linux-ext4 - Re: [PATCH 1/3] fs_parser: handle parameters that can be empty and don't have a value

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Date: Fri, 1 Mar 2024 14:31:49 +0100
From: Christian Brauner <brauner@...nel.org>
To: Luis Henriques <lhenriques@...e.de>
Cc: Theodore Ts'o <tytso@....edu>, 
	Andreas Dilger <adilger.kernel@...ger.ca>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Jan Kara <jack@...e.cz>, Miklos Szeredi <miklos@...redi.hu>, 
	Amir Goldstein <amir73il@...il.com>, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] fs_parser: handle parameters that can be empty and
 don't have a value

On Thu, Feb 29, 2024 at 04:30:08PM +0000, Luis Henriques wrote:
> Currently, only parameters that have the fs_parameter_spec 'type' set to
> NULL are handled as 'flag' types.  However, parameters that have the
> 'fs_param_can_be_empty' flag set and their value is NULL should also be
> handled as 'flag' type, as their type is set to 'fs_value_is_flag'.
> 
> Signed-off-by: Luis Henriques <lhenriques@...e.de>
> ---
>  fs/fs_parser.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fs_parser.c b/fs/fs_parser.c
> index edb3712dcfa5..53f6cb98a3e0 100644
> --- a/fs/fs_parser.c
> +++ b/fs/fs_parser.c
> @@ -119,7 +119,8 @@ int __fs_parse(struct p_log *log,
>  	/* Try to turn the type we were given into the type desired by the
>  	 * parameter and give an error if we can't.
>  	 */
> -	if (is_flag(p)) {
> +	if (is_flag(p) ||
> +	    (!param->string && (p->flags & fs_param_can_be_empty))) {
>  		if (param->type != fs_value_is_flag)
>  			return inval_plog(log, "Unexpected value for '%s'",
>  				      param->key);

If the parameter was derived from FSCONFIG_SET_STRING in fsconfig() then
param->string is guaranteed to not be NULL. So really this is only
about:

FSCONFIG_SET_FD
FSCONFIG_SET_BINARY
FSCONFIG_SET_PATH
FSCONFIG_SET_PATH_EMPTY

and those values being used without a value. What filesystem does this?
I don't see any.

The tempting thing to do here is to to just remove fs_param_can_be_empty
from every helper that isn't fs_param_is_string() until we actually have
a filesystem that wants to use any of the above as flags. Will lose a
lot of code that isn't currently used.