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build: warn on potentially uninitialized reads

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Enable -Wconditional-uninitialized to warn on potentially uninitialized
reads.

Fix the sole such warning in Bitcoin Core in GetRdRand(): r1 would be
set to 0 on rdrand failure, so initializing it to 0 is a non-functional
change.

From "Intel 64 and IA-32 ArchitecturesSoftware Developer's Manual" [1],
page 1711: "CF=1 indicates that the data in the destination is valid.
Otherwise CF=0 and the data in the destination operand will be returned
as zeros for the specified width."

[1] https://software.intel.com/sites/default/files/managed/39/c5/325462-sdm-vol-1-2abcd-3abcd.pdf
This commit is contained in:
Vasil Dimov 2020-05-01 14:05:48 +02:00
parent ddc0a600b3
commit 71f183a49b
No known key found for this signature in database
GPG key ID: 54DF06F64B55CBBF
3 changed files with 8 additions and 3 deletions
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2
configure.ac
View file

@ -344,6 +344,7 @@ 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
@ -358,6 +359,7 @@ 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]])
dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
dnl unknown options if any other warning is produced. Test the -Wfoo case, and

2
src/Makefile.leveldb.include
View file

@ -36,7 +36,7 @@ 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/port_stdcxx.h

7
src/random.cpp
View file

@ -116,7 +116,10 @@ static uint64_t GetRdRand() noexcept
// RdRand may very rarely fail. Invoke it up to 10 times in a loop to reduce this risk.
#ifdef __i386__
uint8_t ok;
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;
for (int i = 0; i < 10; ++i) {
__asm__ volatile (".byte 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %eax
if (ok) break;
@ -128,7 +131,7 @@ static uint64_t GetRdRand() noexcept
return (((uint64_t)r2) << 32) | r1;
#elif defined(__x86_64__) || defined(__amd64__)
uint8_t ok;
uint64_t r1;
uint64_t r1 = 0; // See above why we initialize to 0.
for (int i = 0; i < 10; ++i) {
__asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %rax
if (ok) break;