fscrypt: compute max_lblk_bits from s_maxbytes and block size

For a given filesystem, the number of bits used by the maximum file
logical block number is computable from the maximum file size and the
block size. These values are always present in struct super_block.
Therefore, compute it this way instead of using the value from
fscrypt_operations::get_ino_and_lblk_bits. Since filesystems always
have to set the super_block fields anyway, this avoids having to provide
this information redundantly via fscrypt_operations.

This change is in preparation for adding support for sub-block data
units. For that, the value that is needed will become "the maximum file
data unit index". A hardcoded value won't suffice for that; it will
need to be computed anyway.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Eric Biggers <[e-mail]>

fs/crypto/fscrypt_private.h

···
 void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
 			 const struct fscrypt_info *ci);
 
+/*
+ * Return the number of bits used by the maximum file logical block number that
+ * is possible on the given filesystem.
+ */
+static inline int
+fscrypt_max_file_lblk_bits(const struct super_block *sb)
+{
+	return fls64(sb->s_maxbytes - 1) - sb->s_blocksize_bits;
+}
+
 /* fname.c */
 bool __fscrypt_fname_encrypted_size(const union fscrypt_policy *policy,
 				    u32 orig_len, u32 max_len,

fs/crypto/inline_crypt.c

···
 
 static unsigned int fscrypt_get_dun_bytes(const struct fscrypt_info *ci)
 {
-	struct super_block *sb = ci->ci_inode->i_sb;
+	const struct super_block *sb = ci->ci_inode->i_sb;
 	unsigned int flags = fscrypt_policy_flags(&ci->ci_policy);
-	int ino_bits = 64, lblk_bits = 64;
 
 	if (flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY)
 		return offsetofend(union fscrypt_iv, nonce);
···
 		return sizeof(__le32);
 
 	/* Default case: IVs are just the file logical block number */
-	if (sb->s_cop->get_ino_and_lblk_bits)
-		sb->s_cop->get_ino_and_lblk_bits(sb, &ino_bits, &lblk_bits);
-	return DIV_ROUND_UP(lblk_bits, 8);
+	return DIV_ROUND_UP(fscrypt_max_file_lblk_bits(sb), 8);
 }
 
 /*

fs/crypto/policy.c

···
 
 static bool supported_iv_ino_lblk_policy(const struct fscrypt_policy_v2 *policy,
 					 const struct inode *inode,
-					 const char *type,
-					 int max_ino_bits, int max_lblk_bits)
+					 const char *type, int max_ino_bits)
 {
 	struct super_block *sb = inode->i_sb;
 	int ino_bits = 64, lblk_bits = 64;
···
 			     type, sb->s_id);
 		return false;
 	}
-	if (lblk_bits > max_lblk_bits) {
+
+	/*
+	 * IV_INO_LBLK_64 and IV_INO_LBLK_32 both require that file logical
+	 * block numbers fit in 32 bits.
+	 */
+	if (fscrypt_max_file_lblk_bits(sb) > 32) {
 		fscrypt_warn(inode,
-			     "Can't use %s policy on filesystem '%s' because its block numbers are too long",
+			     "Can't use %s policy on filesystem '%s' because its maximum file size is too large",
 			     type, sb->s_id);
 		return false;
 	}
···
 		return false;
 
 	if ((policy->flags & FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64) &&
-	    !supported_iv_ino_lblk_policy(policy, inode, "IV_INO_LBLK_64",
-					  32, 32))
+	    !supported_iv_ino_lblk_policy(policy, inode, "IV_INO_LBLK_64", 32))
 		return false;
 
 	/*
···
 	 * implementation limit is 32 bits.
 	 */
 	if ((policy->flags & FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32) &&
-	    !supported_iv_ino_lblk_policy(policy, inode, "IV_INO_LBLK_32",
-					  32, 32))
+	    !supported_iv_ino_lblk_policy(policy, inode, "IV_INO_LBLK_32", 32))
 		return false;
 
 	if (memchr_inv(policy->__reserved, 0, sizeof(policy->__reserved))) {