Re: [PATCH] libfs: fix infinite directory reads for offset dir

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Hi!

在 2024/7/31 21:10, Filipe Manana 写道:
On Wed, Jul 31, 2024 at 1:51 PM yangerkun <yangerkun@xxxxxxxxxxxxxxx> wrote:

Hi!

在 2024/7/31 19:51, Jan Kara 写道:
On Wed 31-07-24 12:38:35, yangerkun wrote:
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.

The patch looks good! Just I'm not sure about the llseek part. As far as I
understand it was added due to this sentence in the standard:

"If a file is removed from or added to the directory after the most recent
call to opendir() or rewinddir(), whether a subsequent call to readdir()
returns an entry for that file is unspecified."

So if the offset used in offset_dir_llseek() is 0, then we should update
last_index. But otherwise I'd leave it alone because IMHO it would do more
harm than good.

IIUC, what you means is that we should only reset the private_data to
new last_index when we call rewinddir(which will call lseek to set
offset of dir file to 0)?

Yeah, I prefer the logic you describle! Besides, we may also change
btrfs that do the same(e60aa5da14d0 ("btrfs: refresh dir last index
during a rewinddir(3) call")). Filipe, how do you think?

What problem does it solve?
The standard doesn't forbid it, and I can't see anything wrong with it.


Yeah, I didn't find any obvious bugs for this behavior. It's a choose
does this seems better, and I think it's also ok to keep btrfs not
change now. Thanks for your reply!



Thanks,
Erkun.

                                                               Honza


Fixes: a2e459555c5f ("shmem: stable directory offsets")
Signed-off-by: yangerkun <yangerkun@xxxxxxxxxx>
---
   fs/libfs.c | 34 +++++++++++++++++++++++-----------
   1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 8aa34870449f..38b306738c00 100644
--- 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 +487,7 @@ 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;
+    file->private_data = (void *)ctx->next_offset;
      return vfs_setpos(file, offset, LONG_MAX);
   }

@@ -507,7 +518,7 @@ 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 +526,21 @@ 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 +567,19 @@ 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,
--
2.39.2







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