Re: [Xen-devel] Clang/LLVM version requirements

[Tim Deegan <tim@xxxxxxx>]

At 11:04 +0100 on 07 Sep (1347015863), Jan Beulich wrote:
> >>> On 07.09.12 at 10:50, Tim Deegan <tim@xxxxxxx> wrote:
> > Jan, would you object to some other way of checking for .bss in reloc.c?
> 
> No, if you have a better idea.
> 
> > Could we just objcopy it out so any BSS symbols make it fail at link
> > time?
> 
> Would that reliably fail with all linker versions?
> 
> > In any case we probably ought to check for stray .data symbols too.
> 
> Not really, no. Those would still be present in reloc.bin, and
> hence make it into reloc.S.

But they would break the test, because the magic alignment happens in
.text, right?  Anyway, compile-time failure seems more useful.  How
about this, based on the similar checks for init-only files?


# HG changeset patch
# Parent 5691e4cc17da7fe8664a67f1d07c3755c0ca34ed
x86: check for BSS in reloc code at compile time.

This is a more helpful 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@xxxxxxx>

diff -r 5691e4cc17da xen/arch/x86/boot/build32.mk
--- a/xen/arch/x86/boot/build32.mk      Thu Sep 13 10:23:17 2012 +0200
+++ b/xen/arch/x86/boot/build32.mk      Thu Sep 13 11:04:29 2012 +0100
@@ -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 "$$name" in \
+                       .bss|.bss.*) \
+                               test $$sz != 0 || continue; \
+                               echo "Error: non-empty BSS: 0x$$sz" >&2; \
+                               exit $$(expr $$idx + 1);; \
+                       esac; \
+               done
 
 reloc.o: $(BASEDIR)/include/asm-x86/config.h
 .PRECIOUS: %.bin %.lnk
diff -r 5691e4cc17da xen/arch/x86/boot/reloc.c
--- a/xen/arch/x86/boot/reloc.c Thu Sep 13 10:23:17 2012 +0200
+++ b/xen/arch/x86/boot/reloc.c Thu Sep 13 11:04:29 2012 +0100
@@ -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;

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