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

Luis Henriques
2024-03-11T10:26:13Z

Jan Kara <jack@suse.cz> writes:

> On Fri 08-03-24 10:12:13, Luis Henriques wrote:
>> 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.
>
> 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...

Oh, interesting. FTR I'm using mount from util-linux 2.39.3, but I
haven't tried this with older versions.

Cheers,
--
Luís

>
> But anyway, I get the point. Thanks for educating me :)
>
> Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR