fs: make plain ->i_state access fail to compile

... to make sure all accesses are properly validated.

Merely renaming the var to __i_state still lets the compiler make the
following suggestion:
error: 'struct inode' has no member named 'i_state'; did you mean '__i_state'?

Unfortunately some people will add the __'s and call it a day.

In order to make it harder to mess up in this way, hide it behind a
struct. The resulting error message should be convincing in terms of
checking what to do:
error: invalid operands to binary & (have 'struct inode_state_flags' and 'int')

Of course people determined to do a plain access can still do it, but
nothing can be done for that case.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

include/linux/fs.h

@@ -785,6 +785,13 @@ enum inode_state_flags_enum {
 #define I_DIRTY (I_DIRTY_INODE | I_DIRTY_PAGES)
 #define I_DIRTY_ALL (I_DIRTY | I_DIRTY_TIME)
 
+/*
+ * Use inode_state_read() & friends to access.
+ */
+struct inode_state_flags {
+	enum inode_state_flags_enum __state;
+};
+
 /*
  * Keep mostly read-only and often accessed (especially for
  * the RCU path lookup and 'stat' data) fields at the beginning
@@ -843,7 +850,7 @@ struct inode {
 #endif
 
 	/* Misc */
-	enum inode_state_flags_enum i_state;
+	struct inode_state_flags i_state;
 	/* 32-bit hole */
 	struct rw_semaphore	i_rwsem;
 
@@ -909,19 +916,19 @@ struct inode {
  */
 static inline enum inode_state_flags_enum inode_state_read_once(struct inode *inode)
 {
-	return READ_ONCE(inode->i_state);
+	return READ_ONCE(inode->i_state.__state);
 }
 
 static inline enum inode_state_flags_enum inode_state_read(struct inode *inode)
 {
 	lockdep_assert_held(&inode->i_lock);
-	return inode->i_state;
+	return inode->i_state.__state;
 }
 
 static inline void inode_state_set_raw(struct inode *inode,
 				       enum inode_state_flags_enum flags)
 {
-	WRITE_ONCE(inode->i_state, inode->i_state | flags);
+	WRITE_ONCE(inode->i_state.__state, inode->i_state.__state | flags);
 }
 
 static inline void inode_state_set(struct inode *inode,
@@ -934,7 +941,7 @@ static inline void inode_state_set(struct inode *inode,
 static inline void inode_state_clear_raw(struct inode *inode,
 					 enum inode_state_flags_enum flags)
 {
-	WRITE_ONCE(inode->i_state, inode->i_state & ~flags);
+	WRITE_ONCE(inode->i_state.__state, inode->i_state.__state & ~flags);
 }
 
 static inline void inode_state_clear(struct inode *inode,
@@ -947,7 +954,7 @@ static inline void inode_state_clear(struct inode *inode,
 static inline void inode_state_assign_raw(struct inode *inode,
 					  enum inode_state_flags_enum flags)
 {
-	WRITE_ONCE(inode->i_state, flags);
+	WRITE_ONCE(inode->i_state.__state, flags);
 }
 
 static inline void inode_state_assign(struct inode *inode,
@@ -962,7 +969,7 @@ static inline void inode_state_replace_raw(struct inode *inode,
 					   enum inode_state_flags_enum setflags)
 {
 	enum inode_state_flags_enum flags;
-	flags = inode->i_state;
+	flags = inode->i_state.__state;
 	flags &= ~clearflags;
 	flags |= setflags;
 	inode_state_assign_raw(inode, flags);