[PATCH 1/4] vfs: introduce function to map unique ino/dev pairs

[Mark Fasheh <mfasheh@suse.de>]

There are places in the VFS where we export an ino/dev pair to userspace.
/proc/PID/maps is a good example - we directly expose inode->i_ino and
inode->i_sb->s_dev to userspace there.  Many filesystems don't put a unique
value in inode->i_ino and instead rely on ->getattr to provide the real
inode number to userspace.  super->s_dev is similar - some filesystems
expose a difference device from what's put in super->s_dev when queried via
stat/statx.

Ultimately this makes it impossible for a user (or software) to match one of
those reported pairs to the right inode.

We can fix this by adding a helper function, vfs_map_unique_ino_dev(), which
will query the owning filesystem (via ->getattr) to get the correct ino/dev
pair.  Later patches will update those places which simply dump inode->i_ino
and super->s_dev to use the helper.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
---
 fs/stat.c          | 23 +++++++++++++++++++++++
 include/linux/fs.h |  2 ++
 2 files changed, 25 insertions(+)

diff --git a/fs/stat.c b/fs/stat.c
index f8e6fb2c3657..80ea42505219 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -84,6 +84,29 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
 }
 EXPORT_SYMBOL(vfs_getattr_nosec);
 
+void vfs_map_unique_ino_dev(struct dentry *dentry, u64 *ino, dev_t *dev)
+{
+	int ret;
+	struct path path;
+	struct kstat stat;
+
+	path.mnt = NULL;
+	path.dentry = dentry;
+	/* ->dev is always returned, so we only need to specify ino here */
+	ret = vfs_getattr_nosec(&path, &stat, STATX_INO, 0);
+	if (ret) {
+		struct inode *inode = d_inode(dentry);
+		/* Fallback to old behavior in case of getattr error */
+		*ino = inode->i_ino;
+		*dev = inode->i_sb->s_dev;
+		return ret;
+	}
+	*ino = stat.ino;
+	*dev = stat.dev;
+	return 0;
+}
+EXPORT_SYMBOL(vfs_map_unique_ino_dev);
+
 /*
  * vfs_getattr - Get the enhanced basic attributes of a file
  * @path: The file of interest
diff --git a/include/linux/fs.h b/include/linux/fs.h
index d78d146a98da..b80789472438 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3077,6 +3077,8 @@ extern void kfree_link(void *);
 extern void generic_fillattr(struct inode *, struct kstat *);
 extern int vfs_getattr_nosec(const struct path *, struct kstat *, u32, unsigned int);
 extern int vfs_getattr(const struct path *, struct kstat *, u32, unsigned int);
+extern void vfs_map_unique_ino_dev(struct dentry *dentry, u64 *ino, dev_t *dev);
+
 void __inode_add_bytes(struct inode *inode, loff_t bytes);
 void inode_add_bytes(struct inode *inode, loff_t bytes);
 void __inode_sub_bytes(struct inode *inode, loff_t bytes);
-- 
2.15.1