Re: [PATCH] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG

[Sami Tolvanen]

Hi Al,

On Fri, Jul 16, 2021 at 4:00 PM Alexander Lobakin <alobakin@xxxxx> wrote:
>
> From:   Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Date:   Fri, 16 Jul 2021 13:45:45 -0700
>
> > With CONFIG_LTO_CLANG, we currently link modules into native
> > code just before modpost, which means with TRIM_UNUSED_KSYMS
> > enabled, we still look at the LLVM bitcode in the .o files when
> > generating the list of used symbols. As the bitcode doesn't
> > yet have calls to compiler intrinsics and llvm-nm doesn't see
> > function references that only exist in function-level inline
> > assembly, we currently need a whitelist for TRIM_UNUSED_KSYMS to
> > work with LTO.
> >
> > This change moves module LTO linking to happen earlier, and
> > thus avoids the issue with LLVM bitcode and TRIM_UNUSED_KSYMS
> > entirely, allowing us to also drop the whitelist from
> > gen_autoksyms.sh.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1369
> > Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> > ---
> >  scripts/Makefile.build    | 25 ++++++++++++++++++++++++-
> >  scripts/Makefile.lib      |  7 +++++++
> >  scripts/Makefile.modfinal | 21 ++-------------------
> >  scripts/Makefile.modpost  | 22 +++-------------------
> >  scripts/gen_autoksyms.sh  | 12 ------------
> >  5 files changed, 36 insertions(+), 51 deletions(-)
> >
> > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > index 10b2f2380d6f..80e0fa810870 100644
> > --- a/scripts/Makefile.build
> > +++ b/scripts/Makefile.build
> > @@ -202,6 +202,7 @@ sub_cmd_record_mcount =                                   \
> >       if [ $(@) != "scripts/mod/empty.o" ]; then      \
> >               $(objtree)/scripts/recordmcount $(RECORDMCOUNT_FLAGS) "$(@)";   \
> >       fi;
> > +/
>
> Seems like a leftover or a random typo here.

Oops, indeed.

> >  recordmcount_source := $(srctree)/scripts/recordmcount.c \
> >                   $(srctree)/scripts/recordmcount.h
> >  else
> > @@ -271,12 +272,34 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $$(objtool_dep) FORCE
> >       $(call if_changed_rule,cc_o_c)
> >       $(call cmd,force_checksrc)
> >
> > +ifdef CONFIG_LTO_CLANG
> > +# Module .o files may contain LLVM bitcode, compile them into native code
> > +# before ELF processing
> > +quiet_cmd_cc_lto_link_modules = LTO [M] $@
> > +cmd_cc_lto_link_modules =                                            \
> > +     $(LD) $(ld_flags) -r -o $@                                      \
> > +             $(shell [ -s $(@:.lto.o=.o.symversions) ] &&            \
> > +                     echo -T $(@:.lto.o=.o.symversions))             \
> > +             --whole-archive $^
> > +
> > +ifdef CONFIG_STACK_VALIDATION
> > +# objtool was skipped for LLVM bitcode, run it now that we have compiled
> > +# modules into native code
> > +cmd_cc_lto_link_modules += ;                                         \
> > +     $(objtree)/tools/objtool/objtool $(objtool_args)                \
>
> Now $(part-of-module) inside $(objtool_args) doesn't get expanded
> properly, because previously it was being called on x.ko, and now
> it's being called on x.lto.o. $(basename $@) returns "x.lto" instead
> of "x", and Make doesn't find "x.lto.o" in $(real-objs-m).
>
> An example of objtool args dump:
>
>   LTO [M] fs/btrfs/btrfs.lto.o
> Call: ./tools/objtool/objtool orc generate --no-fp --no-unreachable --retpoline --uaccess fs/btrfs/btrfs.lto.o
> fs/btrfs/btrfs.lto.o: warning: objtool: static_call: can't find static_call_key symbol: __SCK__might_resched

Curiously I didn't see objtool warnings when building allmodconfig,
but you're obviously correct here. I'll fix this in v2.

> As can be seen, objtools command line no longer contains "--module".
> And this warning about "can't find static_call_key" can appear only
> in case of !module -> no -m|--module param was given.
>
> As a result, modules get broken and the kernel panics after loading
> initramfs.

Thanks for taking a look!

Sami