* [PATCH 05/28] ext2: avoid bogus -Wmaybe-uninitialized warning
2016-10-17 22:03 [PATCH 00/28] Reenable maybe-uninitialized warnings Arnd Bergmann
@ 2016-10-17 22:05 ` Arnd Bergmann
2016-10-18 5:15 ` Christoph Hellwig
2016-10-17 22:19 ` [PATCH 28/28] Kbuild: bring back " Arnd Bergmann
2016-10-18 5:08 ` [PATCH 00/28] Reenable maybe-uninitialized warnings Christoph Hellwig
2 siblings, 1 reply; 6+ messages in thread
From: Arnd Bergmann @ 2016-10-17 22:05 UTC (permalink / raw)
To: Jan Kara
Cc: Linus Torvalds, linux-kernel, Arnd Bergmann, Christoph Hellwig,
Dave Chinner, Ross Zwisler, Dave Chinner, Al Viro, Andrew Morton,
Matthew Wilcox, Carlos Maiolino, linux-ext4
On ARM, we get this false-positive warning since the rework of
the ext2_get_blocks interface:
fs/ext2/inode.c: In function 'ext2_get_block':
include/linux/buffer_head.h:340:16: error: 'bno' may be used uninitialized in this function [-Werror=maybe-uninitialized]
The calling conventions for this function are rather complex, and it's
not surprising that the compiler gets this wrong, I spent a long time
trying to understand how it all fits together myself.
This change to avoid the warning makes sure the compiler sees that we
always set 'bno' pointer whenever we have a positive return code.
The transformation is correct because we always arrive at the 'got_it'
label with a positive count that gets used as the return value, while
any branch to the 'cleanup' label has a negative or zero 'err'.
Fixes: 6750ad71986d ("ext2: stop passing buffer_head to ext2_get_blocks")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <david@fromorbit.com>
---
fs/ext2/inode.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
index d831e24..41b8b44 100644
--- a/fs/ext2/inode.c
+++ b/fs/ext2/inode.c
@@ -622,7 +622,7 @@ static int ext2_get_blocks(struct inode *inode,
u32 *bno, bool *new, bool *boundary,
int create)
{
- int err = -EIO;
+ int err;
int offsets[4];
Indirect chain[4];
Indirect *partial;
@@ -639,7 +639,7 @@ static int ext2_get_blocks(struct inode *inode,
depth = ext2_block_to_path(inode,iblock,offsets,&blocks_to_boundary);
if (depth == 0)
- return (err);
+ return -EIO;
partial = ext2_get_branch(inode, depth, offsets, chain, &err);
/* Simplest case - block found, no allocation needed */
@@ -761,7 +761,6 @@ static int ext2_get_blocks(struct inode *inode,
ext2_splice_branch(inode, iblock, partial, indirect_blks, count);
mutex_unlock(&ei->truncate_mutex);
got_it:
- *bno = le32_to_cpu(chain[depth-1].key);
if (count > blocks_to_boundary)
*boundary = true;
err = count;
@@ -772,6 +771,8 @@ static int ext2_get_blocks(struct inode *inode,
brelse(partial->bh);
partial--;
}
+ if (err > 0)
+ *bno = le32_to_cpu(chain[depth-1].key);
return err;
}
--
2.9.0
^ permalink raw reply related [flat|nested] 6+ messages in thread* [PATCH 28/28] Kbuild: bring back -Wmaybe-uninitialized warning
2016-10-17 22:03 [PATCH 00/28] Reenable maybe-uninitialized warnings Arnd Bergmann
2016-10-17 22:05 ` [PATCH 05/28] ext2: avoid bogus -Wmaybe-uninitialized warning Arnd Bergmann
@ 2016-10-17 22:19 ` Arnd Bergmann
2016-10-18 5:08 ` [PATCH 00/28] Reenable maybe-uninitialized warnings Christoph Hellwig
2 siblings, 0 replies; 6+ messages in thread
From: Arnd Bergmann @ 2016-10-17 22:19 UTC (permalink / raw)
To: Linus Torvalds, Michal Marek
Cc: Nicolas Pitre, Greg Kroah-Hartman, Heiko Carstens, dri-devel,
linux-mtd, Ingo Molnar, linux-s390, Herbert Xu, x86,
Christian Borntraeger, Ilya Dryomov, linux-ext4, linux-media,
Kees Cook, Arnd Bergmann, linux-kbuild, Josh Poimboeuf,
ceph-devel, Mauro Carvalho Chehab, linux-snps-arc, netdev,
linux-kernel, linux-f2fs-devel, netfilter-devel, linux-crypto,
Vineet Gupta
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
diff --git a/Makefile b/Makefile
index 512e47a..43cd3d9 100644
--- a/Makefile
+++ b/Makefile
@@ -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 += $(call cc-option,--param=allow-store-data-races=0)
diff --git a/arch/arc/Makefile b/arch/arc/Makefile
index aa82d13..19cce22 100644
--- a/arch/arc/Makefile
+++ b/arch/arc/Makefile
@@ -71,7 +71,9 @@ cflags-$(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
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index dd779c4..3b1b138 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -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
--
2.9.0
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH 00/28] Reenable maybe-uninitialized warnings
2016-10-17 22:03 [PATCH 00/28] Reenable maybe-uninitialized warnings Arnd Bergmann
2016-10-17 22:05 ` [PATCH 05/28] ext2: avoid bogus -Wmaybe-uninitialized warning Arnd Bergmann
2016-10-17 22:19 ` [PATCH 28/28] Kbuild: bring back " Arnd Bergmann
@ 2016-10-18 5:08 ` Christoph Hellwig
2 siblings, 0 replies; 6+ messages in thread
From: Christoph Hellwig @ 2016-10-18 5:08 UTC (permalink / raw)
To: Arnd Bergmann
Cc: Linus Torvalds, linux-kernel, x86, linux-media,
Mauro Carvalho Chehab, Martin Schwidefsky, linux-s390,
Ilya Dryomov, dri-devel, linux-mtd, Herbert Xu, linux-crypto,
David S. Miller, netdev, Greg Kroah-Hartman, ceph-devel,
linux-f2fs-devel, linux-ext4, netfilter-devel
On Tue, Oct 18, 2016 at 12:03:28AM +0200, Arnd Bergmann wrote:
> This is a set of patches that I hope to get into v4.9 in some form
> in order to turn on the -Wmaybe-uninitialized warnings again.
Hi Arnd,
I jsut complained to Geert that I was introducing way to many
bugs or pointless warnings for some compilers lately, but gcc didn't
warn me about them. From a little research the lack of
-Wmaybe-uninitialized seems to be the reason for it, so I'm all
for re-enabling it.
^ permalink raw reply [flat|nested] 6+ messages in thread