Re: [PATCH] vmlinux.lds.h: Handle clang's module.{c,d}tor sections

[Fangrui Song]

On 2021-07-30, Nick Desaulniers wrote:
> On Fri, Jul 30, 2021 at 3:38 PM Nathan Chancellor <nathan@xxxxxxxxxx> 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).

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@xxxxxxxxxxxxxxx
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1432
> > Link: https://github.com/llvm/llvm-project/commit/7b789562244ee941b7bf2cefeb3fc08a59a01865
> > Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> > ---
> >  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?

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.)

> Do we want to add these conditionally on
> CONFIG_KASAN_GENERIC/CONFIG_KCSAN like we do for SANITIZER_DISCARDS?
>
> >                 TEXT_CFI_JT                                             \
> >         MEM_KEEP(init.text*)                                            \
> >         MEM_KEEP(exit.text*)                                            \
> >
> > base-commit: 4669e13cd67f8532be12815ed3d37e775a9bdc16
> > --
>
>
> --
> Thanks,
> ~Nick Desaulniers