On Mon, Jun 29, 2020 at 10:15:47AM +0200, Ard Biesheuvel wrote: > On Mon, 29 Jun 2020 at 08:18, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > In preparation for building efi/libstub with -mbranch-protection=none > > (EFI does not support branch protection features[1]), add no-op code > > to work around a Clang bug that emits an unwanted .note.gnu.property > > section for object files without code[2]. > > > > [1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@xxxxxxxxxxxxxx > > [2] https://bugs.llvm.org/show_bug.cgi?id=46480 > > > > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> > > Cc: Will Deacon <will@xxxxxxxxxx> > > Cc: Dave Martin <Dave.Martin@xxxxxxx> > > Cc: clang-built-linux@xxxxxxxxxxxxxxxx > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > > --- > > lib/ctype.c | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > diff --git a/lib/ctype.c b/lib/ctype.c > > index c819fe269eb2..21245ed57d90 100644 > > --- a/lib/ctype.c > > +++ b/lib/ctype.c > > @@ -36,3 +36,13 @@ _L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L, /* 224-239 */ > > _L,_L,_L,_L,_L,_L,_L,_P,_L,_L,_L,_L,_L,_L,_L,_L}; /* 240-255 */ > > > > EXPORT_SYMBOL(_ctype); > > + > > +/* > > + * Clang will generate .note.gnu.property sections for object files > > + * without code, even in the presence of -mbranch-protection=none. > > + * To work around this, define an unused static function. > > + * https://bugs.llvm.org/show_bug.cgi?id=46480 > > + */ > > +#ifdef CONFIG_CC_IS_CLANG > > +void __maybe_unused __clang_needs_code_here(void) { } > > +#endif > > -- > > 2.25.1 > > > > I take it we don't need this horrible hack if we build the EFI stub > with branch protections and filter out the .note.gnu.property section > explicitly? Correct. > Sorry to backpedal, but that is probably a better approach after all, > given that the instructions don't hurt, and we will hopefully be able > to arm them once UEFI (as well as PE/COFF) gets around to describing > this in a way that both the firmware and the OS can consume. Okay, will revert to the v3 solution. -- Kees Cook
