@@ -370,7 +370,7 @@ LDFLAGS_MODULE =
CFLAGS_KERNEL =
AFLAGS_KERNEL =
LDFLAGS_vmlinux =
-CFLAGS_GCOV = -fprofile-arcs -ftest-coverage -fno-tree-loop-im
+CFLAGS_GCOV = -fprofile-arcs -ftest-coverage -fno-tree-loop-im -Wno-maybe-uninitialized
CFLAGS_KCOV := $(call cc-option,-fsanitize-coverage=trace-pc,)
@@ -620,7 +620,6 @@ ARCH_CFLAGS :=
include arch/$(SRCARCH)/Makefile
KBUILD_CFLAGS += $(call cc-option,-fno-delete-null-pointer-checks,)
-KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,)
KBUILD_CFLAGS += $(call cc-disable-warning,frame-address,)
ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
@@ -629,15 +628,18 @@ KBUILD_CFLAGS += $(call cc-option,-fdata-sections,)
endif
ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
-KBUILD_CFLAGS += -Os
+KBUILD_CFLAGS += -Os $(call cc-disable-warning,maybe-uninitialized,)
else
ifdef CONFIG_PROFILE_ALL_BRANCHES
-KBUILD_CFLAGS += -O2
+KBUILD_CFLAGS += -O2 $(call cc-disable-warning,maybe-uninitialized,)
else
KBUILD_CFLAGS += -O2
endif
endif
+KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \
+ $(call cc-disable-warning,maybe-uninitialized,))
+
# Tell gcc to never replace conditional load with a non-conditional one
KBUILD_CFLAGS += $(call cc-option,--param=allow-store-data-races=0)
@@ -71,7 +71,9 @@ cflags-$(CONFIG_ARC_DW2_UNWIND) += -fasynchronous-unwind-tables $(cfi)
ifndef CONFIG_CC_OPTIMIZE_FOR_SIZE
# Generic build system uses -O2, we want -O3
# Note: No need to add to cflags-y as that happens anyways
-ARCH_CFLAGS += -O3
+#
+# Disable the false maybe-uninitialized warings gcc spits out at -O3
+ARCH_CFLAGS += -O3 $(call cc-disable-warning,maybe-uninitialized,)
endif
# small data is default for elf32 tool-chain. If not usable, disable it
@@ -17,4 +17,8 @@ endif
ifdef CONFIG_UBSAN_NULL
CFLAGS_UBSAN += $(call cc-option, -fsanitize=null)
endif
+
+ # -fsanitize=* options makes GCC less smart than usual and
+ # increase number of 'maybe-uninitialized false-positives
+ CFLAGS_UBSAN += $(call cc-option, -Wno-maybe-uninitialized)
endif
Traditionally, we have always had warnings about uninitialized variables enabled, as this is part of -Wall, and generally a good idea [1], but it also always produced false positives, mainly because this is a variation of the halting problem and provably impossible to get right in all cases [2]. Various people have identified cases that are particularly bad for false positives, and in commit e74fc973b6e5 ("Turn off -Wmaybe-uninitialized when building with -Os"), I turned off the warning for any build that was done with CC_OPTIMIZE_FOR_SIZE. This drastically reduced the number of false positive warnings in the default build but unfortunately had the side effect of turning the warning off completely in 'allmodconfig' builds, which in turn led to a lot of warnings (both actual bugs, and remaining false positives) to go in unnoticed. With commit 877417e6ffb9 ("Kbuild: change CC_OPTIMIZE_FOR_SIZE definition") enabled the warning again for allmodconfig builds in v4.7 and in v4.8-rc1, I had finally managed to address all warnings I get in an ARM allmodconfig build and most other maybe-uninitialized warnings for ARM randconfig builds. However, commit 6e8d666e9253 ("Disable "maybe-uninitialized" warning globally") was merged at the same time and disabled it completely for all configurations, because of false-positive warnings on x86 that I had not addressed until then. This caused a lot of actual bugs to get merged into mainline, and I sent several dozen patches for these during the v4.9 development cycle. Most of these are actual bugs, some are for correct code that is safe because it is only called under external constraints that make it impossible to run into the case that gcc sees, and in a few cases gcc is just stupid and finds something that can obviously never happen. I have now done a few thousand randconfig builds on x86 and collected all patches that I needed to address every single warning I got (I can provide the combined patch for the other warnings if anyone is interested), so I hope we can get the warning back and let people catch the actual bugs earlier. Note that the majority of the patches I created are for the third kind of problem (stupid false-positives), for one of two reasons: - some of them only get triggered in certain combinations of config options, so we don't always run into them, and - the actual bugs tend to get addressed much quicker as they also lead to incorrect runtime behavior. These 27 patches address the warnings that either occur in one of the more common configurations (defconfig, allmodconfig, or something built by the kbuild robot or kernelci.org), or they are about a real bug. It would be good to get these all into v4.9 if we want to turn on the warning again. I have tested these extensively with gcc-4.9 and gcc-6 and done a bit of testing with gcc-5, and all of these should now be fine. gcc-4.8 is much worse about the false-positive warnings and is also fairly old now, so I'm leaving the warning disabled with that version. gcc-4.7 and older don't understand the -Wno-maybe-uninitialized option and are not affected by this patch either way. I have another (smaller) series of patches for warnings that are both harmless and not as easy to trigger, and I will send them for inclusion in v4.10. Link: https://rusty.ozlabs.org/?p=232 [1] Link: https://gcc.gnu.org/wiki/Better_Uninitialized_Warnings [2] Signed-off-by: Arnd Bergmann <arnd@arndb.de> --- Makefile | 10 ++++++---- arch/arc/Makefile | 4 +++- scripts/Makefile.ubsan | 4 ++++ 3 files changed, 13 insertions(+), 5 deletions(-) Cc: x86@kernel.org Cc: linux-media@vger.kernel.org Cc: Mauro Carvalho Chehab <mchehab@kernel.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: linux-s390@vger.kernel.org Cc: Ilya Dryomov <idryomov@gmail.com> Cc: dri-devel@lists.freedesktop.org Cc: linux-mtd@lists.infradead.org Cc: Herbert Xu <herbert@gondor.apana.org.au> Cc: linux-crypto@vger.kernel.org Cc: "David S. Miller" <davem@davemloft.net> Cc: netdev@vger.kernel.org Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: ceph-devel@vger.kernel.org Cc: linux-f2fs-devel@lists.sourceforge.net Cc: linux-ext4@vger.kernel.org Cc: netfilter-devel@vger.kernel.org