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

Luis Henriques
2024-03-03T21:31:45Z

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
>