Re: [PATCH] cifs: fix skipping to incorrect offset in emit_cached_dirents

[Tom Talpey <tom@talpey.com>]

On 10/11/2022 2:46 AM, Ronnie Sahlberg wrote:
> When application has done lseek() to a different offset on a directory fd
> we skipped one entry too many before we start emitting directory entries
> from the cache.
> 
> We need to also make sure that when we are starting to emit directory
> entries from the cache, the ->pos sequence might have holes and skip
> some indices.
> 
> Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
> ---
>   fs/cifs/readdir.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/cifs/readdir.c b/fs/cifs/readdir.c
> index 8e060c00c969..7170614434a1 100644
> --- a/fs/cifs/readdir.c
> +++ b/fs/cifs/readdir.c
> @@ -847,14 +847,19 @@ static bool emit_cached_dirents(struct cached_dirents *cde,
>   	int rc;
>   
>   	list_for_each_entry(dirent, &cde->entries, entry) {
> -		if (ctx->pos >= dirent->pos)
> +		if (ctx->pos > dirent->pos)
>   			continue;
> +		/*
> +		 * There may be holes in the ->pos sequence
> +		 * so always force ctx->pos to the current position.
> +		 */

The comment is a bit vague by referring to "->pos", because
it's the same name in both ctx and dirent.

But I have a second question, does squeezing out the holes
affect a later possible lseek on the dirhandle? I'm having
trouble tracking down where that might happen.

>   		ctx->pos = dirent->pos;
>   		rc = dir_emit(ctx, dirent->name, dirent->namelen,
>   			      dirent->fattr.cf_uniqueid,
>   			      dirent->fattr.cf_dtype);
>   		if (!rc)
>   			return rc;

It's weird that "rc" is an integer, but dir_emit() returns a bool.
It's also confusing that this isn't coded as

		if (!rc)
			return false;

Other than those questions, it looks like a clever fix.

Tom.
> +		ctx->pos++;
>   	}
>   	return true;
>   }
> @@ -1202,10 +1207,10 @@ int cifs_readdir(struct file *file, struct dir_context *ctx)
>   				 ctx->pos, tmp_buf);
>   			cifs_save_resume_key(current_entry, cifsFile);
>   			break;
> -		} else
> -			current_entry =
> -				nxt_dir_entry(current_entry, end_of_smb,
> -					cifsFile->srch_inf.info_level);
> +		}
> +		current_entry =
> +			nxt_dir_entry(current_entry, end_of_smb,
> +				      cifsFile->srch_inf.info_level);
>   	}
>   	kfree(tmp_buf);
>