On Wed, Dec 05, 2018 at 09:09:56AM +0100, Ard Biesheuvel wrote: > (+ Arnd) > > On Wed, 5 Dec 2018 at 09:06, Nathan Chancellor <natechancel...@gmail.com> > wrote: > > > > On Wed, Dec 05, 2018 at 08:37:05AM +0100, Ard Biesheuvel wrote: > > > On Wed, 5 Dec 2018 at 02:42, Nathan Chancellor <natechancel...@gmail.com> > > > wrote: > > > > > > > > This flag is not supported by lld: > > > > > > > > ld.lld: error: unknown argument: --pic-veneer > > > > > > > > Signed-off-by: Nathan Chancellor <natechancel...@gmail.com> > > > > > > Hi Nate, > > > > > > Does this mean ld.lld is guaranteed to produce position independent > > > veneers if you build kernels that are bigger than the typical range of > > > a relative branch? > > > > > > > Hi Ard, > > > > Honestly, I'm not quite sure. I saw your commit that introduced this > > flag and I wasn't quite sure what to make of it for lld. What > > configuration would I use to verify and what would I check for? > > > > Try building allyesconfig, and check the resulting binary for veneers > (which have 'veneer' in the symbol name, at least when ld.bfd emits > them). These veneers should not take the [virtual] address of the > branch target directly, but take a PC relative offset (as in the > example in the commit log of that patch you are referring to) >
Alright, compiling with allyesconfig is a little rough at the moment (bug reports I will file in due time) but I was able to do it. Here's the disassembly specifically for the functions you had in your commit, my assembly knowledge is pretty much non-existent unfortunately so I am not sure what to make of it (it doesn't look like there is a virtual address for pc in that mix?). I am happy to provide any more information that is needed. c03030cc <__enable_mmu>: c03030cc: e3c00002 bic r0, r0, #2 c03030d0: e3c00b02 bic r0, r0, #2048 ; 0x800 c03030d4: e3c00a01 bic r0, r0, #4096 ; 0x1000 c03030d8: e3a05051 mov r5, #81 ; 0x51 c03030dc: ee035f10 mcr 15, 0, r5, cr3, cr0, {0} c03030e0: ee024f10 mcr 15, 0, r4, cr2, cr0, {0} c03030e4: eafff3c5 b c0300000 <__turn_mmu_on> c03030e8: e320f000 nop {0} c03030ec: e320f000 nop {0} c03030f0: e320f000 nop {0} c03030f4: e320f000 nop {0} c03030f8: e320f000 nop {0} c03030fc: e320f000 nop {0} c0300000 <__turn_mmu_on>: c0300000: e1a00000 nop ; (mov r0, r0) c0300004: ee070f95 mcr 15, 0, r0, cr7, cr5, {4} c0300008: ee010f10 mcr 15, 0, r0, cr1, cr0, {0} c030000c: ee103f10 mrc 15, 0, r3, cr0, cr0, {0} c0300010: ee070f95 mcr 15, 0, r0, cr7, cr5, {4} c0300014: e1a03003 mov r3, r3 c0300018: e1a0300d mov r3, sp c030001c: e1a0f003 mov pc, r3 Thanks, Nathan > > Additionally, I have filed an LLVM bug for the lld developers to > > check and see if this is a flag they should support: > > > > https://bugs.llvm.org/show_bug.cgi?id=39886 > > > > Thanks for the quick reply, > > Nathan > > > > > > --- > > > > arch/arm/Makefile | 2 +- > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > diff --git a/arch/arm/Makefile b/arch/arm/Makefile > > > > index e2a0baf36766..4fab2aa29570 100644 > > > > --- a/arch/arm/Makefile > > > > +++ b/arch/arm/Makefile > > > > @@ -10,7 +10,7 @@ > > > > # > > > > # Copyright (C) 1995-2001 by Russell King > > > > > > > > -LDFLAGS_vmlinux := --no-undefined -X --pic-veneer > > > > +LDFLAGS_vmlinux := --no-undefined -X $(call > > > > ld-option,--pic-veneer) > > > > ifeq ($(CONFIG_CPU_ENDIAN_BE8),y) > > > > LDFLAGS_vmlinux += --be8 > > > > KBUILD_LDFLAGS_MODULE += --be8 > > > > -- > > > > 2.20.0.rc1 > > > >