Re: [PATCH v8 09/12] xen: add runtime parameter access support to hypfs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.05.2020 16:56, Jürgen Groß wrote:
> On 14.05.20 14:10, Jan Beulich wrote:
>> On 14.05.2020 13:48, Jürgen Groß wrote:
>>> On 14.05.20 12:20, Jan Beulich wrote:
>>>> On 08.05.2020 17:34, Juergen Gross wrote:
>>>>> --- a/xen/common/grant_table.c
>>>>> +++ b/xen/common/grant_table.c
>>>>> @@ -85,8 +85,43 @@ struct grant_table {
>>>>>        struct grant_table_arch arch;
>>>>>    };
>>>>>    -static int parse_gnttab_limit(const char *param, const char *arg,
>>>>> -                              unsigned int *valp)
>>>>> +unsigned int __read_mostly opt_max_grant_frames = 64;
>>>>> +static unsigned int __read_mostly opt_max_maptrack_frames = 1024;
>>>>> +
>>>>> +#ifdef CONFIG_HYPFS
>>>>> +#define GRANT_CUSTOM_VAL_SZ  12
>>>>> +static char __read_mostly opt_max_grant_frames_val[GRANT_CUSTOM_VAL_SZ];
>>>>> +static char __read_mostly 
>>>>> opt_max_maptrack_frames_val[GRANT_CUSTOM_VAL_SZ];
>>>>> +
>>>>> +static void update_gnttab_par(struct param_hypfs *par, unsigned int val,
>>>>> +                              char *parval)
>>>>> +{
>>>>> +    snprintf(parval, GRANT_CUSTOM_VAL_SZ, "%u", val);
>>>>> +    custom_runtime_set_var_sz(par, parval, GRANT_CUSTOM_VAL_SZ);
>>>>> +}
>>>>> +
>>>>> +static void __init gnttab_max_frames_init(struct param_hypfs *par)
>>>>> +{
>>>>> +    update_gnttab_par(par, opt_max_grant_frames, 
>>>>> opt_max_grant_frames_val);
>>>>> +}
>>>>> +
>>>>> +static void __init max_maptrack_frames_init(struct param_hypfs *par)
>>>>> +{
>>>>> +    update_gnttab_par(par, opt_max_maptrack_frames,
>>>>> +                      opt_max_maptrack_frames_val);
>>>>> +}
>>>>> +#else
>>>>> +#define opt_max_grant_frames_val    NULL
>>>>> +#define opt_max_maptrack_frames_val NULL
>>>>
>>>> This looks latently dangerous to me (in case new uses of these
>>>> two identifiers appeared), but I guess my alternative suggestion
>>>> will be at best controversial, too:
>>>>
>>>> #define update_gnttab_par(par, val, unused) update_gnttab_par(par, val)
>>>> #define parse_gnttab_limit(par, arg, valp, unused) parse_gnttab_limit(par, 
>>>> arg, valp)
>>>>
>>>> (placed right here)
>>>
>>> Who else would use those identifiers not related to hypfs?
>>
>> I can't see an obvious possible use, but people get creative, i.e.
>> you never know. Passing NULL into a function without it being
>> blindingly obvious that it won't ever get (de)referenced is an at
>> least theoretical risk imo.
> 
> Hmm, what about using a special type for those content variables?
> Something like:
> 
> #ifdef CONFIG_HYPFS
> #define hypfs_string_var            char *
> #else
> #define hypfs_string_var            char
> #define opt_max_grant_frames_val    0
> #define opt_max_maptrack_frames_val 0
> #endif
> 
> And then use that as type for function parameters? This should make
> dereferencing pretty hard.
> 
> Other than that I have no really good idea how to avoid this problem.

IOW (as suspected) you don't like my suggestion? Personally I think
yours, having more #define-s, is at least not better than mine.

Jan