linux-kernel - [PATCH v4 4/9] fs: Create sb_same

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Message-Id: <20250813-tonyk-overlayfs-v4-4-357ccf2e12ad@igalia.com>
Date: Wed, 13 Aug 2025 19:36:40 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Miklos Szeredi <miklos@...redi.hu>, Amir Goldstein <amir73il@...il.com>, 
 Theodore Tso <tytso@....edu>, Gabriel Krisman Bertazi <krisman@...nel.org>
Cc: linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-fsdevel@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
 kernel-dev@...lia.com, 
 André Almeida <andrealmeid@...lia.com>
Subject: [PATCH v4 4/9] fs: Create sb_same_encoding() helper

For cases where a file lookup can look in different filesystems (like in
overlayfs), both super blocks must have the same encoding and the same
flags. To help with that, create a sb_same_encoding() function.

Reviewed-by: Amir Goldstein <amir73il@...il.com>
Signed-off-by: André Almeida <andrealmeid@...lia.com>
---
Changes from v3:
- Improve wording

Changes from v2:
- Simplify the code. Instead of `if (cond) return true`, just do `return
  cond`;
---
 include/linux/fs.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 20102d81e18a59d5daaed06855d1f168979b4fa7..64d24e89bc5593915158b40f0442e6d8ef3d968d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3753,6 +3753,24 @@ static inline bool sb_has_encoding(const struct super_block *sb)
 	return !!sb_encoding(sb);
 }
 
+/*
+ * Compare if two super blocks have the same encoding and flags
+ */
+static inline bool sb_same_encoding(const struct super_block *sb1,
+				    const struct super_block *sb2)
+{
+#if IS_ENABLED(CONFIG_UNICODE)
+	if (sb1->s_encoding == sb2->s_encoding)
+		return true;
+
+	return (sb1->s_encoding && sb2->s_encoding &&
+	       (sb1->s_encoding->version == sb2->s_encoding->version) &&
+	       (sb1->s_encoding_flags == sb2->s_encoding_flags));
+#else
+	return true;
+#endif
+}
+
 int may_setattr(struct mnt_idmap *idmap, struct inode *inode,
 		unsigned int ia_valid);
 int setattr_prepare(struct mnt_idmap *, struct dentry *, struct iattr *);

-- 
2.50.1