From 0d49e47978d5f2d0b108b0ddc65943763e88f729 Mon Sep 17 00:00:00 2001
From: "William A. Kennington III" <wak@google.com>
Date: Mon, 8 Apr 2019 20:27:26 -0700
Subject: message/unpack: Fix undefined behavior

It's UB to shift an integer by the size of that integer. More
specifically, if you disable compiler optimization and try and unpack a
32 bit bitset you will end up with a 0x0 mask. Avoid UB by replacing
shift subtract with a negate shift.

Tested:
    Unit tests pass now.

Change-Id: I03a6f866a51c955b57787d641da9180841747e4c
Signed-off-by: William A. Kennington III <wak@google.com>
---
 include/ipmid/message/unpack.hpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/ipmid/message/unpack.hpp b/include/ipmid/message/unpack.hpp
index d96928f..94f80f1 100644
--- a/include/ipmid/message/unpack.hpp
+++ b/include/ipmid/message/unpack.hpp
@@ -207,7 +207,9 @@ struct UnpackSingle<std::bitset<N>>
         {
             return -1;
         }
-        fixed_uint_t<details::bitStreamSize> bitmask = ((1 << count) - 1);
+        fixed_uint_t<details::bitStreamSize> bitmask =
+            ~fixed_uint_t<details::bitStreamSize>(0) >>
+            (details::bitStreamSize - count);
         t |= (p.bitStream & bitmask).convert_to<unsigned long long>();
         p.bitStream >>= count;
         p.bitCount -= count;
-- 
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