fsverity: add support for info in fs-specific part of inode

Add an inode_info_offs field to struct fsverity_operations, and update
fs/verity/ to support it.  When set to a nonzero value, it specifies the
offset to the fsverity_info pointer within the filesystem-specific part
of the inode structure, to be used instead of inode::i_verity_info.

Since this makes inode::i_verity_info no longer necessarily used, update
comments that mentioned it.

This is a prerequisite for a later commit that removes
inode::i_verity_info, saving memory and improving cache efficiency on
filesystems that don't support fsverity.

Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/20250810075706.172910-9-ebiggers@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 34 insertions, 10 deletions

diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index 1eb7eae580be..e0f132cb7839 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -28,6 +28,12 @@
 
 /* Verity operations for filesystems */
 struct fsverity_operations {
+	/**
+	 * The offset of the pointer to struct fsverity_info in the
+	 * filesystem-specific part of the inode, relative to the beginning of
+	 * the common part of the inode (the 'struct inode').
+	 */
+	ptrdiff_t inode_info_offs;
 
 	/**
 	 * Begin enabling verity on the given file.
@@ -124,15 +130,33 @@ struct fsverity_operations {
 
 #ifdef CONFIG_FS_VERITY
 
+static inline struct fsverity_info **
+fsverity_info_addr(const struct inode *inode)
+{
+	if (inode->i_sb->s_vop->inode_info_offs == 0)
+		return (struct fsverity_info **)&inode->i_verity_info;
+	return (void *)inode + inode->i_sb->s_vop->inode_info_offs;
+}
+
 static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
 {
 	/*
-	 * Pairs with the cmpxchg_release() in fsverity_set_info().
-	 * I.e., another task may publish ->i_verity_info concurrently,
-	 * executing a RELEASE barrier.  We need to use smp_load_acquire() here
-	 * to safely ACQUIRE the memory the other task published.
+	 * Since this function can be called on inodes belonging to filesystems
+	 * that don't support fsverity at all, and fsverity_info_addr() doesn't
+	 * work on such filesystems, we have to start with an IS_VERITY() check.
+	 * Checking IS_VERITY() here is also useful to minimize the overhead of
+	 * fsverity_active() on non-verity files.
+	 */
+	if (!IS_VERITY(inode))
+		return NULL;
+
+	/*
+	 * Pairs with the cmpxchg_release() in fsverity_set_info().  I.e.,
+	 * another task may publish the inode's verity info concurrently,
+	 * executing a RELEASE barrier.  Use smp_load_acquire() here to safely
+	 * ACQUIRE the memory the other task published.
 	 */
-	return smp_load_acquire(&inode->i_verity_info);
+	return smp_load_acquire(fsverity_info_addr(inode));
 }
 
 /* enable.c */
@@ -156,11 +180,11 @@ void __fsverity_cleanup_inode(struct inode *inode);
  * fsverity_cleanup_inode() - free the inode's verity info, if present
  * @inode: an inode being evicted
  *
- * Filesystems must call this on inode eviction to free ->i_verity_info.
+ * Filesystems must call this on inode eviction to free the inode's verity info.
  */
 static inline void fsverity_cleanup_inode(struct inode *inode)
 {
-	if (inode->i_verity_info)
+	if (*fsverity_info_addr(inode))
 		__fsverity_cleanup_inode(inode);
 }
 
@@ -267,12 +291,12 @@ static inline bool fsverity_verify_page(struct page *page)
  * fsverity_active() - do reads from the inode need to go through fs-verity?
  * @inode: inode to check
  *
- * This checks whether ->i_verity_info has been set.
+ * This checks whether the inode's verity info has been set.
  *
  * Filesystems call this from ->readahead() to check whether the pages need to
  * be verified or not.  Don't use IS_VERITY() for this purpose; it's subject to
  * a race condition where the file is being read concurrently with
- * FS_IOC_ENABLE_VERITY completing.  (S_VERITY is set before ->i_verity_info.)
+ * FS_IOC_ENABLE_VERITY completing.  (S_VERITY is set before the verity info.)
  *
  * Return: true if reads need to go through fs-verity, otherwise false
  */
@@ -287,7 +311,7 @@ static inline bool fsverity_active(const struct inode *inode)
  * @filp: the struct file being set up
  *
  * When opening a verity file, deny the open if it is for writing.  Otherwise,
- * set up the inode's ->i_verity_info if not already done.
+ * set up the inode's verity info if not already done.
  *
  * When combined with fscrypt, this must be called after fscrypt_file_open().
  * Otherwise, we won't have the key set up to decrypt the verity metadata.