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Re: [PATCH v2 7/7] qobject atomics osdep: Make a few macros more hygieni
From: |
Eric Blake |
Subject: |
Re: [PATCH v2 7/7] qobject atomics osdep: Make a few macros more hygienic |
Date: |
Wed, 20 Sep 2023 15:10:58 -0500 |
User-agent: |
NeoMutt/20230517 |
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
- [PATCH v2 6/7] block: Clean up local variable shadowing, (continued)
- [PATCH v2 6/7] block: Clean up local variable shadowing, Markus Armbruster, 2023/09/20
- [PATCH v2 5/7] block/vdi: Clean up local variable shadowing, Markus Armbruster, 2023/09/20
- [PATCH v2 1/7] migration/rdma: Fix save_page method to fail on polling error, Markus Armbruster, 2023/09/20
- [PATCH v2 4/7] block/dirty-bitmap: Clean up local variable shadowing, Markus Armbruster, 2023/09/20
- [PATCH v2 2/7] migration: Clean up local variable shadowing, Markus Armbruster, 2023/09/20
- [PATCH v2 7/7] qobject atomics osdep: Make a few macros more hygienic, Markus Armbruster, 2023/09/20
- [PATCH v2 3/7] ui: Clean up local variable shadowing, Markus Armbruster, 2023/09/20