[PATCH v6 09/14] aarch64: ensure objects are BTI compatible

[Szabolcs Nagy]

The 07/06/2020 14:37, Adhemerval Zanella via Libc-alpha wrote:
> On 01/07/2020 11:39, Szabolcs Nagy wrote:
> > When glibc is built with branch protection (i.e. with a gcc configured
> > with --enable-standard-branch-protection), all glibc binaries should
> > be BTI compatible and marked as such.
> > 
> > It is easy to link BTI incompatible objects by accident and this is
> > silent currently which is usually not the expectation, so this is
> > changed into a link error. (There is no linker flag for failing on
> > BTI incompatible inputs so all warnings are turned into fatal errors
> > outside the test system when building glibc with branch protection.)
> > 
> > Unfortunately, outlined atomic functions are not BTI compatible in
> > libgcc (PR libgcc/96001), so to build glibc with current gcc use
> > 'CC=gcc -mno-outline-atomics', this should be fixed in libgcc soon
> > and then glibc can be built and tested without such workarounds.
> 
> Is the outlined atomic present in any current released gcc? If so
> should we add a configure handler to add -mno-outline-atomics for
> such cases?

outlined atomics is supported for a while now (since
gcc-9 i think) but it is now the default in gcc-10.

i am testing patches to fix libgcc in gcc trunk and
then backport to gcc-10 branch.

my preference is to only support fixed gcc and use
the -mno-outline-atomics as a temporary workaround
so we can test the patches with gcc-10.1.0

it's not a good workaround for using the resulting
toolchain for building user code with branch protection
(which will have the same problem: silently dropping
bti protection) so i think it's better to fail the build.

> 
> Besides that LGTM, thanks.
> 
> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> 
> > ---
> >  sysdeps/aarch64/Makefile     | 8 ++++++++
> >  sysdeps/aarch64/configure    | 2 ++
> >  sysdeps/aarch64/configure.ac | 1 +
> >  3 files changed, 11 insertions(+)
> > 
> > diff --git a/sysdeps/aarch64/Makefile b/sysdeps/aarch64/Makefile
> > index 5ae8b082b0..7f82fc055e 100644
> > --- a/sysdeps/aarch64/Makefile
> > +++ b/sysdeps/aarch64/Makefile
> > @@ -1,5 +1,13 @@
> >  long-double-fcts = yes
> >  
> > +ifeq (yes,$(aarch64-bti))
> > +# Mark linker output BTI compatible, it warns on non-BTI inputs.
> > +sysdep-LDFLAGS += -Wl,-z,force-bti
> > +# Make warnings fatal outside the test system.
> > +LDFLAGS-lib.so += -Wl,--fatal-warnings
> > +LDFLAGS-rtld += -Wl,-z,force-bti,--fatal-warnings
> > +endif
> > +
> >  ifeq ($(subdir),elf)
> >  sysdep-dl-routines += dl-bti
> >  endif
> 
> Ok.
> 
> > diff --git a/sysdeps/aarch64/configure b/sysdeps/aarch64/configure
> > index 70477a7fa5..c637540436 100644
> > --- a/sysdeps/aarch64/configure
> > +++ b/sysdeps/aarch64/configure
> > @@ -210,6 +210,8 @@ EOF
> >  fi
> >  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_aarch64_bti" >&5
> >  $as_echo "$libc_cv_aarch64_bti" >&6; }
> > +config_vars="$config_vars
> > +aarch64-bti = $libc_cv_aarch64_bti"
> >  if test $libc_cv_aarch64_bti = yes; then
> >    $as_echo "#define HAVE_AARCH64_BTI 1" >>confdefs.h
> >  
> > diff --git a/sysdeps/aarch64/configure.ac b/sysdeps/aarch64/configure.ac
> > index 798f494740..2c2817514d 100644
> > --- a/sysdeps/aarch64/configure.ac
> > +++ b/sysdeps/aarch64/configure.ac
> > @@ -36,6 +36,7 @@ EOF
> >      libc_cv_aarch64_bti=yes
> >    fi
> >    rm -rf conftest.*])
> > +LIBC_CONFIG_VAR([aarch64-bti], [$libc_cv_aarch64_bti])
> >  if test $libc_cv_aarch64_bti = yes; then
> >    AC_DEFINE(HAVE_AARCH64_BTI)
> >  fi
> > 
> 
> Ok.

--