Re: [PATCH RFC] libfs: getdents() should return 0 after reaching EOD

[Christian Brauner <brauner@kernel.org>]

On Tue, Nov 14, 2023 at 10:49:37AM -0500, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> The new directory offset helpers don't conform with the convention
> of getdents() returning no more entries once a directory file
> descriptor has reached the current end-of-directory.
> 
> To address this, copy the logic from dcache_readdir() to mark the
> open directory file descriptor once EOD has been reached. Rewinding
> resets the mark.
> 
> Reported-by: Tavian Barnes <tavianator@tavianator.com>
> Closes: https://lore.kernel.org/linux-fsdevel/20231113180616.2831430-1-tavianator@tavianator.com/
> Fixes: 6faddda69f62 ("libfs: Add directory operations for stable offsets")
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/libfs.c |   13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index e9440d55073c..1c866b087f0c 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -428,7 +428,7 @@ 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)
>  {
>  	struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
>  	XA_STATE(xas, &so_ctx->xa, ctx->pos);
> @@ -437,7 +437,8 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  	while (true) {
>  		dentry = offset_find_next(&xas);
>  		if (!dentry)
> -			break;
> +			/* readdir has reached the current EOD */
> +			return (void *)0x10;
>  
>  		if (!offset_dir_emit(ctx, dentry)) {
>  			dput(dentry);
> @@ -447,6 +448,7 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  		dput(dentry);
>  		ctx->pos = xas.xa_index + 1;
>  	}
> +	return NULL;
>  }
>  
>  /**
> @@ -479,7 +481,12 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
>  	if (!dir_emit_dots(file, ctx))
>  		return 0;
>  
> -	offset_iterate_dir(d_inode(dir), ctx);
> +	if (ctx->pos == 2)
> +		file->private_data = NULL;
> +	else if (file->private_data == (void *)0x10)
> +		return 0;
> +
> +	file->private_data = offset_iterate_dir(d_inode(dir), ctx);

I think it's usually best practice to only modify the file->private_data
pointer during f_op->open and f_op->close but not override
file->private_data once the file is visible to other threads. I think
here it might not matter because access to file->private_data is
serialized on f_pos_lock and it's not used by anything else.