Re: [PATCH v9 06/16] kbuild: lto: add a default list of used symbols

[Sami Tolvanen]

On Fri, Dec 11, 2020 at 11:32 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Fri, Dec 11, 2020 at 10:46:23AM -0800, Sami Tolvanen wrote:
> > With CONFIG_LTO_CLANG, LLVM bitcode has not yet been compiled into a
> > binary when the .mod files are generated, which means they don't yet
> > contain references to certain symbols that will be present in the final
> > binaries. This includes intrinsic functions, such as memcpy, memmove,
> > and memset [1], and stack protector symbols [2]. This change adds a
> > default symbol list to use with CONFIG_TRIM_UNUSED_KSYMS when Clang's
> > LTO is used.
> >
> > [1] https://llvm.org/docs/LangRef.html#standard-c-c-library-intrinsics
> > [2] https://llvm.org/docs/LangRef.html#llvm-stackprotector-intrinsic
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> > ---
> >  init/Kconfig                | 1 +
> >  scripts/lto-used-symbollist | 5 +++++
> >  2 files changed, 6 insertions(+)
> >  create mode 100644 scripts/lto-used-symbollist
> >
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 0872a5a2e759..e88c919c1bf1 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -2297,6 +2297,7 @@ config TRIM_UNUSED_KSYMS
> >  config UNUSED_KSYMS_WHITELIST
> >       string "Whitelist of symbols to keep in ksymtab"
> >       depends on TRIM_UNUSED_KSYMS
> > +     default "scripts/lto-used-symbollist" if LTO_CLANG
> >       help
> >         By default, all unused exported symbols will be un-exported from the
> >         build when TRIM_UNUSED_KSYMS is selected.
> > diff --git a/scripts/lto-used-symbollist b/scripts/lto-used-symbollist
> > new file mode 100644
> > index 000000000000..38e7bb9ebaae
> > --- /dev/null
> > +++ b/scripts/lto-used-symbollist
> > @@ -0,0 +1,5 @@
> > +memcpy
> > +memmove
> > +memset
> > +__stack_chk_fail
> > +__stack_chk_guard
> > --
> > 2.29.2.576.ga3fc446d84-goog
> >
>
> bikeshed: Should this filename use some kind of extension, like
> lto-user-symbols.txt or .list, to make it more human-friendly?

Sure, I can rename this in the next version. Does anyone have strong
opinions about the name and/or extension?

Sami