bpf-lsm: Introduce new helper bpf_ima_file_hash()

ima_file_hash() has been modified to calculate the measurement of a file on
demand, if it has not been already performed by IMA or the measurement is
not fresh. For compatibility reasons, ima_inode_hash() remains unchanged.

Keep the same approach in eBPF and introduce the new helper
bpf_ima_file_hash() to take advantage of the modified behavior of
ima_file_hash().

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220302111404.193900-4-roberto.sassu@huawei.com

include/uapi/linux/bpf.h

@@ -5119,6 +5119,16 @@ union bpf_attr {
  *		0 on success.
  *		**-EINVAL** for invalid input
  *		**-EOPNOTSUPP** for unsupported protocol
+ *
+ * long bpf_ima_file_hash(struct file *file, void *dst, u32 size)
+ *	Description
+ *		Returns a calculated IMA hash of the *file*.
+ *		If the hash is larger than *size*, then only *size*
+ *		bytes will be copied to *dst*
+ *	Return
+ *		The **hash_algo** is returned on success,
+ *		**-EOPNOTSUP** if the hash calculation failed or **-EINVAL** if
+ *		invalid arguments are passed.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5314,6 +5324,7 @@ union bpf_attr {
 	FN(xdp_store_bytes),		\
 	FN(copy_from_user_task),	\
 	FN(skb_set_tstamp),		\
+	FN(ima_file_hash),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper