Patch "tracing: Fix freak link error caused by branch tracer" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

tracing: Fix freak link error caused by branch tracer

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tracing-fix-freak-link-error-caused-by-branch-tracer.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Arnd Bergmann <[email protected]>
Date: Fri, 12 Feb 2016 22:26:42 +0100
Subject: tracing: Fix freak link error caused by branch tracer

From: Arnd Bergmann <[email protected]>

commit b33c8ff4431a343561e2319f17c14286f2aa52e2 upstream.

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.

---
 include/linux/compiler.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -131,7 +131,7 @@ void ftrace_likely_update(struct ftrace_
  */
 #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			\

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/dma-debug-switch-check-from-_text-to-_stext.patch queue-3.14/tracing-fix-freak-link-error-caused-by-branch-tracer.patch