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

Christian Brauner
2024-03-02T17:56:57Z

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:

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);
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