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[PATCH 4.9 034/108] log2: make order_base_2() behave correctly on const input value zero

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4.9-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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[PATCH 4.9 000/108] 4.9.33-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 021/108] i2c: piix4: Request the SMBUS semaphore inside the mutex Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 006/108] ibmvnic: Fix endian error when requesting device capabilities Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 034/108] log2: make order_base_2() behave correctly on const input value zero Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 010/108] ipv6: Handle IPv4-mapped src to in6addr_any dst. Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 031/108] jump label: pass kbuild_cflags when checking for asm goto support Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
    Re: [PATCH 4.9 031/108] jump label: pass kbuild_cflags when checking  for asm goto support Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org> - 2017-06-19 17:30 +0200
  [PATCH 4.9 007/108] net: xilinx_emaclite: fix freezes due to unordered I/O Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 016/108] ibmvnic: Initialize completion variables before starting work Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 017/108] NET: mkiss: Fix panic Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 004/108] netfilter: nf_conntrack_sip: fix wrong memory initialisation Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 040/108] fscache: Clear outstanding writes when disabling a cookie Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:30 +0200
  [PATCH 4.9 066/108] nfs: Fix "Dont increment lock sequence ID after NFS4ERR_MOVED" Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-15 20:40 +0200
  Re: [PATCH 4.9 000/108] 4.9.33-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2017-06-16 00:30 +0200
  Re: [PATCH 4.9 000/108] 4.9.33-stable review Guenter Roeck <linux@roeck-us.net> - 2017-06-16 02:50 +0200

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