From 638fdf25fd25baead020c9f578193f075f931c51 Mon Sep 17 00:00:00 2001
From: Kittywhiskers Van Gogh <63189531+kittywhiskers@users.noreply.github.com>
Date: Fri, 1 May 2020 14:05:48 +0200
Subject: [PATCH] bitcoin#18843: warn on potentially uninitialized reads

---
 configure.ac                 | 2 ++
 src/Makefile.leveldb.include | 2 +-
 src/random.cpp               | 7 +++++--
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/configure.ac b/configure.ac
index 8887a423fd..579e9770b9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -367,6 +367,7 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=unused-variable],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unused-variable"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=date-time],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=date-time"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=return-type],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=return-type"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Werror=conditional-uninitialized],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=conditional-uninitialized"],,[[$CXXFLAG_WERROR]])
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then
@@ -381,6 +382,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wredundant-decls],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wunused-variable],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunused-variable"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wdate-time],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdate-time"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wconditional-uninitialized],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wconditional-uninitialized"],,[[$CXXFLAG_WERROR]])
 
   ## Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   ## unknown options if any other warning is produced. Test the -Wfoo case, and
diff --git a/src/Makefile.leveldb.include b/src/Makefile.leveldb.include
index 833f3d2a10..f4828361fd 100644
--- a/src/Makefile.leveldb.include
+++ b/src/Makefile.leveldb.include
@@ -30,7 +30,7 @@ LEVELDB_CPPFLAGS_INT += -DLEVELDB_PLATFORM_POSIX
 endif
 
 leveldb_libleveldb_a_CPPFLAGS = $(AM_CPPFLAGS) $(LEVELDB_CPPFLAGS_INT) $(LEVELDB_CPPFLAGS)
-leveldb_libleveldb_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
+leveldb_libleveldb_a_CXXFLAGS = $(filter-out -Wconditional-uninitialized -Werror=conditional-uninitialized, $(AM_CXXFLAGS)) $(PIE_FLAGS)
 
 leveldb_libleveldb_a_SOURCES=
 leveldb_libleveldb_a_SOURCES += leveldb/port/atomic_pointer.h
diff --git a/src/random.cpp b/src/random.cpp
index c2c1a863bd..61c21f040f 100644
--- a/src/random.cpp
+++ b/src/random.cpp
@@ -100,7 +100,10 @@ static bool GetHWRand(unsigned char* ent32) {
         // Not all assemblers support the rdrand instruction, write it in hex.
 #ifdef __i386__
         for (int iter = 0; iter < 4; ++iter) {
-            uint32_t r1, r2;
+            // Initialize to 0 to silence a compiler warning that r1 or r2 may be used
+            // uninitialized. Even if rdrand fails (!ok) it will set the output to 0,
+            // but there is no way that the compiler could know that.
+            uint32_t r1 = 0, r2 = 0;
             __asm__ volatile (".byte 0x0f, 0xc7, 0xf0;" // rdrand %eax
                               ".byte 0x0f, 0xc7, 0xf2;" // rdrand %edx
                               "setc %2" :
@@ -110,7 +113,7 @@ static bool GetHWRand(unsigned char* ent32) {
             WriteLE32(ent32 + 8 * iter + 4, r2);
         }
 #else
-        uint64_t r1, r2, r3, r4;
+        uint64_t r1 = 0, r2 = 0, r3 = 0, r4 = 0; // See above why we initialize to 0.
         __asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf0, " // rdrand %rax
                                 "0x48, 0x0f, 0xc7, 0xf3, " // rdrand %rbx
                                 "0x48, 0x0f, 0xc7, 0xf1, " // rdrand %rcx