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

[Eric Blake]

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)

> + * 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...

>  })
> +#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.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org