x86: check for data and BSS in reloc code at compile time.

This is a more useful failure mode than hanging at boot time, and
incidentally fixes the clang/LLVM build by removing a .subsection rune.

Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Committed-by: Tim Deegan <tim@xen.org>

2 files changed, 10 insertions, 7 deletions

diff --git a/xen/arch/x86/boot/build32.mk b/xen/arch/x86/boot/build32.mk
index 5401b07050..a2d4b34244 100644
--- a/xen/arch/x86/boot/build32.mk
+++ b/xen/arch/x86/boot/build32.mk
@@ -20,6 +20,15 @@ CFLAGS := $(filter-out -flto,$(CFLAGS))
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c -fpic $< -o $@
+	$(OBJDUMP) -h $@ | sed -n '/[0-9]/{s,00*,0,g;p}' |\
+		while read idx name sz rest; do \
+			case "$$name" in \
+			.data|.data.*|.rodata|.rodata.*|.bss|.bss.*) \
+				test $$sz != 0 || continue; \
+				echo "Error: non-empty $$name: 0x$$sz" >&2; \
+				exit $$(expr $$idx + 1);; \
+			esac; \
+		done
 
 reloc.o: $(BASEDIR)/include/asm-x86/config.h
 .PRECIOUS: %.bin %.lnk
diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c
index ea94bf17a5..fa0fb6bfb3 100644
--- a/xen/arch/x86/boot/reloc.c
+++ b/xen/arch/x86/boot/reloc.c
@@ -18,10 +18,7 @@ asm (
     "    call 1f                       \n"
     "1:  pop  %ebx                     \n"
     "    mov  %eax,alloc-1b(%ebx)      \n"
-    "    mov  $_end,%ecx               \n"  /* check that BSS is empty! */
-    "    sub  $__bss_start,%ecx        \n"
-    "    jz reloc                      \n"
-    "1:  jmp 1b                        \n"
+    "    jmp  reloc                    \n"
     );
 
 /* This is our data.  Because the code must be relocatable, no BSS is
@@ -30,9 +27,6 @@ asm (
 asm (
     "alloc:                            \n"
     "    .long 0                       \n"
-    "    .subsection 1                 \n"
-    "    .p2align 4, 0xcc              \n"
-    "    .subsection 0                 \n"
     );
 
 typedef unsigned int u32;