On Fri, 12 Feb 2016, Arnd Bergmann wrote:
> In my randconfig tests, I came across a bug that involves several
> components:
>
> * gcc-4.9 through at least 5.3
> * CONFIG_GCOV_PROFILE_ALL enabling -fprofile-arcs for all files
> * CONFIG_PROFILE_ALL_BRANCHES overriding every if()
> * The optimized implementation of do_div() that tries to
> replace a library call with an division by multiplication
> * code in drivers/media/dvb-frontends/zl10353.c doing
>
> u32 adc_clock = 450560; /* 45.056 MHz */
> if (state->config.adc_clock)
> adc_clock = state->config.adc_clock;
> do_div(value, adc_clock);
>
> In this case, gcc fails to determine whether the divisor
> in do_div() is __builtin_constant_p(). In particular, it
> concludes that __builtin_constant_p(adc_clock) is false, while
> __builtin_constant_p(!!adc_clock) is true.
>
> That in turn throws off the logic in do_div() that also uses
> __builtin_constant_p(), and instead of picking either the
> constant- optimized division, and the code in ilog2() that uses
> __builtin_constant_p() to figure out whether it knows the answer at
> compile time. The result is a link error from failing to find
> multiple symbols that should never have been called based on
> the __builtin_constant_p():
>
> dvb-frontends/zl10353.c:138: undefined reference to `____ilog2_NaN'
> dvb-frontends/zl10353.c:138: undefined reference to `__aeabi_uldivmod'
> ERROR: "____ilog2_NaN" [drivers/media/dvb-frontends/zl10353.ko] undefined!
> ERROR: "__aeabi_uldivmod" [drivers/media/dvb-frontends/zl10353.ko] undefined!
>
> This patch avoids the problem by changing __trace_if() to check
> whether the condition is known at compile-time to be nonzero, rather
> than checking whether it is actually a constant.
>
> I see this one link error in roughly one out of 1600 randconfig builds
> on ARM, and the patch fixes all known instances.
>
> Signed-off-by: Arnd Bergmann <a...@arndb.de>
> Fixes: ab3c9c686e22 ("branch tracer, intel-iommu: fix build with
> CONFIG_BRANCH_TRACER=y")
It is certainly a worthwhile workaround if it solves all the known
cases.
Acked-by: Nicolas Pitre <n...@linaro.org>
This trick doesn't work with the test case I produced to demonstrate
this bug though. So we might be affected again in the future, or maybe
not if we're lucky.
> ---
> include/linux/compiler.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index b5acbb404854..b5ff9881bef8 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -148,7 +148,7 @@ void ftrace_likely_update(struct ftrace_branch_data *f,
> int val, int expect);
> */
> #define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
> #define __trace_if(cond) \
> - if (__builtin_constant_p((cond)) ? !!(cond) : \
> + if (__builtin_constant_p(!!(cond)) ? !!(cond) : \
> ({ \
> int ______r; \
> static struct ftrace_branch_data \
> --
> 2.7.0
>
>
