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

[Dave P Martin <Dave.Martin@arm.com>]

On Mon, May 11, 2015 at 10:05:37AM +0100, Christoffer Dall wrote:
> On Sat, May 09, 2015 at 10:10:56PM +0200, Ard Biesheuvel wrote:
> > On 9 May 2015 at 22:07, Christoffer Dall <christoffer.dall@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@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.

There are a few overlapping confusions.

ldr= will do the right thing *if* the target symbol's type is correctly
annotated.  This means that ldr =some_local_code_symbol does the right
thing for branch target addresses if and only if some_local_code_symbol
is marked with .type %function (or ENDPROC).

The fact that a symbol is in a code section is *not* enough.

For ARM code this never mattered, so local symbols in .S files are
probably under-annotated in general.  BSYM() might have been used to
work around this in some cases.

We should check that all the BSYMs removed by this series from ldr=
and .long/.word etc. point to a correctly annotated symbol, and add
the annotations if not.

Cheers
---Dave