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    Subject[PATCH 3.16.y-ckt 018/142] tracing: Fix freak link error caused by branch tracer
    Date
    3.16.7-ckt26 -stable review patch.  If anyone has any objections, please let me know.

    ---8<------------------------------------------------------------

    From: Arnd Bergmann <arnd@arndb.de>

    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.

    Link: http://lkml.kernel.org/r/1455312410-1058841-1-git-send-email-arnd@arndb.de

    Acked-by: Nicolas Pitre <nico@linaro.org>
    Fixes: ab3c9c686e22 ("branch tracer, intel-iommu: fix build with CONFIG_BRANCH_TRACER=y")
    Signed-off-by: Arnd Bergmann <arnd@arndb.de>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    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 da293bf86575..8f6e7b2d667f 100644
    --- a/include/linux/compiler.h
    +++ b/include/linux/compiler.h
    @@ -138,7 +138,7 @@ 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 \
    \
     
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