| Message ID | 20210824022640.2170859-2-nathan@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Harden clang against unknown flag options | expand |
On 2021-08-23, Nathan Chancellor wrote: >clang does not support -falign-jumps and only recently gained support >for -falign-loops. When one of the configuration options that adds these >flags is enabled, clang warns and all cc-{disable-warning,option} that >follow fail because -Werror gets added to test for the presence of this >warning: [I implemented clang -falign-loops :) It doesn't affect LTO, though. LTO ld.lld may use -Wl,-mllvm,-align-loops=32 for now. ] >clang-14: warning: optimization flag '-falign-jumps=0' is not supported >[-Wignored-optimization-argument] grub made a similar mistake:) It thought the availability of -falign-X implies the availability of other -falign-* https://lists.gnu.org/archive/html/grub-devel/2021-08/msg00076.html >To resolve this, add a couple of cc-option calls when building with >clang; gcc has supported these options since 3.2 so there is no point in >testing for their support. -falign-functions was implemented in clang-7, >-falign-loops was implemented in clang-14, and -falign-jumps has not >been implemented yet. > >Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/ >Reported-by: kernel test robot <lkp@intel.com> >Signed-off-by: Nathan Chancellor <nathan@kernel.org> >--- > arch/x86/Makefile_32.cpu | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > >diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu >index cd3056759880..e8c65f990afd 100644 >--- a/arch/x86/Makefile_32.cpu >+++ b/arch/x86/Makefile_32.cpu >@@ -10,6 +10,12 @@ else > tune = $(call cc-option,-mcpu=$(1),$(2)) > endif > >+ifdef CONFIG_CC_IS_CLANG >+align := -falign-functions=0 $(call cc-option,-falign-jumps=0) $(call cc-option,-falign-loops=0) >+else >+align := -falign-functions=0 -falign-jumps=0 -falign-loops=0 >+endif >+ > cflags-$(CONFIG_M486SX) += -march=i486 > cflags-$(CONFIG_M486) += -march=i486 > cflags-$(CONFIG_M586) += -march=i586 >@@ -25,11 +31,11 @@ cflags-$(CONFIG_MK6) += -march=k6 > # They make zero difference whatsosever to performance at this time. > cflags-$(CONFIG_MK7) += -march=athlon > cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8,-march=athlon) >-cflags-$(CONFIG_MCRUSOE) += -march=i686 -falign-functions=0 -falign-jumps=0 -falign-loops=0 >-cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) -falign-functions=0 -falign-jumps=0 -falign-loops=0 >+cflags-$(CONFIG_MCRUSOE) += -march=i686 $(align) >+cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) $(align) > cflags-$(CONFIG_MWINCHIPC6) += $(call cc-option,-march=winchip-c6,-march=i586) > cflags-$(CONFIG_MWINCHIP3D) += $(call cc-option,-march=winchip2,-march=i586) >-cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) -falign-functions=0 -falign-jumps=0 -falign-loops=0 >+cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) $(align) > cflags-$(CONFIG_MVIAC3_2) += $(call cc-option,-march=c3-2,-march=i686) > cflags-$(CONFIG_MVIAC7) += -march=i686 > cflags-$(CONFIG_MCORE2) += -march=i686 $(call tune,core2) >-- >2.33.0 https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html says "If n is not specified or is zero, use a machine-dependent default." Unless some other files specify -falign-loops=N and expect 0 to reset to the machine default, -falign-jumps=0 -falign-loops=0 -falign-functions=0 should just be dropped. BTW: I believe GCC 8 (likely when fixing another issue with a large refactor https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84100) introduced a bug that -falign-X=0 was essentially -falign-X=1. GCC 11.0 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96247) fixed the bug.
On Mon, Aug 23, 2021 at 07:56:47PM -0700, Fangrui Song wrote: > On 2021-08-23, Nathan Chancellor wrote: > > clang does not support -falign-jumps and only recently gained support > > for -falign-loops. When one of the configuration options that adds these > > flags is enabled, clang warns and all cc-{disable-warning,option} that > > follow fail because -Werror gets added to test for the presence of this > > warning: > > [I implemented clang -falign-loops :) It doesn't affect LTO, though. > LTO ld.lld may use -Wl,-mllvm,-align-loops=32 for now. ] > > > clang-14: warning: optimization flag '-falign-jumps=0' is not supported > > [-Wignored-optimization-argument] > > grub made a similar mistake:) It thought the availability of -falign-X > implies the availability of other -falign-* > https://lists.gnu.org/archive/html/grub-devel/2021-08/msg00076.html > > > To resolve this, add a couple of cc-option calls when building with > > clang; gcc has supported these options since 3.2 so there is no point in > > testing for their support. -falign-functions was implemented in clang-7, > > -falign-loops was implemented in clang-14, and -falign-jumps has not > > been implemented yet. > > > > Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/ > > Reported-by: kernel test robot <lkp@intel.com> > > Signed-off-by: Nathan Chancellor <nathan@kernel.org> > > --- > > arch/x86/Makefile_32.cpu | 12 +++++++++--- > > 1 file changed, 9 insertions(+), 3 deletions(-) > > > > diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu > > index cd3056759880..e8c65f990afd 100644 > > --- a/arch/x86/Makefile_32.cpu > > +++ b/arch/x86/Makefile_32.cpu > > @@ -10,6 +10,12 @@ else > > tune = $(call cc-option,-mcpu=$(1),$(2)) > > endif > > > > +ifdef CONFIG_CC_IS_CLANG > > +align := -falign-functions=0 $(call cc-option,-falign-jumps=0) $(call cc-option,-falign-loops=0) > > +else > > +align := -falign-functions=0 -falign-jumps=0 -falign-loops=0 > > +endif > > + > > cflags-$(CONFIG_M486SX) += -march=i486 > > cflags-$(CONFIG_M486) += -march=i486 > > cflags-$(CONFIG_M586) += -march=i586 > > @@ -25,11 +31,11 @@ cflags-$(CONFIG_MK6) += -march=k6 > > # They make zero difference whatsosever to performance at this time. > > cflags-$(CONFIG_MK7) += -march=athlon > > cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8,-march=athlon) > > -cflags-$(CONFIG_MCRUSOE) += -march=i686 -falign-functions=0 -falign-jumps=0 -falign-loops=0 > > -cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) -falign-functions=0 -falign-jumps=0 -falign-loops=0 > > +cflags-$(CONFIG_MCRUSOE) += -march=i686 $(align) > > +cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) $(align) > > cflags-$(CONFIG_MWINCHIPC6) += $(call cc-option,-march=winchip-c6,-march=i586) > > cflags-$(CONFIG_MWINCHIP3D) += $(call cc-option,-march=winchip2,-march=i586) > > -cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) -falign-functions=0 -falign-jumps=0 -falign-loops=0 > > +cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) $(align) > > cflags-$(CONFIG_MVIAC3_2) += $(call cc-option,-march=c3-2,-march=i686) > > cflags-$(CONFIG_MVIAC7) += -march=i686 > > cflags-$(CONFIG_MCORE2) += -march=i686 $(call tune,core2) > > -- > > 2.33.0 > > https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html says > "If n is not specified or is zero, use a machine-dependent default." > > Unless some other files specify -falign-loops=N and expect 0 to reset to > the machine default, -falign-jumps=0 -falign-loops=0 -falign-functions=0 should just be dropped. Grepping the tree, I see: rg "align-(functions|jumps|loops)" Makefile 977:KBUILD_CFLAGS += -falign-functions=64 arch/x86/Makefile 101: KBUILD_CFLAGS += $(call cc-option,-falign-jumps=1) 104: KBUILD_CFLAGS += $(call cc-option,-falign-loops=1) arch/x86/Makefile_32.cpu 28:cflags-$(CONFIG_MCRUSOE) += -march=i686 -falign-functions=0 -falign-jumps=0 -falign-loops=0 29:cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) -falign-functions=0 -falign-jumps=0 -falign-loops=0 32:cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) -falign-functions=0 -falign-jumps=0 -falign-loops=0 arch/ia64/Makefile 26: -falign-functions=32 -frename-registers -fno-optimize-sibling-calls The two cc-options calls in arch/x86/Makefile are for x86_64 only and the Makefile use of -falign-functions=64 is for DEBUG_FORCE_FUNCTION_ALIGN_64B, which is a debug option so it does not seem like the flags are going to get overridden in a normal case. However, I read the GCC docs as if functions are not aligned by default and -falign-functions / -falign-functions=0 aligns them to a machine specific default, so I am not sure if these flags can just be dropped? These flags have been in the tree for 19 years though and there is very little history that I can find around why they are there. https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/tree/arch/i386/Makefile?id=7a2deb32924142696b8174cdf9b38cd72a11fc96 -O2 turns on -falign-{functions,jumps,loops} by default but the kernel can use -Os, which omits those, so it is possible that is why they are there? Some input from the x86 folks might be helpful around this :) > BTW: I believe GCC 8 (likely when fixing another issue with a large refactor > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84100) introduced a bug > that -falign-X=0 was essentially -falign-X=1. > GCC 11.0 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96247) fixed the bug. Cheers, Nathan
On Mon, Aug 23, 2021 at 7:27 PM Nathan Chancellor <nathan@kernel.org> wrote: > > clang does not support -falign-jumps and only recently gained support > for -falign-loops. When one of the configuration options that adds these > flags is enabled, clang warns and all cc-{disable-warning,option} that > follow fail because -Werror gets added to test for the presence of this > warning: > > clang-14: warning: optimization flag '-falign-jumps=0' is not supported > [-Wignored-optimization-argument] > > To resolve this, add a couple of cc-option calls when building with > clang; gcc has supported these options since 3.2 so there is no point in > testing for their support. -falign-functions was implemented in clang-7, > -falign-loops was implemented in clang-14, and -falign-jumps has not > been implemented yet. > > Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/ > Reported-by: kernel test robot <lkp@intel.com> > Signed-off-by: Nathan Chancellor <nathan@kernel.org> Thanks for the patch! Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > --- > arch/x86/Makefile_32.cpu | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu > index cd3056759880..e8c65f990afd 100644 > --- a/arch/x86/Makefile_32.cpu > +++ b/arch/x86/Makefile_32.cpu > @@ -10,6 +10,12 @@ else > tune = $(call cc-option,-mcpu=$(1),$(2)) > endif > > +ifdef CONFIG_CC_IS_CLANG > +align := -falign-functions=0 $(call cc-option,-falign-jumps=0) $(call cc-option,-falign-loops=0) > +else > +align := -falign-functions=0 -falign-jumps=0 -falign-loops=0 > +endif > + > cflags-$(CONFIG_M486SX) += -march=i486 > cflags-$(CONFIG_M486) += -march=i486 > cflags-$(CONFIG_M586) += -march=i586 > @@ -25,11 +31,11 @@ cflags-$(CONFIG_MK6) += -march=k6 > # They make zero difference whatsosever to performance at this time. > cflags-$(CONFIG_MK7) += -march=athlon > cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8,-march=athlon) > -cflags-$(CONFIG_MCRUSOE) += -march=i686 -falign-functions=0 -falign-jumps=0 -falign-loops=0 > -cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) -falign-functions=0 -falign-jumps=0 -falign-loops=0 > +cflags-$(CONFIG_MCRUSOE) += -march=i686 $(align) > +cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) $(align) > cflags-$(CONFIG_MWINCHIPC6) += $(call cc-option,-march=winchip-c6,-march=i586) > cflags-$(CONFIG_MWINCHIP3D) += $(call cc-option,-march=winchip2,-march=i586) > -cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) -falign-functions=0 -falign-jumps=0 -falign-loops=0 > +cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) $(align) > cflags-$(CONFIG_MVIAC3_2) += $(call cc-option,-march=c3-2,-march=i686) > cflags-$(CONFIG_MVIAC7) += -march=i686 > cflags-$(CONFIG_MCORE2) += -march=i686 $(call tune,core2) > -- > 2.33.0 >
On Mon, Aug 23, 2021 at 07:26:39PM -0700, Nathan Chancellor wrote: A couple of nitpicks: > Subject: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang Make that prefix into "x86/build: " > clang does not support -falign-jumps and only recently gained support > for -falign-loops. When one of the configuration options that adds these > flags is enabled, clang warns and all cc-{disable-warning,option} that > follow fail because -Werror gets added to test for the presence of this > warning: > > clang-14: warning: optimization flag '-falign-jumps=0' is not supported > [-Wignored-optimization-argument] > > To resolve this, add a couple of cc-option calls when building with > clang; gcc has supported these options since 3.2 so there is no point in > testing for their support. -falign-functions was implemented in clang-7, > -falign-loops was implemented in clang-14, and -falign-jumps has not > been implemented yet. > > Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/ Also, there should be a second Link: tag which points to this mail thread so that we can find it later, when we dig for the "why we did that" question :) I.e., Link: 20210824022640.2170859-2-nathan@kernel.org > Reported-by: kernel test robot <lkp@intel.com> > Signed-off-by: Nathan Chancellor <nathan@kernel.org> > --- > arch/x86/Makefile_32.cpu | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) with that: Acked-by: Borislav Petkov <bp@suse.de> Thx.
On 9/16/2021 10:18 AM, Borislav Petkov wrote: > On Mon, Aug 23, 2021 at 07:26:39PM -0700, Nathan Chancellor wrote: > > A couple of nitpicks: > >> Subject: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang > > Make that prefix into "x86/build: " Done, I'll be sure to keep that prefix in mind for future flag-based changes. >> clang does not support -falign-jumps and only recently gained support >> for -falign-loops. When one of the configuration options that adds these >> flags is enabled, clang warns and all cc-{disable-warning,option} that >> follow fail because -Werror gets added to test for the presence of this >> warning: >> >> clang-14: warning: optimization flag '-falign-jumps=0' is not supported >> [-Wignored-optimization-argument] >> >> To resolve this, add a couple of cc-option calls when building with >> clang; gcc has supported these options since 3.2 so there is no point in >> testing for their support. -falign-functions was implemented in clang-7, >> -falign-loops was implemented in clang-14, and -falign-jumps has not >> been implemented yet. >> >> Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/ > > Also, there should be a second Link: tag which points to this mail > thread so that we can find it later, when we dig for the "why we did > that" question :) > > I.e., > > Link: 20210824022640.2170859-2-nathan@kernel.org Sure thing, kind of hard to do that on the initial submission but I will do it for the v2 shortly :) >> Reported-by: kernel test robot <lkp@intel.com> >> Signed-off-by: Nathan Chancellor <nathan@kernel.org> >> --- >> arch/x86/Makefile_32.cpu | 12 +++++++++--- >> 1 file changed, 9 insertions(+), 3 deletions(-) > > with that: > > Acked-by: Borislav Petkov <bp@suse.de> Thank you for the ack. The conflicting changes that I mentioned in the cover letter have been merged in 5.15-rc1 so if you guys want to take these changes via -tip, just holler for an ack from Masahiro on the second patch on v2 (but I am going with the assumption this will be merged via the kbuild tree). Cheers, Nathan
On Thu, Sep 16, 2021 at 11:42:19AM -0700, Nathan Chancellor wrote: > Done, I'll be sure to keep that prefix in mind for future flag-based > changes. Yeah, what you could do in the future is git log <filename> and see the previous prefixes. But not that important - we fix those usually before applying. > Sure thing, kind of hard to do that on the initial submission but I will do > it for the v2 shortly :) Haha, very hard. :-) > Thank you for the ack. The conflicting changes that I mentioned in the cover > letter have been merged in 5.15-rc1 so if you guys want to take these > changes via -tip, just holler for an ack from Masahiro on the second patch > on v2 (but I am going with the assumption this will be merged via the kbuild > tree). I'm fine either way. So whatever Masahiro prefers. Thx.
diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu index cd3056759880..e8c65f990afd 100644 --- a/arch/x86/Makefile_32.cpu +++ b/arch/x86/Makefile_32.cpu @@ -10,6 +10,12 @@ else tune = $(call cc-option,-mcpu=$(1),$(2)) endif +ifdef CONFIG_CC_IS_CLANG +align := -falign-functions=0 $(call cc-option,-falign-jumps=0) $(call cc-option,-falign-loops=0) +else +align := -falign-functions=0 -falign-jumps=0 -falign-loops=0 +endif + cflags-$(CONFIG_M486SX) += -march=i486 cflags-$(CONFIG_M486) += -march=i486 cflags-$(CONFIG_M586) += -march=i586 @@ -25,11 +31,11 @@ cflags-$(CONFIG_MK6) += -march=k6 # They make zero difference whatsosever to performance at this time. cflags-$(CONFIG_MK7) += -march=athlon cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8,-march=athlon) -cflags-$(CONFIG_MCRUSOE) += -march=i686 -falign-functions=0 -falign-jumps=0 -falign-loops=0 -cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) -falign-functions=0 -falign-jumps=0 -falign-loops=0 +cflags-$(CONFIG_MCRUSOE) += -march=i686 $(align) +cflags-$(CONFIG_MEFFICEON) += -march=i686 $(call tune,pentium3) $(align) cflags-$(CONFIG_MWINCHIPC6) += $(call cc-option,-march=winchip-c6,-march=i586) cflags-$(CONFIG_MWINCHIP3D) += $(call cc-option,-march=winchip2,-march=i586) -cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) -falign-functions=0 -falign-jumps=0 -falign-loops=0 +cflags-$(CONFIG_MCYRIXIII) += $(call cc-option,-march=c3,-march=i486) $(align) cflags-$(CONFIG_MVIAC3_2) += $(call cc-option,-march=c3-2,-march=i686) cflags-$(CONFIG_MVIAC7) += -march=i686 cflags-$(CONFIG_MCORE2) += -march=i686 $(call tune,core2)
clang does not support -falign-jumps and only recently gained support for -falign-loops. When one of the configuration options that adds these flags is enabled, clang warns and all cc-{disable-warning,option} that follow fail because -Werror gets added to test for the presence of this warning: clang-14: warning: optimization flag '-falign-jumps=0' is not supported [-Wignored-optimization-argument] To resolve this, add a couple of cc-option calls when building with clang; gcc has supported these options since 3.2 so there is no point in testing for their support. -falign-functions was implemented in clang-7, -falign-loops was implemented in clang-14, and -falign-jumps has not been implemented yet. Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/ Reported-by: kernel test robot <lkp@intel.com> Signed-off-by: Nathan Chancellor <nathan@kernel.org> --- arch/x86/Makefile_32.cpu | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)