib/bpf/bpf_tracing.h`_. In this
+example:
+
+       * ``"lsm/file_mprotect"`` indicates the LSM hook that the program must
+         be attached to
+       * ``mprotect_audit`` is the name of the eBPF program
+
+.. code-block:: c
+
+        SEC("lsm/file_mprotect")
+        int BPF_PROG(mprotect_audit, struct vm_area_struct *vma,
+                     unsigned long reqprot, unsigned long prot, int ret)
+       {
+                /* Ret is the return value from the previous BPF program
+                 * or 0 if it's the first hook.
+                 */
+                if (ret != 0)
+                        return ret;
+
+               int is_heap;
+
+               is_heap = (vma->vm_start >= vma->vm_mm->start_brk &&
+                          vma->vm_end <= vma->vm_mm->brk);
+
+               /* Return an -EPERM or write information to the perf events buffer
+                * for auditing
+                */

return missing?

+       }
+
+The ``__attribute__((preserve_access_index))`` is a clang feature that allows
+the BPF verifier to update the offsets for the access at runtime using the
+:doc:`/bpf/btf` information. Since the BPF verifier is aware of the types, it
+also validates all the accesses made to the various types in the eBPF program.
+
+Loading
+-------
+
+eBPF programs can be loaded with the :manpage:`bpf(2)` syscall's
+``BPF_PROG_LOAD`` operation or more simply by using the the libbpf helper
+``bpf_prog_load_xattr``:
+
+
+.. code-block:: c
+
+       struct bpf_prog_load_attr attr = {
+               .file = "./prog.o",
+       };
+       struct bpf_object *prog_obj;
+       struct bpf_program *prog;
+       int prog_fd;
+
+       bpf_prog_load_xattr(&attr, &prog_obj, &prog_fd);

Can you please update this to not use deprecated/legacy APIs. Please
suggest bpf_object__open/bpf_object__load  and/or BPF skeleton as an
example.

+
+Attachment to LSM Hooks
+-----------------------
+
+The LSM allows attachment of eBPF programs as LSM hooks using :manpage:`bpf(2)`
+syscall's ``BPF_PROG_ATTACH`` operation or more simply by

BPF_PROG_ATTACH is incorrect, it's RAW_TRACEPOINT_OPEN, isn't it?

+using the libbpf helper ``bpf_program__attach_lsm``. In the code shown below
+``prog`` is the eBPF program loaded using ``BPF_PROG_LOAD``:
+
+.. code-block:: c
+
+       struct bpf_link *link;
+
+       link = bpf_program__attach_lsm(prog);
+
+The program can be detached from the LSM hook by *destroying* the ``link``
+link returned by ``bpf_program__attach_lsm``:
+
+.. code-block:: c
+
+       link->destroy();

that's not how it works in C ;)

bpf_link__destroy(link);