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: <20240308230911.r5a4xn6f5vp24hil@quack3>
Date: Sat, 9 Mar 2024 00:09:11 +0100
From: Jan Kara <jack@...e.cz>
To: Luis Henriques <lhenriques@...e.de>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	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 08-03-24 10:12:13, Luis Henriques wrote:
> Christian Brauner <brauner@...nel.org> writes:
> 
> > On Thu, Mar 07, 2024 at 04:13:56PM +0100, Jan Kara wrote:
> >> 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?
> >
> > mount(8) has no way of easily knowing that for something like
> > mount -o usrjquota /dev/sda1 /mnt that "usrjquota" is supposed to be
> > set as an empty string via FSCONFIG_SET_STRING. For mount(8) it is
> > indistinguishable from a flag because it's specified without an
> > argument. So mount(8) passes FSCONFIG_SET_FLAG and it seems strange that
> > we should require mount(8) to know what mount options are strings or no.
> > I've ran into this issue before myself when using the mount api
> > programatically.
> 
> Right.  A simple usecase is to try to do:
> 
>   mount -t ext4 -o usrjquota= /dev/sda1 /mnt/
> 
> It will fail, and this has been broken for a while.

I see. But you have to have new enough mount that is using fsconfig, don't
you? Because for me in my test VM this works just fine...

But anyway, I get the point. Thanks for educating me :)

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