Re: Problems compiling with KCFLAGS="-frecord-gcc-switches"

[Josh Poimboeuf]

On Fri, Sep 29, 2017 at 01:00:56PM -0700, Guenter Roeck wrote:
> Hi Josh,
> 
> when trying to compile an image with KCFLAGS="-frecord-gcc-switches",
> I get the folllowing build warning/error.
> 
> make allmodconfig
> KCFLAGS="-frecord-gcc-switches" make arch/x86/kvm/emulate.o
> ./tools/objtool/objtool check --no-unreachable "arch/x86/kvm/emulate.o"
> 
> arch/x86/kvm/emulate.o: warning:
> 	objtool: .GCC.command.line+0x0: special: can't find new instruction
> 
> Building a full image aborts with:
> 
> WARNING: arch/x86/kvm/kvm.o(__ex_table+0x4c): Section mismatch in reference
> 	from the (unknown reference) (unknown)
> 	to the variable .GCC.command.line:kvm_fastop_exception
> FATAL: The relocation at __ex_table+0x4c references
> section ".GCC.command.line" which is not executable, IOW
> the kernel will fault if it ever tries to
> jump to it.  Something is seriously wrong
> and should be fixed.
> make[2]: *** [arch/x86/kvm/kvm.o] Error 1
> 
> Any idea what might cause this problem ?

I think this is a GCC bug with the -frecord-gcc-switches feature.  Due
to some ambiguity in some inline asm in emulate.c, it's placing some
kernel code in the .GCC.command.line section.

This seems to fix it:


diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index a36254cbf776..d90cdc77e077 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -425,8 +425,10 @@ static int fastop(struct x86_emulate_ctxt *ctxt, void (*fop)(struct fastop *));
 	#op " %al \n\t" \
 	FOP_RET
 
-asm(".global kvm_fastop_exception \n"
-    "kvm_fastop_exception: xor %esi, %esi; ret");
+asm(".pushsection .fixup, \"ax\"\n"
+    ".global kvm_fastop_exception \n"
+    "kvm_fastop_exception: xor %esi, %esi; ret\n"
+    ".popsection");
 
 FOP_START(setcc)
 FOP_SETCC(seto)