EVM: add comment describing why ino field is still unsigned long

Mimi pointed out that we didn't widen the inode number field in struct
h_misc alongside the inode->i_ino widening. While we could make an
equivalent change there, that would require EVM resigning on all 32-bit
hosts.

Instead, leave the field as an unsigned long. This should have no effect
on 64-bit hosts, and allow things to continue working on 32-bit hosts in
the cases where the i_ino fits in 32-bits.

Add a comment explaining why it's being left as unsigned long.

Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260316-iino-u64-v3-1-d1076b8f7a20@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jtlayton

security/integrity/evm/evm_crypto.c

@@ -144,6 +144,12 @@ static void hmac_add_misc(struct shash_desc *desc, struct inode *inode,
 			  char type, char *digest)
 {
 	struct h_misc {
+		/*
+		 * Although inode->i_ino is now u64, this field remains
+		 * unsigned long to allow existing HMAC and signatures from
+		 * 32-bit hosts to continue working when i_ino hasn't changed
+		 * and fits in a u32.
+		 */
 		unsigned long ino;
 		__u32 generation;
 		uid_t uid;