On Fri, Jul 17, 2020 at 5:32 PM H.J. Lu via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Fri, Jul 17, 2020 at 6:27 AM Richard Biener <rguent...@suse.de> wrote:
> >
> > On Fri, 17 Jul 2020, H.J. Lu wrote:
> >
> > > On Fri, Jul 17, 2020 at 12:08 AM Richard Biener <rguent...@suse.de> wrote:
> > > >
> > > > On Wed, 15 Jul 2020, Joseph Myers wrote:
> > > >
> > > > > On Wed, 15 Jul 2020, Richard Biener wrote:
> > > > >
> > > > > > But note one of the issues is that when not cross-compiling we're
> > > > > > using a single libiberty for target and host objects (likewise
> > > > >
> > > > > There shouldn't be a target libiberty, since commit
> > > > > 8499116aa30a46993deff5acf73985df6b16fb8b (re PR regression/47836 (Some
> > > > > Cross Compiler can't build target-libiberty or target-zlib), Wed Jun
> > > > > 22
> > > > > 19:40:45 2011 +0000). If something is causing target libiberty to be
> > > > > built, that's a bug that should be fixed.
> > > > >
> > > > > > That said, giving configury an idea whether it configures for
> > > > > > the host, the target or the build would be required here - Joseph,
> > > > > > is there an existing mechanism for example libiberty can use
> > > > > > here?
> > > > >
> > > > > Makefile.def has some settings specific to host or build, e.g.
> > > > >
> > > > > build_modules= { module= libcpp;
> > > > > extra_configure_flags='--disable-nls
> > > > > am_cv_func_iconv=no';};
> > > > >
> > > > > or
> > > > >
> > > > > host_modules= { module= libiberty; bootstrap=true;
> > > > >
> > > > > extra_configure_flags='@extra_host_libiberty_configure_flags@';};
> > > >
> > > > Ah, OK. Looks like we should be able to add a
> > > > @extra_target_cet_configure_flags@, funnel that to the target_modules
> > > > and keep CET disabled by default in the modules configury.
> > > >
> > > > Similarly (if HJ is correct) we'd need to add
> > > > @extra_{host,build}_cet_configure_flags@ for the purpose of lto-plugin
> > > > which only has a host module (and for bootstrap host == build, so it's
> > > > shared there but we still have separate libiberties for host/build...)
> > > >
> > >
> > > We need -fcf-protection only on object files which will be dlopened on
> > > CET enabled build and host.
> >
> > Why is there a distinction between dlopen and execution? IIRC
> > ld falls back to non-CET operation when dlopening a non-CET shared object?
>
> BTW, ld.so refuses to dlopen a legacy shared object after CET has been
> enabled.
> This behavior can be controlled when configuring glibc:
>
> '--enable-cet'
> '--enable-cet=permissive'
> Enable Intel Control-flow Enforcement Technology (CET) support.
> When the GNU C Library is built with '--enable-cet' or
> '--enable-cet=permissive', the resulting library is protected with
> indirect branch tracking (IBT) and shadow stack (SHSTK). When CET
> is enabled, the GNU C Library is compatible with all existing
> executables and shared libraries. This feature is currently
> supported on i386, x86_64 and x32 with GCC 8 and binutils 2.29 or
> later. Note that when CET is enabled, the GNU C Library requires
> CPUs capable of multi-byte NOPs, like x86-64 processors as well as
> Intel Pentium Pro or newer. With '--enable-cet', it is an error to
> dlopen a non CET enabled shared library in CET enabled application.
> With '--enable-cet=permissive', CET is disabled when dlopening a
> non CET enabled shared library in CET enabled application.
So getting back to this one of the issues is that --enable-cet is used
for both GCC_CET_FLAGS and GCC_CET_HOST_FLAGS where
for the host flag part I'd use --enable-cet=auto but for the target library
part I definitely want to know if --enable-cet cannot be honored.
Your current patch would still prohibit a non-bootstrap build with a host
compiler not supporting CET and requesting CET enabled target libraries,
thus
../configure --enable-cet --disable-bootstrap
would fail. Shouldn't we - for the host part - simply treat 'yes' equal
to 'auto'? If not, then we should have --enable-cet-host. Which would
be somewhat misleading since cc1 isn't built CET enabled, just
lto-plugin.so is, so better --enable-lto-plugin-cet?
Thus I'd go with the simpler of both:
diff --git a/config/cet.m4 b/config/cet.m4
index d9608699cd5..fb4e4275413 100644
--- a/config/cet.m4
+++ b/config/cet.m4
@@ -64,7 +64,7 @@ case "$host" in
save_CFLAGS="$CFLAGS"
CFLAGS="$CFLAGS -fcf-protection"
case "$enable_cet" in
- auto)
+ auto|yes)
# Check if target supports multi-byte NOPs
# and if assembler supports CET insn.
AC_COMPILE_IFELSE(
@@ -80,15 +80,6 @@ asm ("setssbsy");
[enable_cet=yes],
[enable_cet=no])
;;
- yes)
- # Check if assembler supports CET.
- AC_COMPILE_IFELSE(
- [AC_LANG_PROGRAM(
- [],
- [asm ("setssbsy");])],
- [],
- [AC_MSG_ERROR([assembler with CET support is required for
--enable-cet])])
- ;;
esac
CFLAGS="$save_CFLAGS"
;;
is that OK with you?
Thanks,
Richard.
> > But OK, so if we'll allow mixing CET and non-CET objects for the
> > gen* link, simply dropping CET enablement, I guess it will work
> > as it did before.
> >
> > Richard.
>
>
>
> --
> H.J.
