qnx4: Extract dir entry filename processing into helper

Both dir.c and namei.c need to perform the same work to figure out a
directory entry's name and size. Extract this into a helper for use
in the next patch.

Acked-by: Anders Larsen <al@alarsen.net>
Link: https://lore.kernel.org/r/20231130205120.3642477-1-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

fs/qnx4/dir.c

@@ -15,43 +15,6 @@
 #include <linux/buffer_head.h>
 #include "qnx4.h"
 
-/*
- * A qnx4 directory entry is an inode entry or link info
- * depending on the status field in the last byte. The
- * first byte is where the name start either way, and a
- * zero means it's empty.
- *
- * Also, due to a bug in gcc, we don't want to use the
- * real (differently sized) name arrays in the inode and
- * link entries, but always the 'de_name[]' one in the
- * fake struct entry.
- *
- * See
- *
- *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578#c6
- *
- * for details, but basically gcc will take the size of the
- * 'name' array from one of the used union entries randomly.
- *
- * This use of 'de_name[]' (48 bytes) avoids the false positive
- * warnings that would happen if gcc decides to use 'inode.di_name'
- * (16 bytes) even when the pointer and size were to come from
- * 'link.dl_name' (48 bytes).
- *
- * In all cases the actual name pointer itself is the same, it's
- * only the gcc internal 'what is the size of this field' logic
- * that can get confused.
- */
-union qnx4_directory_entry {
-	struct {
-		const char de_name[48];
-		u8 de_pad[15];
-		u8 de_status;
-	};
-	struct qnx4_inode_entry inode;
-	struct qnx4_link_info link;
-};
-
 static int qnx4_readdir(struct file *file, struct dir_context *ctx)
 {
 	struct inode *inode = file_inode(file);
@@ -74,26 +37,25 @@ static int qnx4_readdir(struct file *file, struct dir_context *ctx)
 		ix = (ctx->pos >> QNX4_DIR_ENTRY_SIZE_BITS) % QNX4_INODES_PER_BLOCK;
 		for (; ix < QNX4_INODES_PER_BLOCK; ix++, ctx->pos += QNX4_DIR_ENTRY_SIZE) {
 			union qnx4_directory_entry *de;
+			const char *fname;
 
 			offset = ix * QNX4_DIR_ENTRY_SIZE;
 			de = (union qnx4_directory_entry *) (bh->b_data + offset);
 
-			if (!de->de_name[0])
-				continue;
-			if (!(de->de_status & (QNX4_FILE_USED|QNX4_FILE_LINK)))
+			fname = get_entry_fname(de, &size);
+			if (!fname)
 				continue;
+
 			if (!(de->de_status & QNX4_FILE_LINK)) {
-				size = sizeof(de->inode.di_fname);
 				ino = blknum * QNX4_INODES_PER_BLOCK + ix - 1;
 			} else {
-				size = sizeof(de->link.dl_fname);
 				ino = ( le32_to_cpu(de->link.dl_inode_blk) - 1 ) *
 					QNX4_INODES_PER_BLOCK +
 					de->link.dl_inode_ndx;
 			}
-			size = strnlen(de->de_name, size);
-			QNX4DEBUG((KERN_INFO "qnx4_readdir:%.*s\n", size, name));
-			if (!dir_emit(ctx, de->de_name, size, ino, DT_UNKNOWN)) {
+
+			QNX4DEBUG((KERN_INFO "qnx4_readdir:%.*s\n", size, fname));
+			if (!dir_emit(ctx, fname, size, ino, DT_UNKNOWN)) {
 				brelse(bh);
 				return 0;
 			}

fs/qnx4/qnx4.h

@@ -44,3 +44,63 @@ static inline struct qnx4_inode_entry *qnx4_raw_inode(struct inode *inode)
 {
 	return &qnx4_i(inode)->raw;
 }
+
+/*
+ * A qnx4 directory entry is an inode entry or link info
+ * depending on the status field in the last byte. The
+ * first byte is where the name start either way, and a
+ * zero means it's empty.
+ *
+ * Also, due to a bug in gcc, we don't want to use the
+ * real (differently sized) name arrays in the inode and
+ * link entries, but always the 'de_name[]' one in the
+ * fake struct entry.
+ *
+ * See
+ *
+ *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578#c6
+ *
+ * for details, but basically gcc will take the size of the
+ * 'name' array from one of the used union entries randomly.
+ *
+ * This use of 'de_name[]' (48 bytes) avoids the false positive
+ * warnings that would happen if gcc decides to use 'inode.di_name'
+ * (16 bytes) even when the pointer and size were to come from
+ * 'link.dl_name' (48 bytes).
+ *
+ * In all cases the actual name pointer itself is the same, it's
+ * only the gcc internal 'what is the size of this field' logic
+ * that can get confused.
+ */
+union qnx4_directory_entry {
+	struct {
+		const char de_name[48];
+		u8 de_pad[15];
+		u8 de_status;
+	};
+	struct qnx4_inode_entry inode;
+	struct qnx4_link_info link;
+};
+
+static inline const char *get_entry_fname(union qnx4_directory_entry *de,
+					  int *size)
+{
+	/* Make sure the status byte is in the same place for all structs. */
+	BUILD_BUG_ON(offsetof(struct qnx4_inode_entry, di_status) !=
+			offsetof(struct qnx4_link_info, dl_status));
+	BUILD_BUG_ON(offsetof(struct qnx4_inode_entry, di_status) !=
+			offsetof(union qnx4_directory_entry, de_status));
+
+	if (!de->de_name[0])
+		return NULL;
+	if (!(de->de_status & (QNX4_FILE_USED|QNX4_FILE_LINK)))
+		return NULL;
+	if (!(de->de_status & QNX4_FILE_LINK))
+		*size = sizeof(de->inode.di_fname);
+	else
+		*size = sizeof(de->link.dl_fname);
+
+	*size = strnlen(de->de_name, *size);
+
+	return de->de_name;
+}