The patch titled
Subject: log2: make order_base_2() behave correctly on const input value zero
has been added to the -mm tree. Its filename is
log2-make-order_base_2-behave-correctly-on-const-input-value-zero.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/log2-make-order_base_2-behave-correctly-on-const-input-value-zero.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/log2-make-order_base_2-behave-correctly-on-const-input-value-zero.patch
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From: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Subject: log2: make order_base_2() behave correctly on const input value zero
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.
Link: http://lkml.kernel.org/r/1486058726-7262-1-git-send-email-ard.biesheuvel@xxxxxxxxxx
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Laura Abbott <labbott@xxxxxxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
Cc: Gregory CLEMENT <gregory.clement@xxxxxxxxxxxxxxxxxx>
Cc: James Greenlaigh <james.greenhalgh@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/log2.h | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff -puN include/linux/log2.h~log2-make-order_base_2-behave-correctly-on-const-input-value-zero include/linux/log2.h
--- a/include/linux/log2.h~log2-make-order_base_2-behave-correctly-on-const-input-value-zero
+++ a/include/linux/log2.h
@@ -203,6 +203,17 @@ 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 */
_
Patches currently in -mm which might be from ard.biesheuvel@xxxxxxxxxx are
log2-make-order_base_2-behave-correctly-on-const-input-value-zero.patch
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