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Date:   Sat, 22 Jan 2022 19:41:31 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Amadeusz =?utf-8?B?U8WCYXdpxYRza2k=?= 
        <amadeuszx.slawinski@linux.intel.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Cezary Rojewski <cezary.rojewski@intel.com>
Subject: Re: [PATCH] Makefile: Fix build with scan-build
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On Sat, Jan 22, 2022 at 08:53:00PM +0900, Masahiro Yamada wrote:
> On Sat, Jan 22, 2022 at 1:47 AM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > 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@google.com>
> > > > > > Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
> > > > > > Reviewed-by: Cezary Rojewski <cezary.rojewski@intel.com>
> > > > > > ---
> > > > > >   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
> 
> 
> 
> As far as I understood, the conclusion is
> 
> [1] There is nothing to fix on the Kbuild side.
>      (So, this patch was rejected)
> 
> 
> [2] If you want to use scan-build for kbuild,
>         "scan-build --use-cc=clang" should work properly.
> 
>    The "disable RETPOLINE' workaround is also unneeded.
> 
>   $ scan-build --use-cc=clang make -j16 defconfig bindeb-pkg
> 
>    should be enough
> 
> 
> [3]  If scan-build supports mixed-compilers, it is up to LLVM community.
>       If they are happy, they will carry the pain of maintaining ignored flags.
> 
> 
> 
> The detection of retpoline flags is not working somehow.
> 
> $ /usr/share/clang/scan-build-13/bin/../libexec/ccc-analyzer
> -mindirect-branch=thunk-extern  -c -x c /dev/null -o /dev/null
> could not find clang line
> $ /usr/share/clang/scan-build-13/bin/../libexec/ccc-analyzer
> -mretpoline-external-thunk  -c -x c /dev/null -o /dev/null
> gcc: error: unrecognized command-line option ‘-mretpoline-external-thunk’
> 
> I think this should be fixed.

It looks like this happens for the same reason as the error in the
patch:

https://github.com/llvm/llvm-project/blob/c1988dbf2d191d771e3a396eaa6396500965787d/clang/tools/scan-build/libexec/ccc-analyzer#L604

All '-m' flags get forwarded along from the compiler to clang. Because
clang and GCC use different flags for retpoline, there will always be an
error when ccc-analyzer is used with GCC as the compiler. It sounds like
there is some agreement upstream that this is a problem; how it will be
solved remains to be seen but I don't think the kernel should work
around this, which goes along with point #1.

Cheers,
Nathan