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Subject: Re: [PATCH] vmlinux.lds.h: Handle clang's module.{c,d}tor sections
To:     Fangrui Song <maskray@google.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>, Marco Elver <elver@google.com>,
        linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        kasan-dev@googlegroups.com, clang-built-linux@googlegroups.com,
        stable@vger.kernel.org
References: <20210730223815.1382706-1-nathan@kernel.org>
 <CAKwvOdnJ9VMZfZrZprD6k0oWxVJVSNePUM7fbzFTJygXfO24Pw@mail.gmail.com>
 <20210730225936.ce3hcjdg2sptvbh7@google.com>
From:   Nathan Chancellor <nathan@kernel.org>
Message-ID: <baf67422-8662-02f2-0bbf-6afb141875af@kernel.org>
Date:   Fri, 30 Jul 2021 17:32:33 -0700
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On 7/30/2021 3:59 PM, Fangrui Song wrote:
> On 2021-07-30, Nick Desaulniers wrote:
>> On Fri, Jul 30, 2021 at 3:38 PM Nathan Chancellor <nathan@kernel.org> 
>> wrote:
>>>
>>> A recent change in LLVM causes module_{c,d}tor sections to appear when
>>> CONFIG_K{A,C}SAN are enabled, which results in orphan section warnings
>>> because these are not handled anywhere:
>>>
>>> ld.lld: warning: 
>>> arch/x86/pci/built-in.a(legacy.o):(.text.asan.module_ctor) is being 
>>> placed in '.text.asan.module_ctor'
>>> ld.lld: warning: 
>>> arch/x86/pci/built-in.a(legacy.o):(.text.asan.module_dtor) is being 
>>> placed in '.text.asan.module_dtor'
>>> ld.lld: warning: 
>>> arch/x86/pci/built-in.a(legacy.o):(.text.tsan.module_ctor) is being 
>>> placed in '.text.tsan.module_ctor'
>>
>> ^ .text.tsan.*
> 
> I was wondering why the orphan section warning only arose recently.
> Now I see: the function asan.module_ctor has the SHF_GNU_RETAIN flag, so
> it is in a separate section even with -fno-function-sections (default).

Thanks for the explanation, I will add this to the commit message.

> It seems that with -ffunction-sections the issue should have been caught
> much earlier.
> 
>>>
>>> Place them in the TEXT_TEXT section so that these technologies continue
>>> to work with the newer compiler versions. All of the KASAN and KCSAN
>>> KUnit tests continue to pass after this change.
>>>
>>> Cc: stable@vger.kernel.org
>>> Link: https://github.com/ClangBuiltLinux/linux/issues/1432
>>> Link: 
>>> https://github.com/llvm/llvm-project/commit/7b789562244ee941b7bf2cefeb3fc08a59a01865 
>>>
>>> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
>>> ---
>>>  include/asm-generic/vmlinux.lds.h | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/include/asm-generic/vmlinux.lds.h 
>>> b/include/asm-generic/vmlinux.lds.h
>>> index 17325416e2de..3b79b1e76556 100644
>>> --- a/include/asm-generic/vmlinux.lds.h
>>> +++ b/include/asm-generic/vmlinux.lds.h
>>> @@ -586,6 +586,7 @@
>>>                 
>>> NOINSTR_TEXT                                            \
>>>                 
>>> *(.text..refcount)                                      \
>>>                 
>>> *(.ref.text)                                            \
>>> +               *(.text.asan 
>>> .text.asan.*)                              \
>>
>> Will this match .text.tsan.module_ctor?

No, I forgot to test CONFIG_KCSAN with this version, rather than the 
prior one I had on GitHub so I will send v2 shortly.

> asan.module_ctor is the only function AddressSanitizer synthesizes in 
> the instrumented translation unit.
> There is no function called "asan".
> 
> (Even if a function "asan" exists due to -ffunction-sections
> -funique-section-names, TEXT_MAIN will match .text.asan, so the
> .text.asan pattern will match nothing.)

Sounds good, I will update it to remove the .text.asan and replace it 
with .text.tsan.*

>> Do we want to add these conditionally on
>> CONFIG_KASAN_GENERIC/CONFIG_KCSAN like we do for SANITIZER_DISCARDS?

I do not think there is a point in doing so but I can if others feel 
strongly.

Thank you both for the comments for the comments!

Cheers,
Nathan