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

[yangerkun <yangerkun@xxxxxxxxxxxxxxx>]

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?

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