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 Fri, Mar 01, 2024 at 03:45:27PM +0000, 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.
>
> Thanks for the details. Let me see if I get this right. So you're saying that
> someone is doing:
>
> fsconfig(..., FSCONFIG_SET_FLAG, "usrjquota", NULL, 0); // [1]
>
> ? Is so that is a vital part of the explanation. So please put that in the
> commit message.

Right, I guess I should have added a simple usecase for that in the commit
message.  I.e. add a simple 'mount' command with this parameter without
any value.

> Then ext4 defines:
>
> 	fsparam_string_empty ("usrjquota",		Opt_usrjquota),
>
> So [1] gets us:
>
>         param->type == fs_value_is_flag
>         param->string == NULL
>
> Now we enter into
> fs_parse()
> -> __fs_parse()
>    -> fs_lookup_key() for @param and that does:
>
>         bool want_flag = param->type == fs_value_is_flag;
>
>         *negated = false;
>         for (p = desc; p->name; p++) {
>                 if (strcmp(p->name, name) != 0)
>                         continue;
>                 if (likely(is_flag(p) == want_flag))
>                         return p;
>                 other = p;
>         }
>
> So we don't have a flag parameter defined so the only real match we get is
> @other for:
>
>         fsparam_string_empty ("usrjquota",		Opt_usrjquota),
>
> What happens now is that you call p->type == fs_param_is_string() and that
> rejects it as bad parameter because param->type == fs_value_is_flag !=
> fs_value_is_string as required. So you dont end up getting Opt_userjquota
> called with param->string NULL, right? So there's not NULL deref or anything,
> right?
>
> You just fail to set usrjquota. Ok, so I think the correct fix is to do
> something like the following in ext4:
>
>         fsparam_string_empty ("usrjquota",      Opt_usrjquota),
>         fs_param_flag        ("usrjquota",      Opt_usrjquota_flag),
>
> and then in the switch you can do:
>
> switch (opt)
> case Opt_usrjquota:
>         // string thing
> case Opt_usrjquota_flag:
>         // flag thing
>
> And I really think we should kill all empty handling for non-string types and
> only add that when there's a filesystem that actually needs it.

Yeah, that looks like the right fix.  I see you sent out a patch doing
something like this, so I'll comment there instead.

Cheers,
-- 
Luís