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

[Guenter Roeck]

On 09/29/2017 01:58 PM, Josh Poimboeuf wrote:
> On Fri, Sep 29, 2017 at 01:38:43PM -0700, Guenter Roeck wrote:
> > 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 ?
> >
> > Here is another interesting problem, seen when building arm64 allmodconfig
> > -CONFIG_CPU_BIG_ENDIAN +CONFIG_EFI.
> >
> > kallsyms failure:
> > 	relative symbol value 0xffffff9008073000 out of range in relative mode
> >
> > This is due to symbols such as
> >
> > 000000000000000e n __efistub_$d
> >
> > in the symbol table. Those are not filtered out by kallsyms, resulting in a
> > relative "base" address of 0x0e, and all other symbols are out of range.
> > Those symbols are only generated in efi/libstubs.
> >
> > Any idea what might be going on there, and how to fix it ?
> > An easy fix would be something like
> >
> > -       else if (stype == 'N')
> > +       else if (toupper(stype) == 'N')
> >
> > in kallsyms, but that doesn't seem like a clean solution to me.
>
> Not sure... What does 'n' mean?  I don't see it in the man page for nm.

Good question. As far as I can see, gcc uses it to mark symbols associated
with -frecord-gcc-switches, presumably in the .GCC.command.line section.

Guenter