[lttng-dev] [PATCH lttng-modules] Fix: BUILD_BUG_ON with compile time constant on < v2.6.38

[Mathieu Desnoyers]

merged thanks!

Mathieu

----- On Jun 7, 2018, at 12:24 PM, Michael Jeanson mjeanson at efficios.com wrote:

> See upstream commits :
> 
>  commit 8c87df457cb58fe75b9b893007917cf8095660a0
>  Author: Jan Beulich <JBeulich at novell.com>
>  Date:   Tue Sep 22 16:43:52 2009 -0700
> 
>    BUILD_BUG_ON(): fix it and a couple of bogus uses of it
> 
>    gcc permitting variable length arrays makes the current construct used for
>    BUILD_BUG_ON() useless, as that doesn't produce any diagnostic if the
>    controlling expression isn't really constant.  Instead, this patch makes
>    it so that a bit field gets used here.  Consequently, those uses where the
>    condition isn't really constant now also need fixing.
> 
>    Note that in the gfp.h, kmemcheck.h, and virtio_config.h cases
>    MAYBE_BUILD_BUG_ON() really just serves documentation purposes - even if
>    the expression is compile time constant (__builtin_constant_p() yields
>    true), the array is still deemed of variable length by gcc, and hence the
>    whole expression doesn't have the intended effect.
> 
>  commit 7ef88ad561457c0346355dfd1f53e503ddfde719
>  Author: Rusty Russell <rusty at rustcorp.com.au>
>  Date:   Mon Jan 24 14:45:10 2011 -0600
> 
>    BUILD_BUG_ON: make it handle more cases
> 
>    BUILD_BUG_ON used to use the optimizer to do code elimination or fail
>    at link time; it was changed to first the size of a negative array (a
>    nicer compile time error), then (in
>    8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
> 
>    This forced us to change some non-constant cases to MAYBE_BUILD_BUG_ON();
>    as Jan points out in that commit, it didn't work as intended anyway.
> 
>    bitfields: needs a literal constant at parse time, and can't be put under
>            "if (__builtin_constant_p(x))" for example.
>    negative array: can handle anything, but if the compiler can't tell it's
>            a constant, silently has no effect.
>    link time: breaks link if the compiler can't determine the value, but the
>            linker output is not usually as informative as a compiler error.
> 
>    If we use the negative-array-size method *and* the link time trick,
>    we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
>    branches, and maximal ability for the compiler to detect errors at
>    build time.
> 
>    We also document it thoroughly.
> 
> Signed-off-by: Michael Jeanson <mjeanson at efficios.com>
> ---
> lib/bug.h | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/bug.h b/lib/bug.h
> index ecd4928..3de40ca 100644
> --- a/lib/bug.h
> +++ b/lib/bug.h
> @@ -21,6 +21,14 @@
>  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>  */
> 
> +#include <linux/version.h>
> +
> +#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,38))
> +#define LTTNG_BUILD_BUG_ON(cond) BUILD_BUG_ON(cond)
> +#else
> +#define LTTNG_BUILD_BUG_ON(cond) MAYBE_BUILD_BUG_ON(cond)
> +#endif
> +
> /**
>  * BUILD_RUNTIME_BUG_ON - check condition at build (if constant) or runtime
>  * @condition: the condition which should be false.
> @@ -33,7 +41,7 @@
> #define BUILD_RUNTIME_BUG_ON(condition)				\
> 	do {							\
> 		if (__builtin_constant_p(condition))		\
> -			BUILD_BUG_ON(condition);		\
> +			LTTNG_BUILD_BUG_ON(condition);		\
> 		else						\
> 			BUG_ON(condition);			\
> 	} while (0)
> --
> 2.17.1

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com