Re: [PATCH] vmlinux.lds.h: Define SANTIZER_DISCARDS with CONFIG_GCOV_KERNEL=y

[Fangrui Song]

On 2021-01-29, Nick Desaulniers wrote:
> On Fri, Jan 29, 2021 at 12:11 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
> > clang produces .eh_frame sections when CONFIG_GCOV_KERNEL is enabled,
> > even when -fno-asynchronous-unwind-tables is in KBUILD_CFLAGS:
> >
> > $ make CC=clang vmlinux
> > ...
> > ld: warning: orphan section `.eh_frame' from `init/main.o' being placed in section `.eh_frame'
> > ld: warning: orphan section `.eh_frame' from `init/version.o' being placed in section `.eh_frame'
> > ld: warning: orphan section `.eh_frame' from `init/do_mounts.o' being placed in section `.eh_frame'
> > ld: warning: orphan section `.eh_frame' from `init/do_mounts_initrd.o' being placed in section `.eh_frame'
> > ld: warning: orphan section `.eh_frame' from `init/initramfs.o' being placed in section `.eh_frame'
> > ld: warning: orphan section `.eh_frame' from `init/calibrate.o' being placed in section `.eh_frame'
> > ld: warning: orphan section `.eh_frame' from `init/init_task.o' being placed in section `.eh_frame'
> > ...
> >
> > $ rg "GCOV_KERNEL|GCOV_PROFILE_ALL" .config
> > CONFIG_GCOV_KERNEL=y
> > CONFIG_ARCH_HAS_GCOV_PROFILE_ALL=y
> > CONFIG_GCOV_PROFILE_ALL=y
> >
> > This was already handled for a couple of other options in
> > commit d812db78288d ("vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted
> > sections") and there is an open LLVM bug for this issue. Take advantage
> > of that section for this config as well so that there are no more orphan
> > warnings.
> >
> > Link: https://bugs.llvm.org/show_bug.cgi?id=46478
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1069
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>
> I suspect we're going to need to add module level attributes in LLVM
> IR for these options, then check those when synthesizing new function
> definitions within LLVM.  At least we'll be able to point to this file
> and say "hey, this is a general problem in LLVM, and here are 3
> specific cases now where it's a problem."  Not a large problem, but
> would help us save some bytes in the final object.  LLVM is not
> producing data in this section for all code, just these synthesized
> routines.

Maybe. There are also a long list of security features which may impose
additional requirements. Adding a module flag metadata for each such
feature will be a long battle. For .eh_frame, I think it is
important/generic enough and can benefit other applications that
deserves special handling (and I can look into it). For .init_array, I
am not too sure....

> > ---
> >  include/asm-generic/vmlinux.lds.h | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index b2b3d81b1535..f753fd449436 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -988,12 +988,13 @@
> >  #endif
> >
> >  /*
> > - * Clang's -fsanitize=kernel-address and -fsanitize=thread produce
> > + * Clang's -fsanitize=kernel-address, -fsanitize=thread,
> > + * and -fprofile-arcs -ftest-coverage produce unwanted
> >   * unwanted sections (.eh_frame and .init_array.*), but
> >   * CONFIG_CONSTRUCTORS wants to keep any .init_array.* sections.
> >   * https://bugs.llvm.org/show_bug.cgi?id=46478
> >   */
> > -#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KCSAN)
> > +#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KCSAN) || defined(CONFIG_GCOV_KERNEL)
> >  # ifdef CONFIG_CONSTRUCTORS
> >  #  define SANITIZER_DISCARDS                                           \
> >         *(.eh_frame)
> >
> > base-commit: bec4c2968fce2f44ce62d05288a633cd99a722eb
> > --
> > 2.30.0

Drop -ftest-coverage. -ftest-coverage just produces .gcno and does not
affect code generation.

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>