[klibc] Work around ELF loader bugs when executables have only BSS

Some of klibc's test programs have BSS but no static data when built
to use the shared library.  This is apparently very unusual, and
existing ELF loaders don't always handle it correctly.

I previously found that QEMU's ELF loader rejected such executables,
though this was eventually fixed.

Now a change in binutils for arm64 has revealed a bug in the kernel
ELF loader, that's causing it to reject those same executables.  While
this might get fixed in one way or another, I think it's a sign that
this case is just not tested and it would be best to avoid it.

Therefore:

- Rename interp.S to shared-stub.S
- Add a single byte of static data to it

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

5 files changed, 20 insertions, 11 deletions

diff --git a/klibc.spec.in b/klibc.spec.in
index 7d78b9ab9d5be5..db10363ed31110 100644
--- a/klibc.spec.in
+++ b/klibc.spec.in
@@ -90,7 +90,7 @@ install -m 444 usr/kinit/README %{buildroot}%{bindocdir}/README.kinit
 rm -rf $RPM_BUILD_ROOT
 
 #
-# Note: libc.so and interp.o are technically -devel files, but
+# Note: libc.so and shared-stub.o are technically -devel files, but
 # put them in this package until we can make really, really sure
 # the dependency system can avoid confusion.  (In fact, it would be
 # good to eventually get them out of here, so that multiple runtimes
@@ -100,7 +100,7 @@ rm -rf $RPM_BUILD_ROOT
 %defattr(-,root,root,-)
 /lib/klibc-*.so
 %{klibcdir}/lib/*.so
-%{klibcdir}/lib/interp.o
+%{klibcdir}/lib/shared-stub.o
 
 %files devel
 %defattr(-,root,root,-)
diff --git a/scripts/Kbuild.klibc b/scripts/Kbuild.klibc
index 64da31ac420f93..accf7f1b1c0e49 100644
--- a/scripts/Kbuild.klibc
+++ b/scripts/Kbuild.klibc
@@ -82,7 +82,7 @@ KLIBCCFLAGS       :=
 
 # Defaults for arch to override
 KLIBCARCHINCFLAGS =
-KLIBCCRTSHARED    := $(KLIBCOBJ)/interp.o
+KLIBCCRTSHARED    := $(KLIBCOBJ)/shared-stub.o
 
 LD_IS_LLD         := $(shell $(LD) --version 2>&1 | grep LLD)
 LD_IMAGE_BASE_OPT := $(if $(LD_IS_LLD),--image-base,-Ttext-segment)
diff --git a/usr/klibc/.gitignore b/usr/klibc/.gitignore
index f24c971efe09bd..5f4a9af713f2b1 100644
--- a/usr/klibc/.gitignore
+++ b/usr/klibc/.gitignore
@@ -1,5 +1,5 @@
 errlist.c
-.interp.o.d
+.shared-stub.o.d
 klibc*.so
 klib.list
 .klib.list.cmd
diff --git a/usr/klibc/Kbuild b/usr/klibc/Kbuild
index 5933f89a1a7b14..973b0c8179d85b 100644
--- a/usr/klibc/Kbuild
+++ b/usr/klibc/Kbuild
@@ -99,17 +99,17 @@ LIBC     := libc.a
 SOLIB    := libc.so
 SOHASH   := klibc.so
 CRT0     := arch/$(KLIBCARCHDIR)/crt0.o
-INTERP_O := interp.o
+SHARED_STUB := shared-stub.o
 
 always   := $(LIBC)
 ifdef KLIBCSHAREDFLAGS
-always   += $(SOLIB) $(SOHASH) $(INTERP_O)
+always   += $(SOLIB) $(SOHASH) $(SHARED_STUB)
 endif
 LIBC     := $(call objectify,$(LIBC))
 SOLIB    := $(call objectify,$(SOLIB))
 SOHASH   := $(call objectify,$(SOHASH))
 CRT0     := $(call objectify,$(CRT0))
-INTERP_O := $(call objectify,$(INTERP_O))
+SHARED_STUB := $(call objectify,$(SHARED_STUB))
 
 SOLIBHASH = $(shell cat $(SOLIB).hash)
 
@@ -179,8 +179,8 @@ $(SOHASH): $(SOLIB) $(SOLIB).hash
 
 
 #####
-# build interp.o
-targets += interp.o
+# build shared-stub.o
+targets += shared-stub.o
 
 quiet_cmd_interp = BUILD   $@
       cmd_interp = $(KLIBCCC) $(klibccflags) -D__ASSEMBLY__     \
@@ -188,7 +188,7 @@ quiet_cmd_interp = BUILD   $@
 			     -DSOHASH=\"$(SOLIBHASH)\" \
 			     -c -o $@ $<
 
-$(INTERP_O): $(obj)/interp.S $(SOLIB).hash
+$(SHARED_STUB): $(obj)/shared-stub.S $(SOLIB).hash
 	$(call if_changed,interp)
 
 #####
@@ -199,7 +199,7 @@ install-rule:
 	  $(shell $(install-data) $(f) \
 	          $(INSTALLROOT)$(INSTALLDIR)/$(KLIBCCROSS)lib))
 ifdef KLIBCSHAREDFLAGS
-	$(Q)$(foreach f, $(SOLIB) $(INTERP_O), \
+	$(Q)$(foreach f, $(SOLIB) $(SHARED_STUB), \
 	  $(shell $(install-data) $(f) \
 	          $(INSTALLROOT)$(INSTALLDIR)/$(KLIBCCROSS)lib))
 	$(Q)$(install-lib) $(obj)/klibc-$(SOLIBHASH).so \
diff --git a/usr/klibc/interp.S b/usr/klibc/shared-stub.S
index 4b15cfbdac9544..15c0f02af32da1 100644
--- a/usr/klibc/interp.S
+++ b/usr/klibc/shared-stub.S
@@ -11,3 +11,12 @@
         .ascii SOHASH
         .ascii ".so"
         .byte 0
+
+#
+# Ensure that every executable includes some static data, as
+# ELF loaders tend to assume this - see
+# <https://bugs.debian.org/919921>, <https://bugs.debian.org/1016718>.
+#
+
+	.section ".data"
+	.byte 0