On Fri, Jan 21, 2022 at 12:20:39PM +0100, Amadeusz Sławiński wrote: > On 1/20/2022 12:08 AM, Nathan Chancellor wrote: > > On Wed, Jan 19, 2022 at 02:19:39PM -0700, Nathan Chancellor wrote: > > > On Wed, Jan 19, 2022 at 02:51:47PM +0100, Amadeusz Sławiński wrote: > > > > When building kernel with scan-build for analysis: > > > > $ scan-build make defconfig > > > > $ scan-build make menuconfig # disable RETPOLINE > > > > $ scan-build make -j16 bindeb-pkg > > > > since commit 7d73c3e9c514 ("Makefile: remove stale cc-option checks") > > > > it fails with: > > > > CC scripts/mod/empty.o > > > > could not find clang line > > > > make[4]: *** [scripts/Makefile.build:287: scripts/mod/empty.o] Error 1 > > > > > > > > Seems like changes to how -fconserve-stack support was detected broke > > > > build with scan-build. Revert part of mentioned commit which changed > > > > that. > > > > > > > > Fixes: 7d73c3e9c514 ("Makefile: remove stale cc-option checks") > > > > CC: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > > > > Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@xxxxxxxxxxxxxxx> > > > > Reviewed-by: Cezary Rojewski <cezary.rojewski@xxxxxxxxx> > > > > --- > > > > Makefile | 4 +--- > > > > 1 file changed, 1 insertion(+), 3 deletions(-) > > > > > > > > diff --git a/Makefile b/Makefile > > > > index 765115c99655..1174ccd182f5 100644 > > > > --- a/Makefile > > > > +++ b/Makefile > > > > @@ -991,9 +991,7 @@ KBUILD_CFLAGS += -fno-strict-overflow > > > > KBUILD_CFLAGS += -fno-stack-check > > > > # conserve stack if available > > > > -ifdef CONFIG_CC_IS_GCC > > > > -KBUILD_CFLAGS += -fconserve-stack > > > > -endif > > > > +KBUILD_CFLAGS += $(call cc-option,-fconserve-stack) > > > > # Prohibit date/time macros, which would make the build non-deterministic > > > > KBUILD_CFLAGS += -Werror=date-time > > > > -- > > > > 2.25.1 > > > > > > > > > > Okay, I think I understand why this happens... > > > > > > scan-build points CC to its CC wrapper [1], ccc-analyzer, which builds the > > > code with a compiler [2] then runs clang for the static analyzer [3]. > > > The problem is that the default compiler for ccc-analyzer is GCC, which > > > means that CONFIG_CC_IS_GCC gets set and flags that are supported by GCC > > > but not clang will cause the clang analyzer part of ccc-analyzer to > > > error because ccc-analyzer just passes all '-f' flags along [4]. > > > > > > Prior to 7d73c3e9c514, there was no error because cc-option would run > > > the flag against ccc-analyzer, which would error out for the reason I > > > just described, which would prevent the flag from getting added to > > > KBUILD_CFLAGS. > > > > > > Now, -fconserve-stack gets passed along to both gcc and clang but clang > > > does not recognize it and errors out. > > > > > > This should be fixed in clang, which already has the machinery to > > > recognize but ignore GCC flags for compatibility reasons (which is > > > probably how gcc and clang can use the same flags). I have pushed a > > > patch to Phabricator for review: > > > > > > https://reviews.llvm.org/D117717 > > > > > > You need to disable CONFIG_RETPOLINE for the same reason but I don't > > > think working around that in clang is as simple. > > > > > > Until that fix can proliferate through distributions and such, this is > > > not an unreasonable workaround (unless Masahiro or Nick have a better > > > idea) but I would really like a comment so that we can revert this once > > > that fix is more widely available (it is unlikely that clang will > > > actually support this option). > > > > > > [1]: https://github.com/llvm/llvm-project/blob/3062a1469da0569e714aa4634b29345f6d8c874c/clang/tools/scan-build/bin/scan-build#L1080 > > > [2]: https://github.com/llvm/llvm-project/blob/fd0782a37bbf7dd4ece721df92c703a381595661/clang/tools/scan-build/libexec/ccc-analyzer#L457 > > > [3]: https://github.com/llvm/llvm-project/blob/fd0782a37bbf7dd4ece721df92c703a381595661/clang/tools/scan-build/libexec/ccc-analyzer#L783 > > > [4]: https://github.com/llvm/llvm-project/blob/fd0782a37bbf7dd4ece721df92c703a381595661/clang/tools/scan-build/libexec/ccc-analyzer#L661-L665 > > > > Thinking more about this after Fangrui commented on the clang patch > > above, using scan-build with GCC as the compiler is going to be hard to > > support, as we are basically trying to support using two different > > compilers with a unified set of '-f' flags, which I see as problematic > > for a few reasons. > > > > 1. It restricts our ability to do cc-option cleanups like Nick did. > > > > We should be eliminating cc-option calls that we know are specific to > > one compiler because checking the Kconfig variables (CONFIG_CC_IS_...) > > is much cheaper than invoking the compiler. > > > > 2. Necessary GCC specific flags will get dropped. > > > > Adding back the call to cc-option will allow the build to succeed but it > > drops the flag from KBUILD_CFLAGS. If there were ever a time where an > > '-f' flag was needed to get a working kernel with GCC, it would not get > > added because clang would reject it. > > > > We already have a static-analyzer target that requires using CC=clang so > > I think there is some precedent here to say we require the kernel to be > > built with clang to use the static analyzer. The fact that it did prior > > to 7d73c3e9c514 can just be chalked up to luck. > > > > $ make -j"$(nproc)" LLVM=1 defconfig bindeb-pkg static-analyzer > > > > would be the equivalent command to the original patch. > > > > You can still use scan-build with the '--use-cc=clang' flag, which will > > use clang for the compilation and analysis, if you so prefer. > > > > Masahiro and Nick may have further thoughts and I am open to other > > opinions but my vote is to say this is an issue we won't fix or > > workaround. > > > > Cheers, > > Nathan > > > Thank you for detailed explanation. Well I guess question then is: how much > scan-build is supported? And if it should even support mixing clang and gcc? > Alternatively maybe use clang as default if CC environment variable is not > set? It probably shouldn't, as least not in the way that it currently does. Someone on the LLVM review I created suggested it should add a filter for flags that clang does not support from GCC. I think changing the default would be another good fix but doesn't fix the issue if someone does actually wants to use GCC for building. > What I like about scan-build is that it generates html report file. Ah, that is a good point. > '--use-cc=clang' worked fine for me. > > I've also tried > > $ make -j"$(nproc)" LLVM=1 defconfig bindeb-pkg static-analyzer > although there seems to be no static-analyzer target, I guess you meant > clang-analyzer instead, but although it seems to generate a lot of text on > terminal, it doesn't seem that useful to me. Not sure if this is expected? Yes, my apologies, it should have been clang-analyzer. > Quoting a piece of log: > ./include/linux/xarray.h:54:2: error: expected '(' after 'asm' > [clang-diagnostic-error] > WARN_ON((long)v < 0); > ^ > ./include/asm-generic/bug.h:123:3: note: expanded from macro 'WARN_ON' > __WARN(); \ > ^ > ./include/asm-generic/bug.h:96:19: note: expanded from macro '__WARN' > #define __WARN() __WARN_FLAGS(BUGFLAG_TAINT(TAINT_WARN)) > ^ > ./arch/x86/include/asm/bug.h:79:2: note: expanded from macro '__WARN_FLAGS' > _BUG_FLAGS(ASM_UD2, BUGFLAG_WARNING|(flags)); \ > ^ > ./arch/x86/include/asm/bug.h:27:2: note: expanded from macro '_BUG_FLAGS' > asm_inline volatile("1:\t" ins "\n" \ > ^ > ./include/linux/compiler_types.h:281:24: note: expanded from macro > 'asm_inline' > #define asm_inline asm __inline > ^ > ./include/linux/xarray.h:1616:2: error: expected '(' after 'asm' > [clang-diagnostic-error] > BUG_ON(order > 0); > ^ > ./include/asm-generic/bug.h:65:57: note: expanded from macro 'BUG_ON' > #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0) > ^ > ./arch/x86/include/asm/bug.h:66:2: note: expanded from macro 'BUG' > _BUG_FLAGS(ASM_UD2, 0); \ > ^ > ./arch/x86/include/asm/bug.h:27:2: note: expanded from macro '_BUG_FLAGS' > asm_inline volatile("1:\t" ins "\n" \ > ^ > ./include/linux/compiler_types.h:281:24: note: expanded from macro > 'asm_inline' > #define asm_inline asm __inline > ^ > Found compiler error(s). > 21 errors generated. > Error while processing /home/xxxxxxxx/linux/drivers/hid/hid-ezkey.c. > error: too many errors emitted, stopping now [clang-diagnostic-error] > error: unknown argument: '-fno-stack-clash-protection' > [clang-diagnostic-error] > error: unknown warning option '-Wno-frame-address'; did you mean > '-Wno-address'? [clang-diagnostic-unknown-warning-option] > error: unknown warning option '-Wno-pointer-to-enum-cast'; did you mean > '-Wno-pointer-compare'? [clang-diagnostic-unknown-warning-option] > > > Unless I did something wrong, this doesn't seem that useful to me compared > to what I get from scan-build? I do not see that error but I have little experience with running the clang-analyzer target. It might be due to a difference between scan-build and clang-tidy? Regardless, it seems like you prefer reading the HTML report, so sticking with scan-build with the '--use-cc=clang' flag will be the way to go. Cheers, Nathan