Re: [PATCH v3] libfs: getdents() should return 0 after reaching EOD

[Chuck Lever <chuck.lever@oracle.com>]

On Sat, Nov 18, 2023 at 02:33:19PM -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 |   12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> This patch passes Tavian's reproducer. fstests over NFS shows no
> regression. However, generic/676 fails when running directly against
> a tmpfs mount:
> 
>      QA output created by 676
>     -All tests passed
>     +Unexpected EOF while reading dir.
> 
> I will look into that.

offset_dir_llseek() has to reset the file descriptor's EOD marker,
just like dcache_dir_lseek() does (effectively it resets
cursor->d_child).

We don't hold f_pos_lock in offset_dir_llseek(), though.


> Changes since v2:
> - Go back to marking EOD in file->private_data (with a comment)
> 
> Changes since RFC:
> - Keep file->private_data stable while directory descriptor remains open
> 
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index e9440d55073c..851e29fdd7e7 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 +437,7 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  	while (true) {
>  		dentry = offset_find_next(&xas);
>  		if (!dentry)
> -			break;
> +			return ERR_PTR(-ENOENT);
>  
>  		if (!offset_dir_emit(ctx, dentry)) {
>  			dput(dentry);
> @@ -447,6 +447,7 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  		dput(dentry);
>  		ctx->pos = xas.xa_index + 1;
>  	}
> +	return NULL;
>  }
>  
>  /**
> @@ -479,7 +480,12 @@ 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);
> +	/* In this case, ->private_data is protected by f_pos_lock */
> +	if (ctx->pos == 2)
> +		file->private_data = NULL;
> +	else if (file->private_data == ERR_PTR(-ENOENT))
> +		return 0;
> +	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
>  	return 0;
>  }
>  
> 
> 

-- 
Chuck Lever