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Re: [Xen-devel] [PATCH v6 01/12] xen: allow only sizeof(bool) variables for boolean_param()

On 09.03.20 14:06, Jan Beulich wrote:
On 09.03.2020 14:01, Jürgen Groß wrote:
On 09.03.20 12:55, Jan Beulich wrote:
On 09.03.2020 12:43, Julien Grall wrote:
On 26/02/2020 12:46, Juergen Gross wrote:
@@ -46,7 +48,8 @@ extern const struct kernel_param __param_start[], 
        __kparam __setup_##_var = \
            { .name = __setup_str_##_var, \
              .type = OPT_BOOL, \
-          .len = sizeof(_var), \
+          .len = sizeof(_var) + \
+                 BUILD_BUG_ON_ZERO(sizeof(_var) != sizeof(bool)), \

   From my understanding, sizeof(bool) is not necessarily 1 (it can be
greater). While this is fine to use it in Xen, I think we want it to
always be one when exposed in the hypfs.

I don't think so: We want variable of type 'bool' to be updated
consistently (i.e. by a write to the full variable). Hence I
think sizeof(bool) is correct here. I can see though that the
hypercall interface then gains a dependency on the hypervisor's
representation of 'bool', but I think such ought to be taken
care of in the function carrying out the write, not in the
macro here.

So you think I should special case bool entries when returning the
size information? Or do you think its fine to have the hypervisor's
size reported and let the lib do the size handling correctly?

Either way would be fine by me, but I think not having callers have
a (required) way to know the hypervisor's sizeof(bool) would be a
more clean interface overall.

The size is reported via the dirent, so this is fine. And when you have
a look into patch 5 you'll see that the writing of the bool is merged
with the uint writing in the lib code. I should remove the safety check
regarding sizeof(bool) matching the size reported in the dirent, however
(this is a leftover I forgot to remove).


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