Re: [PATCH v2] arch: um: Mark the stack non-executable to fix a binutils warning

[David Gow <davidgow@google.com>]

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On Thu, Sep 22, 2022 at 1:47 AM 'Nick Desaulniers' via KUnit
Development <kunit-dev@googlegroups.com> wrote:
>
> On Tue, Sep 20, 2022 at 11:49 PM David Gow <davidgow@google.com> wrote:
> >
> > Since binutils 2.39, ld will print a warning if any stack section is
> > executable, which is the default for stack sections on files without a
> > .note.GNU-stack section.
> >
> > This was fixed for x86 in commit ffcf9c5700e4 ("x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments"),
> > but remained broken for UML, resulting in several warnings:
> >
> > /usr/bin/ld: warning: arch/x86/um/vdso/vdso.o: missing .note.GNU-stack section implies executable stack
> > /usr/bin/ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
> > /usr/bin/ld: warning: .tmp_vmlinux.kallsyms1 has a LOAD segment with RWX permissions
> > /usr/bin/ld: warning: .tmp_vmlinux.kallsyms1.o: missing .note.GNU-stack section implies executable stack
> > /usr/bin/ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
> > /usr/bin/ld: warning: .tmp_vmlinux.kallsyms2 has a LOAD segment with RWX permissions
> > /usr/bin/ld: warning: .tmp_vmlinux.kallsyms2.o: missing .note.GNU-stack section implies executable stack
> > /usr/bin/ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
> > /usr/bin/ld: warning: vmlinux has a LOAD segment with RWX permissions
> >
> > Link both the VDSO and vmlinux with -z noexecstack, fixing the warnings
> > about .note.GNU-stack sections. In addition, pass --no-warn-rwx-segments
> > to dodge the remaining warnings about LOAD segments with RWX permissions
> > in the kallsyms objects. (Note that this flag is apparently not
> > available on lld, so hide it behind a test for BFD, which is what the
> > x86 patch does.)
> >
> > Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ffcf9c5700e49c0aee42dcba9a12ba21338e8136
> > Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=ba951afb99912da01a6e8434126b8fac7aa75107
> > Signed-off-by: David Gow <davidgow@google.com>
> > Reviewed-by: Lukas Straub <lukasstraub2@web.de>
> > Tested-by: Lukas Straub <lukasstraub2@web.de>
> > Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
> > ---
> >
> > Note that this still doesn't seem to be working properly with make
> > LLVM=1. It doesn't appear to break anything, and still is an improvement
> > for gcc, so seems worthwhile anyway...
>
> Hi David,
> Do you have more info about this comment?  Perhaps there's more
> hermiticity issues with ARCH=um when a linker is explicitly specified
> via LD= or HOSTLD=, or implied via LLVM=1.
>
> Looking at our CI for ARCH=um builds...
> https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/3095779516/jobs/5012260390
> warnings from /usr/bin/ld. That's...unexpected.
> I've filed https://github.com/ClangBuiltLinux/linux/issues/1715 to
> follow up on this.
>

Thanks for following up on this.

Yeah: what I'm seeing seems to match  it does look like ld is being
used instead of lld.

I tried -fuse-ld=lld via the following patch, though, and got a whole
bunch of "error: relocation R_X86_64_64 cannot be used against local
symbol; recompile with -fPIC" errors, so I think there's probably more
issues with UML and LLD that need working out. (I had no luck with
CONFIG_STATIC_LINK=y, nor with an attempt to use $(LD) directly,
instead of via $(CC)).

---
diff --git a/arch/um/Makefile b/arch/um/Makefile
index f1d4d67157be..01d9eae736be 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -139,6 +139,9 @@ ifeq ($(CONFIG_LD_IS_BFD),y)
LDFLAGS_EXECSTACK += $(call ld-option,--no-warn-rwx-segments)
endif

+# Since we're using CC as the driver, we need to force LLD if it is requested.
+LINK-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
+
LD_FLAGS_CMDLINE = $(foreach opt,$(KBUILD_LDFLAGS),-Wl,$(opt))

# Used by link-vmlinux.sh which has special support for um link

---

I'll update the GitHub issue with these notes as well, but I
definitely think it'd be best not to make UML use lld here until it
actually builds something: even with the warnings, ld is at least
latting us run at all.

Cheers,
-- David

> >
> > Changes since v1:
> > http://lists.infradead.org/pipermail/linux-um/2022-August/004234.html
> > - Pass the -z noexecstack and --no-warn-rwx-segments flags as LDFLAGS,
> >   rather than as CFLAGS via -Wl
> > - Check that --no-warn-rwx-segments exists with the ld-option function
> >   (Thanks Richard)
> > - Add Lukas and Randy's tags.
> >
> >
> >  arch/um/Makefile          | 8 ++++++++
> >  arch/x86/um/vdso/Makefile | 2 +-
> >  2 files changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/um/Makefile b/arch/um/Makefile
> > index f2fe63bfd819..f1d4d67157be 100644
> > --- a/arch/um/Makefile
> > +++ b/arch/um/Makefile
> > @@ -132,10 +132,18 @@ export LDS_ELF_FORMAT := $(ELF_FORMAT)
> >  # The wrappers will select whether using "malloc" or the kernel allocator.
> >  LINK_WRAPS = -Wl,--wrap,malloc -Wl,--wrap,free -Wl,--wrap,calloc
> >
> > +# Avoid binutils 2.39+ warnings by marking the stack non-executable and
> > +# ignorning warnings for the kallsyms sections.
> > +LDFLAGS_EXECSTACK = -z noexecstack
> > +ifeq ($(CONFIG_LD_IS_BFD),y)
> > +LDFLAGS_EXECSTACK += $(call ld-option,--no-warn-rwx-segments)
> > +endif
> > +
> >  LD_FLAGS_CMDLINE = $(foreach opt,$(KBUILD_LDFLAGS),-Wl,$(opt))
> >
> >  # Used by link-vmlinux.sh which has special support for um link
> >  export CFLAGS_vmlinux := $(LINK-y) $(LINK_WRAPS) $(LD_FLAGS_CMDLINE)
> > +export LDFLAGS_vmlinux := $(LDFLAGS_EXECSTACK)
> >
> >  # When cleaning we don't include .config, so we don't include
> >  # TT or skas makefiles and don't clean skas_ptregs.h.
> > diff --git a/arch/x86/um/vdso/Makefile b/arch/x86/um/vdso/Makefile
> > index 8c0396fd0e6f..6fbe97c52c99 100644
> > --- a/arch/x86/um/vdso/Makefile
> > +++ b/arch/x86/um/vdso/Makefile
> > @@ -65,7 +65,7 @@ quiet_cmd_vdso = VDSO    $@
> >                        -Wl,-T,$(filter %.lds,$^) $(filter %.o,$^) && \
> >                  sh $(srctree)/$(src)/checkundef.sh '$(NM)' '$@'
> >
> > -VDSO_LDFLAGS = -fPIC -shared -Wl,--hash-style=sysv
> > +VDSO_LDFLAGS = -fPIC -shared -Wl,--hash-style=sysv -z noexecstack
> >  GCOV_PROFILE := n
> >
> >  #
> > --
> > 2.37.3.968.ga6b4b080e4-goog
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers
>
> --
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