fanotify: Drop use of flex array in fanotify_fh

We use flex array 'buf' in fanotify_fh to contain the file handle
content. However the buffer is not a proper flex array. It has a
preconfigured fixed size. Furthermore if fixed size is not big enough,
we use external separately allocated buffer. Hence don't pretend buf is
a flex array since we have to use special accessors anyway and instead
just modify the accessors to do the pointer math without flex array.
This fixes warnings that flex array is not the last struct member in
fanotify_fid_event or fanotify_error_event.

Signed-off-by: Jan Kara <[email protected]>
Reviewed-by: Amir Goldstein <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: jankara

fs/notify/fanotify/fanotify.c

@@ -415,7 +415,7 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
 {
 	int dwords, type = 0;
 	char *ext_buf = NULL;
-	void *buf = fh->buf;
+	void *buf = fh + 1;
 	int err;
 
 	fh->type = FILEID_ROOT;

fs/notify/fanotify/fanotify.h

@@ -25,7 +25,7 @@ enum {
  * stored in either the first or last 2 dwords.
  */
 #define FANOTIFY_INLINE_FH_LEN	(3 << 2)
-#define FANOTIFY_FH_HDR_LEN	offsetof(struct fanotify_fh, buf)
+#define FANOTIFY_FH_HDR_LEN	sizeof(struct fanotify_fh)
 
 /* Fixed size struct for file handle */
 struct fanotify_fh {
@@ -34,7 +34,6 @@ struct fanotify_fh {
 #define FANOTIFY_FH_FLAG_EXT_BUF 1
 	u8 flags;
 	u8 pad;
-	unsigned char buf[];
 } __aligned(4);
 
 /* Variable size struct for dir file handle + child file handle + name */
@@ -92,7 +91,7 @@ static inline char **fanotify_fh_ext_buf_ptr(struct fanotify_fh *fh)
 	BUILD_BUG_ON(FANOTIFY_FH_HDR_LEN % 4);
 	BUILD_BUG_ON(__alignof__(char *) - 4 + sizeof(char *) >
 		     FANOTIFY_INLINE_FH_LEN);
-	return (char **)ALIGN((unsigned long)(fh->buf), __alignof__(char *));
+	return (char **)ALIGN((unsigned long)(fh + 1), __alignof__(char *));
 }
 
 static inline void *fanotify_fh_ext_buf(struct fanotify_fh *fh)
@@ -102,7 +101,7 @@ static inline void *fanotify_fh_ext_buf(struct fanotify_fh *fh)
 
 static inline void *fanotify_fh_buf(struct fanotify_fh *fh)
 {
-	return fanotify_fh_has_ext_buf(fh) ? fanotify_fh_ext_buf(fh) : fh->buf;
+	return fanotify_fh_has_ext_buf(fh) ? fanotify_fh_ext_buf(fh) : fh + 1;
 }
 
 static inline int fanotify_info_dir_fh_len(struct fanotify_info *info)
@@ -278,7 +277,7 @@ static inline void fanotify_init_event(struct fanotify_event *event,
 #define FANOTIFY_INLINE_FH(name, size)					\
 struct {								\
 	struct fanotify_fh name;					\
-	/* Space for object_fh.buf[] - access with fanotify_fh_buf() */	\
+	/* Space for filehandle - access with fanotify_fh_buf() */	\
 	unsigned char _inline_fh_buf[size];				\
 }