From: Ben Hutchings <ben@decadent.org.uk>
Date: Mon, 12 Jan 2015 04:54:59 +0000 (+0000)
Subject: vfs: Avoid ABI change for dentry union changes
X-Git-Tag: archive/raspbian/4.9.13-1+rpi1~26^2^2~3
X-Git-Url: https://dgit.raspbian.org/%22http:/www.example.com/%22mailto:kde%40ewsoftware.de//%22style.css/%22/%22http:/www.example.com/%22mailto:kde%40ewsoftware.de/%22style.css/%22?a=commitdiff_plain;h=679822ba873c4939e3f853e47de65545fe2d6c48;p=linux-4.9.git

vfs: Avoid ABI change for dentry union changes

Commit 946e51f2bf37f1656916eb75bd0742ba33983c28 ("move d_rcu from
overlapping d_child to overlapping d_alias") looks disruptive and
it is an API change since the union is named.  However, it doesn't
actually move anything that modules need, so it is not an ABI
change and we can safely hide it from genksysms.

Verify this by adding an unused function with some BUILD_BUG_ONs
to assert the size and alignment of fields remain the same.


Gbp-Pq: Topic debian
Gbp-Pq: Name vfs-avoid-abi-change-for-dentry-union-changes.patch
---

diff --git a/fs/dcache.c b/fs/dcache.c
index e2dc0874a1f0..66f9823cf2b6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -255,6 +255,24 @@ static void dentry_free(struct dentry *dentry)
 		call_rcu(&dentry->d_u.d_rcu, __d_free);
 }
 
+/*
+ * bwh: Assert that dentry union changes didn't change the structure
+ * layout other than to move d_rcu.
+ */
+static void __always_unused dcache_abi_check(void)
+{
+	struct dentry dentry;
+	union {
+		struct list_head d_child;
+		struct rcu_head d_rcu;
+	} old_d_u;
+	BUILD_BUG_ON(sizeof(dentry.d_child) != sizeof(old_d_u) ||
+		     __alignof__(dentry.d_child) != __alignof__(old_d_u));
+	BUILD_BUG_ON(sizeof(dentry.d_u.d_alias) != sizeof(dentry.d_u) ||
+		     __alignof__(dentry.d_u.d_alias) !=
+		     __alignof__(dentry.d_u));
+}
+
 /**
  * dentry_rcuwalk_barrier - invalidate in-progress rcu-walk lookups
  * @dentry: the target dentry
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 6cddc8708a95..d6aa25772e09 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -124,15 +124,31 @@ struct dentry {
 	void *d_fsdata;			/* fs-specific data */
 
 	struct list_head d_lru;		/* LRU list */
+#ifdef __GENKSYMS__
+	/*
+	 * bwh: The union changes here don't move anything other than
+	 * d_rcu (which modules definitely should not touch).  This is
+	 * checked by dcache_abi_check().
+	 */
+	union {
+#endif
 	struct list_head d_child;	/* child of parent list */
+#ifdef __GENKSYMS__
+		struct rcu_head d_rcu;
+	} d_u;
+#endif
 	struct list_head d_subdirs;	/* our children */
 	/*
 	 * d_alias and d_rcu can share memory
 	 */
+#ifndef __GENKSYMS__
 	union {
+#endif
 		struct hlist_node d_alias;	/* inode alias list */
+#ifndef __GENKSYMS__
 	 	struct rcu_head d_rcu;
 	} d_u;
+#endif
 };
 
 /*