Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
Eric Blake writes:
> On Fri, Sep 01, 2023 at 10:48:26AM +0200, Markus Armbruster wrote:
>> > Indeed, not fully understanding the preprocessor makes for some
>> > interesting death traps.
>>
>> We use ALL_CAPS for macros to signal "watch out for traps".
>>
>
>> >> -#define QOBJECT(obj) ({ \
>> >> -typeof(obj) _obj = (obj); \
>> >> -_obj ? container_of(&(_obj)->base, QObject, base) : NULL; \
>> >> +#define QOBJECT_INTERNAL(obj, l) ({ \
>> >> +typeof(obj) PASTE(_obj, l) = (obj); \
>> >> +PASTE(_obj, l) \
>> >> +? container_of(&(PASTE(_obj, l))->base, QObject, base) : NULL; \
>> >> })
>> >> +#define QOBJECT(obj) QOBJECT_INTERNAL((obj), __COUNTER__)
>> >
>> > Slick! Every call to QOBJECT() defines a unique temporary variable
>> > name. But as written, QOBJECT(o) expands to this (when __COUNTER__ is
>> > 1):
>> >
>> > ({
>> > typeof((o)) _obj1 = ((o));
>> > _obj1 ? container_of(&(_obj1)->base, QObject, base) : NULL;
>> > })
>> >
>> > which has three sets of redundant parens that could be elided. Why do
>> > you need to add () around obj when forwarding to QOBJECT_INTERNAL()?
>>
>> Habit: enclose macro arguments in parenthesis unless there's a specific
>> need not to.
>>
>> > The only way the expansion of the text passed through the 'obj'
>> > parameter can contain a comma is if the user has already parenthesized
>> > it on their end (not that I expect a comma expression to be a frequent
>> > argument to QOBJECT(), but who knows). Arguing that it is to silence
>> > a syntax checker is weak; since we must NOT add parens around the
>> > parameters to QOBJECT_INTERNAL (calling PASTE((_obj), (l)) is
>> > obviously wrong).
>> >
>> > Meanwhile, the repetition of three calls to PASTE() is annoying. How
>> > about:
>> >
>> > #define QOBJECT_INTERNAL_2(_obj, _tmp) ({ \
>> > typeof _obj _tmp = _obj; \
>> > _tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
>> > )}
>> > #define QOBJECT_INTERNAL_1(_arg, _tmp) QOBJECT_INTERNAL_2(_arg, _tmp)
>> > #define QOBJECT_INTERNAL(_arg, _ctr) \
>> > QOBJECT_INTERNAL_1(_arg, PASTE(_obj, _ctr))
>> > #define QOBJECT(obj) QOBJECT_INTERNAL((obj), __COUNTER__)
>> >
>> > or:
>> >
>> > #define QOBJECT_INTERNAL_2(_obj, _tmp) ({ \
>> > typeof(_obj) _tmp = (_obj); \
>> > _tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
>> > )}
>> > #define QOBJECT_INTERNAL_1(_arg, _tmp) QOBJECT_INTERNAL_2(_arg, _tmp)
>> > #define QOBJECT_INTERNAL(_arg, _ctr) \
>> > QOBJECT_INTERNAL_1(_arg, PASTE(_obj, _ctr))
>> > #define QOBJECT(obj) QOBJECT_INTERNAL(obj, __COUNTER__)
>> >
>> > where, in either case, QOBJECT(o) should then expand to
>> >
>> > ({
>> > typeof (o) _obj1 = (o);
>> > _obj1 ? container_of(&_obj1->base, QObject, base) : NULL;
>> > })
>>
>> The idea is to have the innermost macro take the variable name instead
>> of the counter. "PASTE(_obj, l)" then becomes "_tmp" there, which is
>> more legible. However, we pay for it by going through two more macros.
>>
>> Can you explain why you need two more?
>
> Likewise habit, on my part (and laziness - I wrote the previous email
> without testing anything). I've learned over the years that pasting is
> hard (you can't mix ## with a macro that you want expanded without 2
> layers of indirection), so I started out by adding two layers of
> QOBJECT_INTERNAL, then wrote QOBJECT to forward to QOBJECT_INTERNAL.
> But now that you asked, I actually spent the time to test with the
> preprocessor, and your hunch is indeed correct: I over-compensated
> becaues of my habit.
>
> $cat foo.c
> #define PASTE(_a, _b) _a##_b
>
> #define QOBJECT_INTERNAL_2(_obj, _tmp) ({ \
> typeof _obj _tmp = _obj; \
> _tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
> )}
> #define QOBJECT_INTERNAL_1(_arg, _tmp) QOBJECT_INTERNAL_2(_arg, _tmp)
> #define QOBJECT_INTERNAL(_arg, _ctr) \
> QOBJECT_INTERNAL_1(_arg, PASTE(_obj, _ctr))
> #define QOBJECT(obj) QOBJECT_INTERNAL((obj), __COUNTER__)
>
> QOBJECT(o)
>
> #define Q_INTERNAL_1(_obj, _tmp) ({ \
> typeof _obj _tmp = _obj; \
> _tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
> )}
> #define Q_INTERNAL(_arg, _ctr) \
> Q_INTERNAL_1(_arg, PASTE(_obj, _ctr))
> #define Q(obj) Q_INTERNAL((obj), __COUNTER__)
>
> Q(o)
> $ gcc -E foo.c
> # 0 "foo.c"
> # 0 ""
> # 0 ""
> # 1 "/usr/include/stdc-predef.h" 1 3 4
> # 0 "" 2
> # 1 "foo.c"
> # 12 "foo.c"
> ({ typeof (o) _obj0 = (o); _obj0 ? container_of(&_obj0->base, QObject, base)
> : NULL; )}
> # 22 "foo.c"
> ({ typeof (o) _obj1 = (o); _obj1 ? container_of(&_obj1->base, QObject, base)
> : NULL; )}
>
> So the important part was merely ensuring that __COUNTER__ has a
> chance to be expanded PRIOR to the point where ## gets its argument,
> and one less layer of indirection was still sufficient for that.
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
On 9/1/23 16:50, Markus Armbruster wrote:
Cédric Le Goater writes:
On 8/31/23 16:30, Eric Blake wrote:
On Thu, Aug 31, 2023 at 03:25:46PM +0200, Markus Armbruster wrote:
[This paragraph written last: Bear with my stream of consciousness
review below, where I end up duplicating some of the conslusions you
reached before the point where I saw where the patch was headed]
Variables declared in macros can shadow other variables. Much of the
time, this is harmless, e.g.:
#define _FDT(exp) \
do { \
int ret = (exp); \
if (ret < 0) { \
error_report("error creating device tree: %s: %s", \
#exp, fdt_strerror(ret)); \
exit(1); \
} \
} while (0)
Which is why I've seen some projects require a strict namespace
separation: if all macro parameters and any identifiers declared in
macros use either a leading or a trailing _ (I prefer a trailing one,
to avoid risking conflicts with libc reserved namespace; but leading
is usually okay), and all other identifiers avoid that namespace, then
you will never have shadowing by calling a macro from normal code.
I started fixing the _FDT() macro since it is quite noisy at compile.
Same for qemu_fdt_setprop_cells(). So are we ok with names like 'ret_'
and 'i_' ? I used a 'local_' prefix for now but I can change.
I believe identifiers with a '_' suffix are just fine in macros. We
have quite a few of them already.
ok
I also have a bunch of fixes for ppc.
Appreciated!
count me on for the ppc files :
hw/ppc/pnv_psi.c
hw/ppc/spapr.c
hw/ppc/spapr_drc.c
hw/ppc/spapr_pci.c
include/hw/ppc/fdt.h
and surely some other files under target/ppc/
This one was taken care of by Phil:
include/sysemu/device_tree.h
C.
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
Cédric Le Goater writes:
> On 8/31/23 16:30, Eric Blake wrote:
>> On Thu, Aug 31, 2023 at 03:25:46PM +0200, Markus Armbruster wrote:
>> [This paragraph written last: Bear with my stream of consciousness
>> review below, where I end up duplicating some of the conslusions you
>> reached before the point where I saw where the patch was headed]
>>
>>> Variables declared in macros can shadow other variables. Much of the
>>> time, this is harmless, e.g.:
>>>
>>> #define _FDT(exp) \
>>> do { \
>>> int ret = (exp); \
>>> if (ret < 0) { \
>>> error_report("error creating device tree: %s: %s", \
>>> #exp, fdt_strerror(ret)); \
>>> exit(1); \
>>> } \
>>> } while (0)
>> Which is why I've seen some projects require a strict namespace
>> separation: if all macro parameters and any identifiers declared in
>> macros use either a leading or a trailing _ (I prefer a trailing one,
>> to avoid risking conflicts with libc reserved namespace; but leading
>> is usually okay), and all other identifiers avoid that namespace, then
>> you will never have shadowing by calling a macro from normal code.
>
> I started fixing the _FDT() macro since it is quite noisy at compile.
> Same for qemu_fdt_setprop_cells(). So are we ok with names like 'ret_'
> and 'i_' ? I used a 'local_' prefix for now but I can change.
I believe identifiers with a '_' suffix are just fine in macros. We
have quite a few of them already.
> I also have a bunch of fixes for ppc.
Appreciated!
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
On 1/9/23 14:59, Cédric Le Goater wrote:
On 8/31/23 16:30, Eric Blake wrote:
On Thu, Aug 31, 2023 at 03:25:46PM +0200, Markus Armbruster wrote:
[This paragraph written last: Bear with my stream of consciousness
review below, where I end up duplicating some of the conslusions you
reached before the point where I saw where the patch was headed]
Variables declared in macros can shadow other variables. Much of the
time, this is harmless, e.g.:
#define
_FDT(exp) \
do
{ \
int ret =
(exp); \
if (ret < 0)
{ \
error_report("error creating device tree: %s: %s", \
#exp,
fdt_strerror(ret)); \
exit(1); \
} \
} while (0)
Which is why I've seen some projects require a strict namespace
separation: if all macro parameters and any identifiers declared in
macros use either a leading or a trailing _ (I prefer a trailing one,
to avoid risking conflicts with libc reserved namespace; but leading
is usually okay), and all other identifiers avoid that namespace, then
you will never have shadowing by calling a macro from normal code.
I started fixing the _FDT() macro since it is quite noisy at compile.
Same for qemu_fdt_setprop_cells(). So are we ok with names like 'ret_'
and 'i_' ? I used a 'local_' prefix for now but I can change.
See
https://lore.kernel.org/qemu-devel/20230831225607.30829-12-phi...@linaro.org/
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
On Fri, Sep 01, 2023 at 10:48:26AM +0200, Markus Armbruster wrote:
> > Indeed, not fully understanding the preprocessor makes for some
> > interesting death traps.
>
> We use ALL_CAPS for macros to signal "watch out for traps".
>
> >> -#define QOBJECT(obj) ({ \
> >> -typeof(obj) _obj = (obj); \
> >> -_obj ? container_of(&(_obj)->base, QObject, base) : NULL; \
> >> +#define QOBJECT_INTERNAL(obj, l) ({ \
> >> +typeof(obj) PASTE(_obj, l) = (obj); \
> >> +PASTE(_obj, l) \
> >> +? container_of(&(PASTE(_obj, l))->base, QObject, base) : NULL; \
> >> })
> >> +#define QOBJECT(obj) QOBJECT_INTERNAL((obj), __COUNTER__)
> >
> > Slick! Every call to QOBJECT() defines a unique temporary variable
> > name. But as written, QOBJECT(o) expands to this (when __COUNTER__ is
> > 1):
> >
> > ({
> > typeof((o)) _obj1 = ((o));
> > _obj1 ? container_of(&(_obj1)->base, QObject, base) : NULL;
> > })
> >
> > which has three sets of redundant parens that could be elided. Why do
> > you need to add () around obj when forwarding to QOBJECT_INTERNAL()?
>
> Habit: enclose macro arguments in parenthesis unless there's a specific
> need not to.
>
> > The only way the expansion of the text passed through the 'obj'
> > parameter can contain a comma is if the user has already parenthesized
> > it on their end (not that I expect a comma expression to be a frequent
> > argument to QOBJECT(), but who knows). Arguing that it is to silence
> > a syntax checker is weak; since we must NOT add parens around the
> > parameters to QOBJECT_INTERNAL (calling PASTE((_obj), (l)) is
> > obviously wrong).
> >
> > Meanwhile, the repetition of three calls to PASTE() is annoying. How
> > about:
> >
> > #define QOBJECT_INTERNAL_2(_obj, _tmp) ({ \
> > typeof _obj _tmp = _obj; \
> > _tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
> > )}
> > #define QOBJECT_INTERNAL_1(_arg, _tmp) QOBJECT_INTERNAL_2(_arg, _tmp)
> > #define QOBJECT_INTERNAL(_arg, _ctr) \
> > QOBJECT_INTERNAL_1(_arg, PASTE(_obj, _ctr))
> > #define QOBJECT(obj) QOBJECT_INTERNAL((obj), __COUNTER__)
> >
> > or:
> >
> > #define QOBJECT_INTERNAL_2(_obj, _tmp) ({ \
> > typeof(_obj) _tmp = (_obj); \
> > _tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
> > )}
> > #define QOBJECT_INTERNAL_1(_arg, _tmp) QOBJECT_INTERNAL_2(_arg, _tmp)
> > #define QOBJECT_INTERNAL(_arg, _ctr) \
> > QOBJECT_INTERNAL_1(_arg, PASTE(_obj, _ctr))
> > #define QOBJECT(obj) QOBJECT_INTERNAL(obj, __COUNTER__)
> >
> > where, in either case, QOBJECT(o) should then expand to
> >
> > ({
> > typeof (o) _obj1 = (o);
> > _obj1 ? container_of(&_obj1->base, QObject, base) : NULL;
> > })
>
> The idea is to have the innermost macro take the variable name instead
> of the counter. "PASTE(_obj, l)" then becomes "_tmp" there, which is
> more legible. However, we pay for it by going through two more macros.
>
> Can you explain why you need two more?
Likewise habit, on my part (and laziness - I wrote the previous email
without testing anything). I've learned over the years that pasting is
hard (you can't mix ## with a macro that you want expanded without 2
layers of indirection), so I started out by adding two layers of
QOBJECT_INTERNAL, then wrote QOBJECT to forward to QOBJECT_INTERNAL.
But now that you asked, I actually spent the time to test with the
preprocessor, and your hunch is indeed correct: I over-compensated
becaues of my habit.
$cat foo.c
#define PASTE(_a, _b) _a##_b
#define QOBJECT_INTERNAL_2(_obj, _tmp) ({ \
typeof _obj _tmp = _obj; \
_tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
)}
#define QOBJECT_INTERNAL_1(_arg, _tmp) QOBJECT_INTERNAL_2(_arg, _tmp)
#define QOBJECT_INTERNAL(_arg, _ctr) \
QOBJECT_INTERNAL_1(_arg, PASTE(_obj, _ctr))
#define QOBJECT(obj) QOBJECT_INTERNAL((obj), __COUNTER__)
QOBJECT(o)
#define Q_INTERNAL_1(_obj, _tmp) ({ \
typeof _obj _tmp = _obj; \
_tmp ? container_of(&_tmp->base, QObject, base) : NULL; \
)}
#define Q_INTERNAL(_arg, _ctr) \
Q_INTERNAL_1(_arg, PASTE(_obj, _ctr))
#define Q(obj) Q_INTERNAL((obj), __COUNTER__)
Q(o)
$ gcc -E foo.c
# 0 "foo.c"
# 0 ""
# 0 ""
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "" 2
# 1 "foo.c"
# 12 "foo.c"
({ typeof (o) _obj0 = (o); _obj0 ? container_of(&_obj0->base, QObject, base) :
NULL; )}
# 22 "foo.c"
({ typeof (o) _obj1 = (o); _obj1 ? container_of(&_obj1->base, QObject, base) :
NULL; )}
So the important part was merely ensuring that __COUNTER__ has a
chance to be expanded PRIOR to the point where ## gets its argument,
and one less layer of indirection was still sufficient for that.
>
> Complication: the innermost macro needs to take all its variable names
> as arguments. The macro above has just one. Macros below have two.
>
> Not quite sure which version is easier
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
On 8/31/23 16:30, Eric Blake wrote:
On Thu, Aug 31, 2023 at 03:25:46PM +0200, Markus Armbruster wrote:
[This paragraph written last: Bear with my stream of consciousness
review below, where I end up duplicating some of the conslusions you
reached before the point where I saw where the patch was headed]
Variables declared in macros can shadow other variables. Much of the
time, this is harmless, e.g.:
#define _FDT(exp) \
do { \
int ret = (exp); \
if (ret < 0) { \
error_report("error creating device tree: %s: %s", \
#exp, fdt_strerror(ret)); \
exit(1); \
} \
} while (0)
Which is why I've seen some projects require a strict namespace
separation: if all macro parameters and any identifiers declared in
macros use either a leading or a trailing _ (I prefer a trailing one,
to avoid risking conflicts with libc reserved namespace; but leading
is usually okay), and all other identifiers avoid that namespace, then
you will never have shadowing by calling a macro from normal code.
I started fixing the _FDT() macro since it is quite noisy at compile.
Same for qemu_fdt_setprop_cells(). So are we ok with names like 'ret_'
and 'i_' ? I used a 'local_' prefix for now but I can change.
I also have a bunch of fixes for ppc.
C.
Harmless shadowing in h_client_architecture_support():
target_ulong ret;
[...]
ret = do_client_architecture_support(cpu, spapr, vec, fdt_bufsize);
if (ret == H_SUCCESS) {
_FDT((fdt_pack(spapr->fdt_blob)));
[...]
}
return ret;
However, we can get in trouble when the shadowed variable is used in a
macro argument:
#define QOBJECT(obj) ({ \
typeof(obj) o = (obj); \
o ? container_of(&(o)->base, QObject, base) : NULL; \
})
QOBJECT(o) expands into
({
--->typeof(o) o = (o);
o ? container_of(&(o)->base, QObject, base) : NULL;
})
Unintended variable name capture at --->. We'd be saved by
-Winit-self. But I could certainly construct more elaborate death
traps that don't trigger it.
Indeed, not fully understanding the preprocessor makes for some
interesting death traps.
To reduce the risk of trapping ourselves, we use variable names in
macros that no sane person would use elsewhere. Here's our actual
definition of QOBJECT():
#define QOBJECT(obj) ({ \
typeof(obj) _obj = (obj); \
_obj ? container_of(&(_obj)->base, QObject, base) : NULL; \
})
Yep, goes back to the policy I've seen of enforcing naming conventions
that ensure macros can't shadow normal code.
Works well enough until we nest macro calls. For instance, with
#define qobject_ref(obj) ({ \
typeof(obj) _obj = (obj); \
qobject_ref_impl(QOBJECT(_obj));\
_obj; \
})
the expression qobject_ref(obj) expands into
({
typeof(obj) _obj = (obj);
qobject_ref_impl(
({
--->typeof(_obj) _obj = (_obj);
_obj ? container_of(&(_obj)->base, QObject, base) : NULL;
}));
_obj;
})
Unintended variable name capture at --->.
Yep, you've just proven how macros calling macros requires care, as we
no longer have the namespace protections. If macro calls can only
form a DAG, it is possible to choose unique intermediate declarations
for every macro (although remembering to do that, and still keeping
the macro definition legible, may not be easy). But if you can have
macros that form any sort of nesting loop (and we WANT to allow things
like MIN(MIN(a, b), c) - which is such a loop), dynamic naming becomes
the only solution.
The only reliable way to prevent unintended variable name capture is
-Wshadow.
Yes, I would love to have that enabled eventually.
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.
Sounds foreboding; hopefully not many macros are affected...
Signed-off-by: Markus Armbruster
---
include/qapi/qmp/qobject.h | 8 +---
include/qemu/atomic.h | 11 ++-
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
Eric Blake writes:
> On Thu, Aug 31, 2023 at 03:25:46PM +0200, Markus Armbruster wrote:
>
> [This paragraph written last: Bear with my stream of consciousness
> review below, where I end up duplicating some of the conslusions you
> reached before the point where I saw where the patch was headed]
>
>> Variables declared in macros can shadow other variables. Much of the
>> time, this is harmless, e.g.:
>>
>> #define _FDT(exp) \
>> do { \
>> int ret = (exp); \
>> if (ret < 0) { \
>> error_report("error creating device tree: %s: %s", \
>> #exp, fdt_strerror(ret)); \
>> exit(1); \
>> } \
>> } while (0)
>
> Which is why I've seen some projects require a strict namespace
> separation: if all macro parameters and any identifiers declared in
> macros use either a leading or a trailing _ (I prefer a trailing one,
> to avoid risking conflicts with libc reserved namespace; but leading
> is usually okay), and all other identifiers avoid that namespace, then
> you will never have shadowing by calling a macro from normal code.
>
>>
>> Harmless shadowing in h_client_architecture_support():
>>
>> target_ulong ret;
>>
>> [...]
>>
>> ret = do_client_architecture_support(cpu, spapr, vec, fdt_bufsize);
>> if (ret == H_SUCCESS) {
>> _FDT((fdt_pack(spapr->fdt_blob)));
>> [...]
>> }
>>
>> return ret;
>>
>> However, we can get in trouble when the shadowed variable is used in a
>> macro argument:
>>
>> #define QOBJECT(obj) ({ \
>> typeof(obj) o = (obj); \
>> o ? container_of(&(o)->base, QObject, base) : NULL; \
>> })
>>
>> QOBJECT(o) expands into
>>
>> ({
>> --->typeof(o) o = (o);
>> o ? container_of(&(o)->base, QObject, base) : NULL;
>> })
>>
>> Unintended variable name capture at --->. We'd be saved by
>> -Winit-self. But I could certainly construct more elaborate death
>> traps that don't trigger it.
>
> Indeed, not fully understanding the preprocessor makes for some
> interesting death traps.
We use ALL_CAPS for macros to signal "watch out for traps".
>> To reduce the risk of trapping ourselves, we use variable names in
>> macros that no sane person would use elsewhere. Here's our actual
>> definition of QOBJECT():
>>
>> #define QOBJECT(obj) ({ \
>> typeof(obj) _obj = (obj); \
>> _obj ? container_of(&(_obj)->base, QObject, base) : NULL; \
>> })
>
> Yep, goes back to the policy I've seen of enforcing naming conventions
> that ensure macros can't shadow normal code.
>
>>
>> Works well enough until we nest macro calls. For instance, with
>>
>> #define qobject_ref(obj) ({ \
>> typeof(obj) _obj = (obj); \
>> qobject_ref_impl(QOBJECT(_obj));\
>> _obj; \
>> })
>>
>> the expression qobject_ref(obj) expands into
>>
>> ({
>> typeof(obj) _obj = (obj);
>> qobject_ref_impl(
>> ({
>> --->typeof(_obj) _obj = (_obj);
>> _obj ? container_of(&(_obj)->base, QObject, base) : NULL;
>> }));
>> _obj;
>> })
>>
>> Unintended variable name capture at --->.
>
> Yep, you've just proven how macros calling macros requires care, as we
> no longer have the namespace protections. If macro calls can only
> form a DAG, it is possible to choose unique intermediate declarations
> for every macro (although remembering to do that, and still keeping
> the macro definition legible, may not be easy). But if you can have
> macros that form any sort of nesting loop (and we WANT to allow things
> like MIN(MIN(a, b), c) - which is such a loop), dynamic naming becomes
> the only solution.
>
>>
>> The only reliable way to prevent unintended variable name capture is
>> -Wshadow.
>
> Yes, I would love to have that enabled eventually.
>
>>
>> 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.
>
> Sounds foreboding; hopefully not many macros are affected...
>
>>
>> Signed-off-by: Markus Armbruster
>> ---
>>
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
Richard Henderson writes: > On 8/31/23 06:25, Markus Armbruster wrote: >> +#define PASTE(a, b) a##b > > We already have glue() in qemu/compiler.h. Missed it, will fix. > The rest of it looks quite sensible. Thanks!
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
On 8/31/23 06:25, Markus Armbruster wrote: +#define PASTE(a, b) a##b We already have glue() in qemu/compiler.h. The rest of it looks quite sensible. r~
Re: [PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
On Thu, Aug 31, 2023 at 03:25:46PM +0200, Markus Armbruster wrote:
[This paragraph written last: Bear with my stream of consciousness
review below, where I end up duplicating some of the conslusions you
reached before the point where I saw where the patch was headed]
> Variables declared in macros can shadow other variables. Much of the
> time, this is harmless, e.g.:
>
> #define _FDT(exp) \
> do { \
> int ret = (exp); \
> if (ret < 0) { \
> error_report("error creating device tree: %s: %s", \
> #exp, fdt_strerror(ret)); \
> exit(1); \
> } \
> } while (0)
Which is why I've seen some projects require a strict namespace
separation: if all macro parameters and any identifiers declared in
macros use either a leading or a trailing _ (I prefer a trailing one,
to avoid risking conflicts with libc reserved namespace; but leading
is usually okay), and all other identifiers avoid that namespace, then
you will never have shadowing by calling a macro from normal code.
>
> Harmless shadowing in h_client_architecture_support():
>
> target_ulong ret;
>
> [...]
>
> ret = do_client_architecture_support(cpu, spapr, vec, fdt_bufsize);
> if (ret == H_SUCCESS) {
> _FDT((fdt_pack(spapr->fdt_blob)));
> [...]
> }
>
> return ret;
>
> However, we can get in trouble when the shadowed variable is used in a
> macro argument:
>
> #define QOBJECT(obj) ({ \
> typeof(obj) o = (obj); \
> o ? container_of(&(o)->base, QObject, base) : NULL; \
> })
>
> QOBJECT(o) expands into
>
> ({
> --->typeof(o) o = (o);
> o ? container_of(&(o)->base, QObject, base) : NULL;
> })
>
> Unintended variable name capture at --->. We'd be saved by
> -Winit-self. But I could certainly construct more elaborate death
> traps that don't trigger it.
Indeed, not fully understanding the preprocessor makes for some
interesting death traps.
>
> To reduce the risk of trapping ourselves, we use variable names in
> macros that no sane person would use elsewhere. Here's our actual
> definition of QOBJECT():
>
> #define QOBJECT(obj) ({ \
> typeof(obj) _obj = (obj); \
> _obj ? container_of(&(_obj)->base, QObject, base) : NULL; \
> })
Yep, goes back to the policy I've seen of enforcing naming conventions
that ensure macros can't shadow normal code.
>
> Works well enough until we nest macro calls. For instance, with
>
> #define qobject_ref(obj) ({ \
> typeof(obj) _obj = (obj); \
> qobject_ref_impl(QOBJECT(_obj));\
> _obj; \
> })
>
> the expression qobject_ref(obj) expands into
>
> ({
> typeof(obj) _obj = (obj);
> qobject_ref_impl(
> ({
> --->typeof(_obj) _obj = (_obj);
> _obj ? container_of(&(_obj)->base, QObject, base) : NULL;
> }));
> _obj;
> })
>
> Unintended variable name capture at --->.
Yep, you've just proven how macros calling macros requires care, as we
no longer have the namespace protections. If macro calls can only
form a DAG, it is possible to choose unique intermediate declarations
for every macro (although remembering to do that, and still keeping
the macro definition legible, may not be easy). But if you can have
macros that form any sort of nesting loop (and we WANT to allow things
like MIN(MIN(a, b), c) - which is such a loop), dynamic naming becomes
the only solution.
>
> The only reliable way to prevent unintended variable name capture is
> -Wshadow.
Yes, I would love to have that enabled eventually.
>
> 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.
Sounds foreboding; hopefully not many macros are affected...
>
> Signed-off-by: Markus Armbruster
> ---
> include/qapi/qmp/qobject.h | 8 +---
> include/qemu/atomic.h | 11 ++-
> include/qemu/osdep.h | 34 +++---
> 3 files changed, 30 insertions(+), 23 deletions(-)
...okay, the size of the diffstat seems
[PATCH 7/7] qobject atomics osdep: Make a few macros more hygienic
Variables declared in macros can shadow other variables. Much of the
time, this is harmless, e.g.:
#define _FDT(exp) \
do { \
int ret = (exp); \
if (ret < 0) { \
error_report("error creating device tree: %s: %s", \
#exp, fdt_strerror(ret)); \
exit(1); \
} \
} while (0)
Harmless shadowing in h_client_architecture_support():
target_ulong ret;
[...]
ret = do_client_architecture_support(cpu, spapr, vec, fdt_bufsize);
if (ret == H_SUCCESS) {
_FDT((fdt_pack(spapr->fdt_blob)));
[...]
}
return ret;
However, we can get in trouble when the shadowed variable is used in a
macro argument:
#define QOBJECT(obj) ({ \
typeof(obj) o = (obj); \
o ? container_of(&(o)->base, QObject, base) : NULL; \
})
QOBJECT(o) expands into
({
--->typeof(o) o = (o);
o ? container_of(&(o)->base, QObject, base) : NULL;
})
Unintended variable name capture at --->. We'd be saved by
-Winit-self. But I could certainly construct more elaborate death
traps that don't trigger it.
To reduce the risk of trapping ourselves, we use variable names in
macros that no sane person would use elsewhere. Here's our actual
definition of QOBJECT():
#define QOBJECT(obj) ({ \
typeof(obj) _obj = (obj); \
_obj ? container_of(&(_obj)->base, QObject, base) : NULL; \
})
Works well enough until we nest macro calls. For instance, with
#define qobject_ref(obj) ({ \
typeof(obj) _obj = (obj); \
qobject_ref_impl(QOBJECT(_obj));\
_obj; \
})
the expression qobject_ref(obj) expands into
({
typeof(obj) _obj = (obj);
qobject_ref_impl(
({
--->typeof(_obj) _obj = (_obj);
_obj ? container_of(&(_obj)->base, QObject, base) : NULL;
}));
_obj;
})
Unintended variable name capture at --->.
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
---
include/qapi/qmp/qobject.h | 8 +---
include/qemu/atomic.h | 11 ++-
include/qemu/osdep.h | 34 +++---
3 files changed, 30 insertions(+), 23 deletions(-)
diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
index 9003b71fd3..7b50fc905d 100644
--- a/include/qapi/qmp/qobject.h
+++ b/include/qapi/qmp/qobject.h
@@ -45,10 +45,12 @@ struct QObject {
struct QObjectBase_ base;
};
-#define QOBJECT(obj) ({ \
-typeof(obj) _obj = (obj); \
-_obj ? container_of(&(_obj)->base, QObject, base) : NULL; \
+#define QOBJECT_INTERNAL(obj, l) ({ \
+typeof(obj) PASTE(_obj, l) = (obj); \
+PASTE(_obj, l) \
+? container_of(&(PASTE(_obj, l))->base, QObject, base) : NULL; \
})
+#define QOBJECT(obj) QOBJECT_INTERNAL((obj), __COUNTER__)
/* 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..3f80ffac69 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -157,13 +157,14 @@
smp_read_barrier_depends();
#endif
-#define qatomic_rcu_read(ptr) \
-({ \
+#define qatomic_rcu_read_internal(ptr, l) \
+({ \
qemu_build_assert(sizeof(*ptr) <= ATOMIC_REG_SIZE); \
-typeof_strip_qual(*ptr) _val; \
-qatomic_rcu_read__nocheck(ptr, &_val); \
-_val; \
+typeof_strip_qual(*ptr) PASTE(_val, l); \
+qatomic_rcu_read__nocheck(ptr, (_val, l));\
+PASTE(_val, l); \
})
