Date:   Fri, 21 Jan 2022 12:20:39 +0100
From:   Amadeusz Sławiński 
        <amadeuszx.slawinski@...ux.intel.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     linux-kbuild@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Cezary Rojewski <cezary.rojewski@...el.com>
Subject: Re: [PATCH] Makefile: Fix build with scan-build

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@...gle.com>
>>> Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@...ux.intel.com>
>>> Reviewed-by: Cezary Rojewski <cezary.rojewski@...el.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?
What I like about scan-build is that it generates html report file.

'--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?

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?

Cheers,
Amadeusz

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