* Re: Question on FIELD_PREP() for static array
[not found] ` <40fd3d963820bf96547fa9b5e8c171c6a339674e.camel@perches.com>
@ 2018-10-10 18:13 ` Johannes Berg
2018-10-11 14:24 ` John Garry
0 siblings, 1 reply; 7+ messages in thread
From: Johannes Berg @ 2018-10-10 18:13 UTC (permalink / raw)
To: Joe Perches, John Garry, Andrew Morton, Andy Shevchenko,
Kalle Valo, jakub.kicinski, yamada.masahiro, Arnd Bergmann, viro,
linux-kernel
Cc: linux-wireless, nbd
On Wed, 2018-10-10 at 10:33 -0700, Joe Perches wrote:
>
> > Specifically it doesn't like the __BF_FIELD_CHECK() in FIELD_PREP().
> >
> > Any ideas on compiler trickery we could do with the FIELD_PREP()
> > definition to avoid this issue (i.e. enforce the check but only use the
> > constant value)?
>
> Perhaps __bf_shf should not use __builtin_ffsll.
__bf_shf() is a constant expression, and is fine in this context.
The problem is the use of the compound statement here:
static int x[2] = {
({ (void)(0); 1; }),
0,
}
similarly fails to compile.
I've recently run into a similar situation, namely in
include/net/netlink.h, and the applicable way to solve it here would be
something like this:
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 3f1ef4450a7c..0680d641923f 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -49,19 +49,16 @@
#define __bf_shf(x) (__builtin_ffsll(x) - 1)
+#define BUILD_BUG_ON_RET_ZERO(cond) (sizeof(char[1 - 2*!!(cond)]) - 1)
+#define BUILD_BUG_ON_NOT_POW2_RET_ZERO(n) BUILD_BUG_ON_RET_ZERO(((n) & ((n) - 1)) != 0)
+
#define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
- ({ \
- BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
- _pfx "mask is not constant"); \
- BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero"); \
- BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
- ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
- _pfx "value too large for the field"); \
- BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull, \
- _pfx "type of reg too small for mask"); \
- __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
- (1ULL << __bf_shf(_mask))); \
- })
+ BUILD_BUG_ON_RET_ZERO(!__builtin_constant_p(_mask)) + \
+ BUILD_BUG_ON_RET_ZERO((_mask) == 0) + \
+ BUILD_BUG_ON_RET_ZERO(__builtin_constant_p(_val) ? \
+ ~((_mask) >> __bf_shf(_mask)) & (_val) : 0) + \
+ BUILD_BUG_ON_RET_ZERO((_mask) > (typeof(_reg))~0ull) + \
+ BUILD_BUG_ON_NOT_POW2_RET_ZERO((_mask) + (1ULL << __bf_shf(_mask)))
/**
* FIELD_FIT() - check if value fits in the field
@@ -85,10 +82,8 @@
* be combined with other fields of the bitfield using logical OR.
*/
#define FIELD_PREP(_mask, _val) \
- ({ \
- __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
- ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
- })
+ (__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ") + \
+ (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)))
/**
* FIELD_GET() - extract a bitfield element
Note that this is an incomplete patch - everything but FIELD_PREP will
not compile with this.
Also, BUILD_BUG_ON_RET_ZERO and BUILD_BUG_ON_NOT_POW2_RET_ZERO should
probably have better names, or perhaps do the positive way that I did in
__NLA_ENSURE, e.g. CONST_ASSERT()/CONST_ASSERT_IS_POWER_OF_2()? I guess
they should go to build_bug.h as well...
johannes
^ permalink raw reply related [flat|nested] 7+ messages in thread
* Re: Question on FIELD_PREP() for static array
2018-10-10 18:13 ` Question on FIELD_PREP() for static array Johannes Berg
@ 2018-10-11 14:24 ` John Garry
2018-10-11 15:23 ` Johannes Berg
0 siblings, 1 reply; 7+ messages in thread
From: John Garry @ 2018-10-11 14:24 UTC (permalink / raw)
To: Johannes Berg, Joe Perches, Andrew Morton, Andy Shevchenko,
Kalle Valo, jakub.kicinski, yamada.masahiro, Arnd Bergmann, viro,
linux-kernel
Cc: linux-wireless, nbd
On 10/10/2018 19:13, Johannes Berg wrote:
> On Wed, 2018-10-10 at 10:33 -0700, Joe Perches wrote:
>>
>>> Specifically it doesn't like the __BF_FIELD_CHECK() in FIELD_PREP().
>>>
>>> Any ideas on compiler trickery we could do with the FIELD_PREP()
>>> definition to avoid this issue (i.e. enforce the check but only use the
>>> constant value)?
>>
thanks guys
>> Perhaps __bf_shf should not use __builtin_ffsll.
>
> __bf_shf() is a constant expression, and is fine in this context.
>
> The problem is the use of the compound statement here:
>
> static int x[2] = {
> ({ (void)(0); 1; }),
> 0,
> }
>
> similarly fails to compile.
>
> I've recently run into a similar situation, namely in
> include/net/netlink.h, and the applicable way to solve it here would be
> something like this:
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 3f1ef4450a7c..0680d641923f 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -49,19 +49,16 @@
>
> #define __bf_shf(x) (__builtin_ffsll(x) - 1)
>
> +#define BUILD_BUG_ON_RET_ZERO(cond) (sizeof(char[1 - 2*!!(cond)]) - 1)
> +#define BUILD_BUG_ON_NOT_POW2_RET_ZERO(n) BUILD_BUG_ON_RET_ZERO(((n) & ((n) - 1)) != 0)
> +
> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
> - ({ \
> - BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
> - _pfx "mask is not constant"); \
> - BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero"); \
> - BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
> - ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
> - _pfx "value too large for the field"); \
> - BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull, \
> - _pfx "type of reg too small for mask"); \
> - __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
> - (1ULL << __bf_shf(_mask))); \
> - })
> + BUILD_BUG_ON_RET_ZERO(!__builtin_constant_p(_mask)) + \
> + BUILD_BUG_ON_RET_ZERO((_mask) == 0) + \
> + BUILD_BUG_ON_RET_ZERO(__builtin_constant_p(_val) ? \
> + ~((_mask) >> __bf_shf(_mask)) & (_val) : 0) + \
> + BUILD_BUG_ON_RET_ZERO((_mask) > (typeof(_reg))~0ull) + \
> + BUILD_BUG_ON_NOT_POW2_RET_ZERO((_mask) + (1ULL << __bf_shf(_mask)))
>
> /**
> * FIELD_FIT() - check if value fits in the field
> @@ -85,10 +82,8 @@
> * be combined with other fields of the bitfield using logical OR.
> */
> #define FIELD_PREP(_mask, _val) \
> - ({ \
> - __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
> - ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
> - })
> + (__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ") + \
> + (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)))
>
> /**
> * FIELD_GET() - extract a bitfield element
>
>
> Note that this is an incomplete patch - everything but FIELD_PREP will
> not compile with this.
>
> Also, BUILD_BUG_ON_RET_ZERO and BUILD_BUG_ON_NOT_POW2_RET_ZERO should
> probably have better names, or perhaps do the positive way that I did in
> __NLA_ENSURE, e.g. CONST_ASSERT()/CONST_ASSERT_IS_POWER_OF_2()? I guess
> they should go to build_bug.h as well...
Seems reasonable. However I did try this and was getting compiler
warnings about VLA, from a non-constant being fed into
BUILD_BUG_ON_RET_ZERO(), related to sizeof char[]:
drivers/iio/adc/meson_saradc.c:375:2: warning: ISO C90 forbids variable
length array [-Wvla]
regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0),
Surely __NLA_ENSURE is getting a similar issue as it uses a similar
principle, no? I see that this is in -next now, but could not this macro
or derivatives being referenced.
>
Much appreciated,
John
> johannes
>
> .
>
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Question on FIELD_PREP() for static array
2018-10-11 14:24 ` John Garry
@ 2018-10-11 15:23 ` Johannes Berg
2018-10-11 16:16 ` John Garry
0 siblings, 1 reply; 7+ messages in thread
From: Johannes Berg @ 2018-10-11 15:23 UTC (permalink / raw)
To: John Garry, Joe Perches, Andrew Morton, Andy Shevchenko,
Kalle Valo, jakub.kicinski, yamada.masahiro, Arnd Bergmann, viro,
linux-kernel
Cc: linux-wireless, nbd
On Thu, 2018-10-11 at 15:24 +0100, John Garry wrote:
>
> > +#define BUILD_BUG_ON_RET_ZERO(cond) (sizeof(char[1 - 2*!!(cond)]) - 1)
> > +#define BUILD_BUG_ON_NOT_POW2_RET_ZERO(n) BUILD_BUG_ON_RET_ZERO(((n) & ((n) - 1)) != 0)
> Seems reasonable. However I did try this and was getting compiler
> warnings about VLA, from a non-constant being fed into
> BUILD_BUG_ON_RET_ZERO(), related to sizeof char[]:
> drivers/iio/adc/meson_saradc.c:375:2: warning: ISO C90 forbids variable
> length array [-Wvla]
> regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0),
Hmm, what's the code there?
I don't see why the compiler should think it's a variable length?
> Surely __NLA_ENSURE is getting a similar issue as it uses a similar
> principle, no? I see that this is in -next now, but could not this macro
> or derivatives being referenced.
Yeah, I have a patch now to reference it, but I don't see anything from
-Wvla with gcc 8.1?
See
https://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git/commit/?id=3d7af878357acd9e37fc156928106f1a969c8942
and its parent.
Do you see -Wvla warnings there? Any idea how I could reproduce them?
johannes
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Question on FIELD_PREP() for static array
2018-10-11 15:23 ` Johannes Berg
@ 2018-10-11 16:16 ` John Garry
2018-10-11 17:26 ` John Garry
0 siblings, 1 reply; 7+ messages in thread
From: John Garry @ 2018-10-11 16:16 UTC (permalink / raw)
To: Johannes Berg, Joe Perches, Andrew Morton, Andy Shevchenko,
Kalle Valo, jakub.kicinski, yamada.masahiro, Arnd Bergmann, viro,
linux-kernel
Cc: linux-wireless, nbd
On 11/10/2018 16:23, Johannes Berg wrote:
Hi
> On Thu, 2018-10-11 at 15:24 +0100, John Garry wrote:
>>
>>> +#define BUILD_BUG_ON_RET_ZERO(cond) (sizeof(char[1 - 2*!!(cond)]) - 1)
>>> +#define BUILD_BUG_ON_NOT_POW2_RET_ZERO(n) BUILD_BUG_ON_RET_ZERO(((n) & ((n) - 1)) != 0)
>
>> Seems reasonable. However I did try this and was getting compiler
>> warnings about VLA, from a non-constant being fed into
>> BUILD_BUG_ON_RET_ZERO(), related to sizeof char[]:
>> drivers/iio/adc/meson_saradc.c:375:2: warning: ISO C90 forbids variable
>> length array [-Wvla]
>> regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0),
>
> Hmm, what's the code there?
Nothing special, it was just a sample. Here'e the code:
regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0),
chan->address);
So val is a variable, and I find if remove both of the
BUILD_BUG_ON_RET_ZERO()' which use __bf_shf() then it goes away.
>
> I don't see why the compiler should think it's a variable length?
>
>> Surely __NLA_ENSURE is getting a similar issue as it uses a similar
>> principle, no? I see that this is in -next now, but could not this macro
>> or derivatives being referenced.
>
> Yeah, I have a patch now to reference it, but I don't see anything from
> -Wvla with gcc 8.1?
I'm using a 7.3.1-based toolchain
>
> See
> https://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git/commit/?id=3d7af878357acd9e37fc156928106f1a969c8942
> and its parent.
>
> Do you see -Wvla warnings there? Any idea how I could reproduce them?
I'll try it, thanks
John
>
> johannes
>
> .
>
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Question on FIELD_PREP() for static array
2018-10-11 16:16 ` John Garry
@ 2018-10-11 17:26 ` John Garry
2018-10-11 19:13 ` Johannes Berg
0 siblings, 1 reply; 7+ messages in thread
From: John Garry @ 2018-10-11 17:26 UTC (permalink / raw)
To: Johannes Berg, Joe Perches, Andrew Morton, Andy Shevchenko,
Kalle Valo, jakub.kicinski, yamada.masahiro, Arnd Bergmann, viro,
linux-kernel
Cc: linux-wireless, nbd
>> Yeah, I have a patch now to reference it, but I don't see anything from
>> -Wvla with gcc 8.1?
>
> I'm using a 7.3.1-based toolchain
>
>>
>> See
>> https://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git/commit/?id=3d7af878357acd9e37fc156928106f1a969c8942
>>
>> and its parent.
>>
>> Do you see -Wvla warnings there? Any idea how I could reproduce them?
>
> I'll try it, thanks
3d7af878357acd9e37f builds ok. However I am using 20181010-next (I'm not
sure what yours is based on), and I just noticed that it includes this
new guy:
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/Makefile?h=next-20181011&id=bc5701d8e27fd8beaf895176982fc6a97878f3b8
When I cherry-pick this onto your codebase@3d7af878357acd9e37f, the
82011 code has warns.
Thanks,
John
>
> John
>
>>
>> johannes
>>
>> .
>>
>
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Question on FIELD_PREP() for static array
2018-10-11 17:26 ` John Garry
@ 2018-10-11 19:13 ` Johannes Berg
2018-10-11 19:22 ` Johannes Berg
0 siblings, 1 reply; 7+ messages in thread
From: Johannes Berg @ 2018-10-11 19:13 UTC (permalink / raw)
To: John Garry, Joe Perches, Andrew Morton, Andy Shevchenko,
Kalle Valo, jakub.kicinski, yamada.masahiro, Arnd Bergmann, viro,
linux-kernel
Cc: linux-wireless, nbd
On Thu, 2018-10-11 at 18:26 +0100, John Garry wrote:
> > > Yeah, I have a patch now to reference it, but I don't see anything from
> > > -Wvla with gcc 8.1?
> >
> > I'm using a 7.3.1-based toolchain
> >
> > >
> > > See
> > > https://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git/commit/?id=3d7af878357acd9e37fc156928106f1a969c8942
> > >
> > > and its parent.
> > >
> > > Do you see -Wvla warnings there? Any idea how I could reproduce them?
> >
> > I'll try it, thanks
>
> 3d7af878357acd9e37f builds ok. However I am using 20181010-next (I'm not
> sure what yours is based on), and I just noticed that it includes this
> new guy:
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/Makefile?h=next-20181011&id=bc5701d8e27fd8beaf895176982fc6a97878f3b
Right, but I added -Wvla by adding subdir-ccflags-y to my Makefile for
this test, and I don't see the warning.
I tried with gcc 7.3.1 too now (Fedora 27) and it doesn't provoke the
warning, even if apply bc5701d8e27fd (manually). I did ensure with V=1
that it shows up on the compiler command line.
This is on x86-64, are you using something else?
Hmm.
However, I have another trick:
#define __NLA_ENSURE(condition) (0 * sizeof(struct { unsigned int x:1 - 2*!(condition);}))
or, in this context,
#define BUILD_BUG_ON_RET_ZERO(cond) (0 * sizeof(struct { unsigned int x:1 - 2*!(condition);}))
What do you think?
johannes
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Question on FIELD_PREP() for static array
2018-10-11 19:13 ` Johannes Berg
@ 2018-10-11 19:22 ` Johannes Berg
0 siblings, 0 replies; 7+ messages in thread
From: Johannes Berg @ 2018-10-11 19:22 UTC (permalink / raw)
To: John Garry, Joe Perches, Andrew Morton, Andy Shevchenko,
Kalle Valo, jakub.kicinski, yamada.masahiro, Arnd Bergmann, viro,
linux-kernel
Cc: linux-wireless, nbd
On Thu, 2018-10-11 at 21:13 +0200, Johannes Berg wrote:
>
> However, I have another trick:
>
> #define __NLA_ENSURE(condition) (0 * sizeof(struct { unsigned int x:1 - 2*!(condition);}))
>
> or, in this context,
>
> #define BUILD_BUG_ON_RET_ZERO(cond) (0 * sizeof(struct { unsigned int x:1 - 2*!(condition);}))
Oops, I forgot to insert the second !, it must be
#define BUILD_BUG_ON_RET_ZERO(cond) (0 * sizeof(struct { unsigned int x:1 - 2*!!(condition);}))
johannes
^ permalink raw reply [flat|nested] 7+ messages in thread
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[not found] ` <40fd3d963820bf96547fa9b5e8c171c6a339674e.camel@perches.com>
2018-10-10 18:13 ` Question on FIELD_PREP() for static array Johannes Berg
2018-10-11 14:24 ` John Garry
2018-10-11 15:23 ` Johannes Berg
2018-10-11 16:16 ` John Garry
2018-10-11 17:26 ` John Garry
2018-10-11 19:13 ` Johannes Berg
2018-10-11 19:22 ` Johannes Berg
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