linux-ext4 - Re: [PATCH 1/3] fs_parser: handle parameters that can be empty and don't have a value

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Date: Fri, 01 Mar 2024 15:45:27 +0000
From: Luis Henriques <lhenriques@...e.de>
To: Christian Brauner <brauner@...nel.org>
Cc: Theodore Ts'o <tytso@....edu>,  Andreas Dilger
 <adilger.kernel@...ger.ca>,  Alexander Viro <viro@...iv.linux.org.uk>,
  Jan Kara <jack@...e.cz>,  Miklos Szeredi <miklos@...redi.hu>,  Amir
 Goldstein <amir73il@...il.com>,  linux-ext4@...r.kernel.org,
  linux-fsdevel@...r.kernel.org,  linux-unionfs@...r.kernel.org,
  linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] fs_parser: handle parameters that can be empty and
 don't have a value

Christian Brauner <brauner@...nel.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@...e.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.

Cheers,
-- 
Luís