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

[Luis Henriques <lhenriques@suse.de>]

Christian Brauner <brauner@kernel.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@kernel.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@suse.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.

(And btw: email here is broken again -- I haven't received Jan's email
yet.  And this reply will likely take a while to reach its recipients.)

Cheers,
-- 
Luís