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

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Message-ID: <20240307151356.ishrtxrsge2i5mjn@quack3>
Date: Thu, 7 Mar 2024 16:13:56 +0100
From: Jan Kara <jack@...e.cz>
To: Luis Henriques <lhenriques@...e.de>
Cc: Christian Brauner <brauner@...nel.org>, 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 Fri 01-03-24 15:45:27, Luis Henriques wrote:
> Christian Brauner <brauner@...nel.org> writes:
> 
> > 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.
> 
> Right, I find it quite confusing and I may be fixing the issue in the
> wrong place.  What I'm seeing with ext4 when I mount a filesystem using
> the option '-o usrjquota' is that fs_parse() will get:
> 
>  * p->type is set to fs_param_is_string
>    ('p' is a struct fs_parameter_spec, ->type is a function)
>  * param->type is set to fs_value_is_flag
>    ('param' is a struct fs_parameter, ->type is an enum)
> 
> This is because ext4 will use the __fsparam macro to set define a
> fs_param_spec as a fs_param_is_string but will also set the
> fs_param_can_be_empty; and the fsconfig() syscall will get that parameter
> as a flag.  That's why param->string will be NULL in this case.

So I'm a bit confused here. Valid variants of these quota options are like
"usrjquota=<filename>" (to set quota file name) or "usrjquota=" (to clear
quota file name). The variant "usrjquota" should ideally be rejected
because it doesn't make a good sense and only adds to confusion. Now as far
as I'm reading fs/ext4/super.c: parse_options() (and as far as my testing
shows) this is what is happening so what is exactly the problem you're
trying to fix?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR