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 Sat, Mar 02, 2024 at 12:46:41PM +0100, Christian Brauner wrote:
>> 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.
>> 
>> 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.
>
> So one option is to do the following:

Thanks a lot of your review (I forgot to thank you in my other reply!).

Now, I haven't yet tested this properly, but I think that's a much simpler
and cleaner way of fixing this issue.  Now, although it needs some
testing, I think the patch has one problem (see comment below).

Do you want me to send out a cleaned-up version[*] of it after some
testing (+ a similar fix for overlayfs)?  Or would you rather handle this
yourself?

(I'll probably won't be able to look into it until mid-week.)

[*] Obviously, keeping a notice that you were the original author.

> From 8bfb142e6caba70704998be072222d6a31d8b97b Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Sat, 2 Mar 2024 18:54:35 +0100
> Subject: [PATCH] [UNTESTED]
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/ext4/super.c           | 32 ++++++++++++++++++--------------
>  include/linux/fs_parser.h |  3 +++
>  2 files changed, 21 insertions(+), 14 deletions(-)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index ebd97442d1d4..bd625f06ec0f 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1724,10 +1724,6 @@ static const struct constant_table ext4_param_dax[] = {
>  	{}
>  };
>  
> -/* String parameter that allows empty argument */
> -#define fsparam_string_empty(NAME, OPT) \
> -	__fsparam(fs_param_is_string, NAME, OPT, fs_param_can_be_empty, NULL)
> -
>  /*
>   * Mount option specification
>   * We don't use fsparam_flag_no because of the way we set the
> @@ -1768,10 +1764,8 @@ static const struct fs_parameter_spec ext4_param_specs[] = {
>  	fsparam_enum	("data",		Opt_data, ext4_param_data),
>  	fsparam_enum	("data_err",		Opt_data_err,
>  						ext4_param_data_err),
> -	fsparam_string_empty
> -			("usrjquota",		Opt_usrjquota),
> -	fsparam_string_empty
> -			("grpjquota",		Opt_grpjquota),
> +	fsparam_string_or_flag ("usrjquota",	Opt_usrjquota),
> +	fsparam_string_or_flag ("grpjquota",	Opt_grpjquota),
>  	fsparam_enum	("jqfmt",		Opt_jqfmt, ext4_param_jqfmt),
>  	fsparam_flag	("grpquota",		Opt_grpquota),
>  	fsparam_flag	("quota",		Opt_quota),
> @@ -2183,15 +2177,25 @@ static int ext4_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  	switch (token) {
>  #ifdef CONFIG_QUOTA
>  	case Opt_usrjquota:
> -		if (!param->string)
> -			return unnote_qf_name(fc, USRQUOTA);
> -		else
> +		if (param->type == fs_value_is_string) {
> +			if (!*param->string)
> +				return unnote_qf_name(fc, USRQUOTA);
> +
>  			return note_qf_name(fc, USRQUOTA, param);
> +		}
> +
> +		// param->type == fs_value_is_flag
> +		return note_qf_name(fc, USRQUOTA, param);
                       ^^^^^^^^^^^^
I think this isn't correct -- the correct function to call in this case is
unnote_qf_name(), not note_qf_name().  The same comment applies to the
grpjquota parameter handling.

Cheers,
-- 
Luís

>  	case Opt_grpjquota:
> -		if (!param->string)
> -			return unnote_qf_name(fc, GRPQUOTA);
> -		else
> +		if (param->type == fs_value_is_string) {
> +			if (!*param->string)
> +				return unnote_qf_name(fc, GRPQUOTA);
> +
>  			return note_qf_name(fc, GRPQUOTA, param);
> +		}
> +
> +		// param->type == fs_value_is_flag
> +		return note_qf_name(fc, GRPQUOTA, param);
>  #endif
>  	case Opt_sb:
>  		if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE) {
> diff --git a/include/linux/fs_parser.h b/include/linux/fs_parser.h
> index 01542c4b87a2..4f45141bea95 100644
> --- a/include/linux/fs_parser.h
> +++ b/include/linux/fs_parser.h
> @@ -131,5 +131,8 @@ static inline bool fs_validate_description(const char *name,
>  #define fsparam_bdev(NAME, OPT)	__fsparam(fs_param_is_blockdev, NAME, OPT, 0, NULL)
>  #define fsparam_path(NAME, OPT)	__fsparam(fs_param_is_path, NAME, OPT, 0, NULL)
>  #define fsparam_fd(NAME, OPT)	__fsparam(fs_param_is_fd, NAME, OPT, 0, NULL)
> +#define fsparam_string_or_flag(NAME, OPT) \
> +	__fsparam(fs_param_is_string, NAME, OPT, fs_param_can_be_empty, NULL), \
> +	fsparam_flag(NAME, OPT)
>  
>  #endif /* _LINUX_FS_PARSER_H */
> -- 
>
> 2.43.0
>