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    Subject[PATCH 4.4 20/46] log2: make order_base_2() behave correctly on const input value zero
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Ard Biesheuvel <ard.biesheuvel@linaro.org>

    commit 29905b52fad0854351f57bab867647e4982285bf upstream.

    The function order_base_2() is defined (according to the comment block)
    as returning zero on input zero, but subsequently passes the input into
    roundup_pow_of_two(), which is explicitly undefined for input zero.

    This has gone unnoticed until now, but optimization passes in GCC 7 may
    produce constant folded function instances where a constant value of
    zero is passed into order_base_2(), resulting in link errors against the
    deliberately undefined '____ilog2_NaN'.

    So update order_base_2() to adhere to its own documented interface.

    [ See

    http://marc.info/?l=linux-kernel&m=147672952517795&w=2

    and follow-up discussion for more background. The gcc "optimization
    pass" is really just broken, but now the GCC trunk problem seems to
    have escaped out of just specially built daily images, so we need to
    work around it in mainline. - Linus ]

    Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/linux/log2.h | 13 ++++++++++++-
    1 file changed, 12 insertions(+), 1 deletion(-)

    --- a/include/linux/log2.h
    +++ b/include/linux/log2.h
    @@ -194,6 +194,17 @@ unsigned long __rounddown_pow_of_two(uns
    * ... and so on.
    */

    -#define order_base_2(n) ilog2(roundup_pow_of_two(n))
    +static inline __attribute_const__
    +int __order_base_2(unsigned long n)
    +{
    + return n > 1 ? ilog2(n - 1) + 1 : 0;
    +}

    +#define order_base_2(n) \
    +( \
    + __builtin_constant_p(n) ? ( \
    + ((n) == 0 || (n) == 1) ? 0 : \
    + ilog2((n) - 1) + 1) : \
    + __order_base_2(n) \
    +)
    #endif /* _LINUX_LOG2_H */

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