Re: [PATCH v2 7/7] qobject atomics osdep: Make a few macros more hygienic

[Markus Armbruster]

Eric Blake <eblake@redhat.com> writes:

> On Wed, Sep 20, 2023 at 08:31:49PM +0200, Markus Armbruster wrote:
> ...
>> The only reliable way to prevent unintended variable name capture is
>> -Wshadow.
>> 
>> One blocker for enabling it is shadowing hiding in function-like
>> macros like
>> 
>>      qdict_put(dict, "name", qobject_ref(...))
>> 
>> qdict_put() wraps its last argument in QOBJECT(), and the last
>> argument here contains another QOBJECT().
>> 
>> Use dark preprocessor sorcery to make the macros that give us this
>> problem use different variable names on every call.
>> 
>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>
> It's changed (for the better) since v1, so I'm re-reviewing.
>
>> ---
>>  include/qapi/qmp/qobject.h | 11 +++++++++--
>>  include/qemu/atomic.h      | 17 ++++++++++++-----
>>  include/qemu/compiler.h    |  3 +++
>>  include/qemu/osdep.h       | 31 +++++++++++++++++++++++--------
>>  4 files changed, 47 insertions(+), 15 deletions(-)
>> 
>> diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
>> index 9003b71fd3..d36cc97805 100644
>> --- a/include/qapi/qmp/qobject.h
>> +++ b/include/qapi/qmp/qobject.h
>> @@ -45,10 +45,17 @@ struct QObject {
>>      struct QObjectBase_ base;
>>  };
>>  
>> -#define QOBJECT(obj) ({                                         \
>> +/*
>> + * Preprocessory sorcery ahead: use a different identifier for the
>
> s/Preprocessory/Preprocessor/ (multiple times in the patch)

Dang!  Will fix.

>> + * local variable in each expansion, so we can nest macro calls
>> + * without shadowing variables.
>> + */
>> +#define QOBJECT_INTERNAL(obj, _obj) ({                          \
>>      typeof(obj) _obj = (obj);                                   \
>> -    _obj ? container_of(&(_obj)->base, QObject, base) : NULL;   \
>> +    _obj                                                        \
>> +        ? container_of(&(_obj)->base, QObject, base) : NULL;    \
>
> As pointed out before, you can write &_obj->base instead of
> &(_obj)->base, now that we know _obj is a single identifier rather
> than an arbitrary expression.  Not strictly necessary since the extra
> () doesn't change semantics...

Makes sense, I just forgot here.

>>  })
>> +#define QOBJECT(obj) QOBJECT_INTERNAL((obj), MAKE_IDENTFIER(_obj))
>>  
>>  /* Required for qobject_to() */
>>  #define QTYPE_CAST_TO_QNull     QTYPE_QNULL
>> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
>> index d95612f7a0..d4cbd01909 100644
>> --- a/include/qemu/atomic.h
>> +++ b/include/qemu/atomic.h
>> @@ -157,13 +157,20 @@
>>      smp_read_barrier_depends();
>>  #endif
>>  
>> -#define qatomic_rcu_read(ptr)                          \
>> -    ({                                                 \
>> +/*
>> + * Preprocessory sorcery ahead: use a different identifier for the
>> + * local variable in each expansion, so we can nest macro calls
>> + * without shadowing variables.
>> + */
>> +#define qatomic_rcu_read_internal(ptr, _val)            \
>> +    ({                                                  \
>>      qemu_build_assert(sizeof(*ptr) <= ATOMIC_REG_SIZE); \
>> -    typeof_strip_qual(*ptr) _val;                      \
>> -    qatomic_rcu_read__nocheck(ptr, &_val);             \
>> -    _val;                                              \
>> +    typeof_strip_qual(*ptr) _val;                       \
>> +    qatomic_rcu_read__nocheck(ptr, &_val);              \
>
> ...but it looks odd for the patch to not be consistent on that front.
>
>> +    _val;                                               \
>>      })
>> +#define qatomic_rcu_read(ptr) \
>> +    qatomic_rcu_read_internal((ptr), MAKE_IDENTFIER(_val))
>>  
>>  #define qatomic_rcu_set(ptr, i) do {                   \
>>      qemu_build_assert(sizeof(*ptr) <= ATOMIC_REG_SIZE); \
>> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
>> index a309f90c76..03236d830c 100644
>> --- a/include/qemu/compiler.h
>> +++ b/include/qemu/compiler.h
>> @@ -37,6 +37,9 @@
>>  #define tostring(s) #s
>>  #endif
>>  
>> +/* Expands into an identifier stemN, where N is another number each time */
>> +#define MAKE_IDENTFIER(stem) glue(stem, __COUNTER__)
>
> I like how this turned out.
>
> With the spelling fix, and optionally with the redundant () dropped,
> you can keep my R-b.

Thanks!