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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

>>> +#define custom_runtime_set_var_sz(parfs, var, sz) \
>>> +    { \
>>> +        (parfs)->hypfs.content = var; \
>>> +        (parfs)->hypfs.e.max_size = sz; \
>>
>> var and sz want parentheses around them.
> 
> Fine with me, but out of curiosity: what can go wrong without? Are
> you thinking of multi-statement arguments?

Well, as just said in the reply on the patch 4 thread, you have
a point about this being the right side of an assignment.
Nevertheless such uses would look more consistent if parenthesized.
The only cases where I see it reasonable to omit parentheses are
- parameters made the operands of # or ##,
- parameters handed on to further macros / functions unaltered,
- parameters representing struct/union field names,
- perhaps other special cases along the lines of the above that
  I can't seem to be able to think of right now.

>>> +/* initfunc needs to set size and content, e.g. via 
>>> custom_runtime_set_var(). */
>>> +#define custom_runtime_only_param(_name, _var, initfunc) \
>>
>> I've started noticing it here, but the issue exists further up
>> (and down) as well - please can you avoid identifiers with
>> leading underscores that are in violation of the C standard?
>> Even more so that here you're not even consistent across
>> macro parameter names.
> 
> Basically I only extended the macro to take another parameter and I
> omitted the underscore exactly for the reason you mentioned.
> 
> In case you like it better I can prepend a patch to the series dropping
> all the leading single underscores from the macros in param.h.

That's code churn I don't view as strictly necessary - adjusting
these can be done when they get touched anyway. But here we have
a whole new body of code.

>>> +#define param_2_parfs(par)  NULL
>>
>> Along the lines of the earlier comment, this looks latently dangerous.
> 
> In which way? How can the empty struct be dereferenced in a way not
> resulting in build time errors, other than using a cast which would
> be obviously wrong for the standard case when CONFIG_HYPFS is defined?

Have the result of the macro passed to a function taking void *, e.g.
memcpy().

Jan