bpf: Add a BPF helper for getting the IMA hash of an inode

Provide a wrapper function to get the IMA hash of an inode. This helper
is useful in fingerprinting files (e.g executables on execution) and
using these fingerprints in detections like an executable unlinking
itself.

Since the ima_inode_hash can sleep, it's only allowed for sleepable
LSM hooks.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201124151210.1081188-3-kpsingh@chromium.org

scripts/bpf_helpers_doc.py

@@ -436,6 +436,7 @@ class PrinterHelpers(Printer):
             'struct xdp_md',
             'struct path',
             'struct btf_ptr',
+            'struct inode',
     ]
     known_types = {
             '...',
@@ -480,6 +481,7 @@ class PrinterHelpers(Printer):
             'struct task_struct',
             'struct path',
             'struct btf_ptr',
+            'struct inode',
     }
     mapped_types = {
             'u8': '__u8',