libfs: fix infinite directory reads for offset dir

After we switch tmpfs dir operations from simple_dir_operations to
simple_offset_dir_operations, every rename happened will fill new dentry
to dest dir's maple tree(&SHMEM_I(inode)->dir_offsets->mt) with a free
key starting with octx->newx_offset, and then set newx_offset equals to
free key + 1. This will lead to infinite readdir combine with rename
happened at the same time, which fail generic/736 in xfstests(detail show
as below).

1. create 5000 files(1 2 3...) under one dir
2. call readdir(man 3 readdir) once, and get one entry
3. rename(entry, "TEMPFILE"), then rename("TEMPFILE", entry)
4. loop 2~3, until readdir return nothing or we loop too many
   times(tmpfs break test with the second condition)

We choose the same logic what commit 9b378f6ad48cf ("btrfs: fix infinite
directory reads") to fix it, record the last_index when we open dir, and
do not emit the entry which index >= last_index. The file->private_data
now used in offset dir can use directly to do this, and we also update
the last_index when we llseek the dir file.

Fixes: a2e459555c5f ("shmem: stable directory offsets")
Signed-off-by: yangerkun <yangerkun@huawei.com>
Link: https://lore.kernel.org/r/20240731043835.1828697-1-yangerkun@huawei.com
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
[brauner: only update last_index after seek when offset is zero like Jan suggested]
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/libfs.c b/fs/libfs.c
index 8aa34870449fe18ce5d44ec525dd8e666de95a23..02602d00939e9688250770ae5195cab2cb5b2663 100644 (file)
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -450,6 +450,14 @@ void simple_offset_destroy(struct offset_ctx *octx)
        mtree_destroy(&octx->mt);
 }
 
+static int offset_dir_open(struct inode *inode, struct file *file)
+{
+       struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
+
+       file->private_data = (void *)ctx->next_offset;
+       return 0;
+}
+
 /**
  * offset_dir_llseek - Advance the read position of a directory descriptor
  * @file: an open directory whose position is to be updated
@@ -463,6 +471,9 @@ void simple_offset_destroy(struct offset_ctx *octx)
  */
 static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 {
+       struct inode *inode = file->f_inode;
+       struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
+
        switch (whence) {
        case SEEK_CUR:
                offset += file->f_pos;
@@ -476,7 +487,8 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
        }
 
        /* In this case, ->private_data is protected by f_pos_lock */
-       file->private_data = NULL;
+       if (!offset)
+               file->private_data = (void *)ctx->next_offset;
        return vfs_setpos(file, offset, LONG_MAX);
 }
 
@@ -507,7 +519,7 @@ 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, long last_index)
 {
        struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
        struct dentry *dentry;
@@ -515,17 +527,21 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
        while (true) {
                dentry = offset_find_next(octx, ctx->pos);
                if (!dentry)
-                       return ERR_PTR(-ENOENT);
+                       return;
+
+               if (dentry2offset(dentry) >= last_index) {
+                       dput(dentry);
+                       return;
+               }
 
                if (!offset_dir_emit(ctx, dentry)) {
                        dput(dentry);
-                       break;
+                       return;
                }
 
                ctx->pos = dentry2offset(dentry) + 1;
                dput(dentry);
        }
-       return NULL;
 }
 
 /**
@@ -552,22 +568,19 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 static int offset_readdir(struct file *file, struct dir_context *ctx)
 {
        struct dentry *dir = file->f_path.dentry;
+       long last_index = (long)file->private_data;
 
        lockdep_assert_held(&d_inode(dir)->i_rwsem);
 
        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);
+       offset_iterate_dir(d_inode(dir), ctx, last_index);
        return 0;
 }
 
 const struct file_operations simple_offset_dir_operations = {
+       .open           = offset_dir_open,
        .llseek         = offset_dir_llseek,
        .iterate_shared = offset_readdir,
        .read           = generic_read_dir,