Re: [PATCH 01/14] fs_parse: add uid & gid option option parsing helpers

On 6/28/24 4:45 AM, Jan Kara wrote:
> On Thu 27-06-24 19:26:24, Eric Sandeen wrote:
>> Multiple filesystems take uid and gid as options, and the code to
>> create the ID from an integer and validate it is standard boilerplate
>> that can be moved into common helper functions, so do that for
>> consistency and less cut&paste.
>>
>> This also helps avoid the buggy pattern noted by Seth Jenkins at
>> https://lore.kernel.org/lkml/CALxfFW4BXhEwxR0Q5LSkg-8Vb4r2MONKCcUCVioehXQKr35eHg@mail.gmail.com/
>> because uid/gid parsing will fail before any assignment in most
>> filesystems.
>>
>> Signed-off-by: Eric Sandeen <[email protected]>
> 
> I like the idea since this seems like a nobrainer but is actually
> surprisingly subtle...
> 
>> diff --git a/fs/fs_parser.c b/fs/fs_parser.c
>> index a4d6ca0b8971..24727ec34e5a 100644
>> --- a/fs/fs_parser.c
>> +++ b/fs/fs_parser.c
>> @@ -308,6 +308,40 @@ int fs_param_is_fd(struct p_log *log, const struct fs_parameter_spec *p,
>>  }
>>  EXPORT_SYMBOL(fs_param_is_fd);
>>  
>> +int fs_param_is_uid(struct p_log *log, const struct fs_parameter_spec *p,
>> +		    struct fs_parameter *param, struct fs_parse_result *result)
>> +{
>> +	kuid_t uid;
>> +
>> +	if (fs_param_is_u32(log, p, param, result) != 0)
>> +		return fs_param_bad_value(log, param);
>> +
>> +	uid = make_kuid(current_user_ns(), result->uint_32);
> 
> But here is the problem: Filesystems mountable in user namespaces need to use
> fc->user_ns for resolving uids / gids (e.g. like fuse_parse_param()).
> Having helpers that work for some filesystems and are subtly broken for
> others is worse than no helpers... Or am I missing something?

Yeah, I should have pointed that out. tmpfs still does that check after the
initial trivial parsing after this change to use the basic helper:

        case Opt_uid:
                kuid = result.uid;
        
                /*
                 * The requested uid must be representable in the
                 * filesystem's idmapping.
                 */
                if (!kuid_has_mapping(fc->user_ns, kuid))
                        goto bad_value;
        
                ctx->uid = kuid;
                break;

I can see your point about risks of a helper that doesn't cover all cases
though.
 
> And the problem with fc->user_ns is that currently __fs_parse() does not
> get fs_context as an argument... So that will need some larger work.

Yup, this was discussed a little when I sent this idea as an RFC, and the
(brief/small) consensus was that it was worth going this far for now.

Getting fc back into __fs_parse looks rather tricky and Al was not keen
on the idea, for some reason.

Thanks,
-Eric
> 								Honza