libfs: Replace simple_offset end-of-directory detection

According to getdents(3), the d_off field in each returned directory
entry points to the next entry in the directory. The d_off field in
the last returned entry in the readdir buffer must contain a valid
offset value, but if it points to an actual directory entry, then
readdir/getdents can loop.

This patch introduces a specific fixed offset value that is placed
in the d_off field of the last entry in a directory. Some user space
applications assume that the EOD offset value is larger than the
offsets of real directory entries, so the largest valid offset value
is reserved for this purpose. This new value is never allocated by
simple_offset_add().

When ->iterate_dir() returns, getdents{64} inserts the ctx->pos
value into the d_off field of the last valid entry in the readdir
buffer. When it hits EOD, offset_readdir() sets ctx->pos to the EOD
offset value so the last entry is updated to point to the EOD marker.

When trying to read the entry at the EOD offset, offset_readdir()
terminates immediately.

It is worth noting that using a Maple tree for directory offset
value allocation does not guarantee a 63-bit range of values --
on platforms where "long" is a 32-bit type, the directory offset
value range is still 0..(2^31 - 1). For broad compatibility with
32-bit user space, the largest tmpfs directory cookie value is now
S32_MAX.

Fixes: 796432e ("libfs: getdents() should return 0 after reaching EOD")
Signed-off-by: Chuck Lever <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: chucklever

fs/libfs.c

@@ -245,9 +245,15 @@ const struct inode_operations simple_dir_inode_operations = {
 };
 EXPORT_SYMBOL(simple_dir_inode_operations);
 
-/* 0 is '.', 1 is '..', so always start with offset 2 or more */
+/* simple_offset_add() never assigns these to a dentry */
 enum {
-	DIR_OFFSET_MIN	= 2,
+	DIR_OFFSET_EOD		= S32_MAX,
+};
+
+/* simple_offset_add() allocation range */
+enum {
+	DIR_OFFSET_MIN		= 2,
+	DIR_OFFSET_MAX		= DIR_OFFSET_EOD - 1,
 };
 
 static void offset_set(struct dentry *dentry, long offset)
@@ -291,7 +297,8 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 		return -EBUSY;
 
 	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
-				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
+				 DIR_OFFSET_MAX, &octx->next_offset,
+				 GFP_KERNEL);
 	if (unlikely(ret < 0))
 		return ret == -EBUSY ? -ENOSPC : ret;
 
@@ -447,8 +454,6 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 		return -EINVAL;
 	}
 
-	/* In this case, ->private_data is protected by f_pos_lock */
-	file->private_data = NULL;
 	return vfs_setpos(file, offset, LONG_MAX);
 }
 
@@ -458,7 +463,7 @@ static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
 	struct dentry *child, *found = NULL;
 
 	rcu_read_lock();
-	child = mas_find(&mas, LONG_MAX);
+	child = mas_find(&mas, DIR_OFFSET_MAX);
 	if (!child)
 		goto out;
 	spin_lock(&child->d_lock);
@@ -479,15 +484,15 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
+static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 {
 	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
 	struct dentry *dentry;
 
 	while (true) {
 		dentry = offset_find_next(octx, ctx->pos);
 		if (!dentry)
-			return ERR_PTR(-ENOENT);
+			goto out_eod;
 
 		if (!offset_dir_emit(ctx, dentry)) {
 			dput(dentry);
@@ -497,7 +502,10 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 		ctx->pos = dentry2offset(dentry) + 1;
 		dput(dentry);
 	}
-	return NULL;
+	return;
+
+out_eod:
+	ctx->pos = DIR_OFFSET_EOD;
 }
 
 /**
@@ -517,6 +525,8 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
  *
  * On return, @ctx->pos contains an offset that will read the next entry
  * in this directory when offset_readdir() is called again with @ctx.
+ * Caller places this value in the d_off field of the last entry in the
+ * user's buffer.
  *
  * Return values:
  *   %0 - Complete
@@ -529,13 +539,8 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
 
 	if (!dir_emit_dots(file, ctx))
 		return 0;
-
-	/* In this case, ->private_data is protected by f_pos_lock */
-	if (ctx->pos == DIR_OFFSET_MIN)
-		file->private_data = NULL;
-	else if (file->private_data == ERR_PTR(-ENOENT))
-		return 0;
-	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
+	if (ctx->pos != DIR_OFFSET_EOD)
+		offset_iterate_dir(d_inode(dir), ctx);
 	return 0;
 }