Add a brief introduction for file system kfuncs:
bpf_get_file_xattr()
bpf_get_fsverity_digest()
The documentation highlights the strategy to avoid recursions of these
kfuncs.
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231129234417.856536-4-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
