[PATCH 1/2] ARM: kvm: fix a bad BSYM() usage

[Christoffer Dall]

On Sat, May 09, 2015 at 10:10:56PM +0200, Ard Biesheuvel wrote:
> On 9 May 2015 at 22:07, Christoffer Dall <christoffer.dall at linaro.org> wrote:
> > On Fri, May 08, 2015 at 05:08:42PM +0100, Russell King wrote:
> >> BSYM() should only be used when refering to local symbols in the same
> >> assembly file which are resolved by the assembler, and not for
> >> linker-fixed up symbols.  The use of BSYM() with panic is incorrect as
> >> the linker is involved in fixing up this relocation, and it knows
> >> whether panic() is ARM or Thumb.
> >>
> >> Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
> >> ---
> >>  arch/arm/kvm/interrupts.S | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
> >> index 79caf79b304a..87847d2c5f99 100644
> >> --- a/arch/arm/kvm/interrupts.S
> >> +++ b/arch/arm/kvm/interrupts.S
> >> @@ -309,7 +309,7 @@ ENTRY(kvm_call_hyp)
> >>  THUMB(       orr     r2, r2, #PSR_T_BIT      )
> >>       msr     spsr_cxsf, r2
> >>       mrs     r1, ELR_hyp
> >> -     ldr     r2, =BSYM(panic)
> >> +     ldr     r2, =panic
> >>       msr     ELR_hyp, r2
> >>       ldr     r0, =\panic_str
> >>       clrex                           @ Clear exclusive monitor
> >> --
> >> 1.8.3.1
> >>
> > Indeed, the linker figures it out as it should.  It does seem like the
> > right result is produced with the BSYM() macro as well so not sure what
> > the harm is.
> >
> 
> BSYM() is defined as 'sym + 1' not 'sym | 1', so if the symbol has the
> thumb bit set already, the result is incorrect.
> 
yeah, but the linker will look at the result of 'sym + 1', so on my
system it ends up with 'sym + 1' after the linker has done its thing
(verified by looking at the disassembly of vmlinux); I assume the
linker logic is that it's branching to a thumb function but the target
is already the +1 so no action necessary, as opposed to just blindly
adding 1.

-Christoffer