Re: [RFC PATCH] ARM: Use logical or instead of addition for badr address calculation

[Guenter Roeck]

Hi Ard,

On 06/19/2018 12:48 AM, Ard Biesheuvel wrote:
> On 19 June 2018 at 07:07, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > Modern assemblers may take the ISA into account when resolving local
> > symbols. This can result in bad address calculations when using badr
> > in the wrong location since the offset + 1 may be added twice, resulting
> > in an even address target for THUMB instructions. This in turn results
> > in an exception at (destination address + 2).
> >
> > Unhandled exception: IPSR = 00000006 LR = fffffff1
> > CPU: 0 PID: 1 Comm: init Not tainted 4.18.0-rc1-00026-gf773e5bdf0c9 #15
> > Hardware name: MPS2 (Device Tree Support)
> > PC is at ret_fast_syscall+0x2/0x58
> > LR is at tty_ioctl+0x2a5/0x528
> > pc : [<21009002>]    lr : [<210d1535>]    psr: 4000000b
> > sp : 21825fa8  ip : 0000001c  fp : 21a95892
> > r10: 00000000  r9 : 21824000  r8 : 210091c0
> > r7 : 00000036  r6 : 21ae1ee0  r5 : 00000000  r4 : 21ae1f3c
> > r3 : 00000000  r2 : 3d9adc25  r1 : 00000000  r0 : 00000000
> > xPSR: 4000000b
> > CPU: 0 PID: 1 Comm: init Not tainted 4.18.0-rc1-00026-gf773e5bdf0c9 #15
> > Hardware name: MPS2 (Device Tree Support)
> > [<2100bd8d>] (unwind_backtrace) from [<2100b13b>] (show_stack+0xb/0xc)
> > [<2100b13b>] (show_stack) from [<2100b87b>] (__invalid_entry+0x4b/0x4c)
> >
> > Fix the problem by using a logical or instead of an addition. This is
> > less efficient but guaranteed to work.
> >
> > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > ---
> > RFC: I don't really like this, but my ARM assembler knowledge is quite
> > limited. Just dropping the "+ 1" from badr doesn't work because some
> > other code needs it (the image hangs completely if I try that).
> > Ultimately I don't even know if the invoke_syscall macro should just
> > have used adr instead of badr (but then how did this ever work ?).
> >
> > Seen with various toolchains based on gcc 7.x and binutils 2.30 when
> > building and testing MPS2 images.
>
> Hello Guenter,
>
> This issue has been discussed before. It appears the binutils people
> suddenly started caring about the thumb annotation of local function
> symbols, resulting in behavior that is not backwards compatible.
>
> https://marc.info/?l=linux-arm-kernel&m=151143776213636&w=2
> https://sourceware.org/bugzilla/show_bug.cgi?id=21458
>
> Can you try the fix below please?
>
> >   arch/arm/include/asm/assembler.h | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> > index 0cd4dccbae78..24c87ff2060f 100644
> > --- a/arch/arm/include/asm/assembler.h
> > +++ b/arch/arm/include/asm/assembler.h
> > @@ -195,7 +195,8 @@
> >          .irp    c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo
> >          .macro  badr\c, rd, sym
> >   #ifdef CONFIG_THUMB2_KERNEL
> > -       adr\c   \rd, \sym + 1
> > +       adr\c   \rd, \sym
> > +       orr     \rd, #1
> >   #else
> >          adr\c   \rd, \sym
> >   #endif
> > --
> > 2.7.4
>
> ----------8<------------
> From: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Date: Tue, 16 Jan 2018 12:12:45 +0000
> Subject: [PATCH] ARM: assembler: prevent ADR from setting the Thumb bit twice
>
> To work around recent issues where ADR references to Thumb function
> symbols may or may not have the Thumb bit set already when they are
> resolved by GAS, reference the symbol indirectly via a local symbol
> typed as 'object', stripping the ARM/Thumb annotation.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index 6ae42ad29518..dd2ff94ad90b 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -195,13 +195,19 @@
>          .irp    c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo
>          .macro  badr\c, rd, sym
>   #ifdef CONFIG_THUMB2_KERNEL
> -       adr\c   \rd, \sym + 1
> +       __badr  \c, \rd, \sym
>   #else
>          adr\c   \rd, \sym
>   #endif
>          .endm
>          .endr
>
> +       /* this needs to be a separate macro or \@ does not work correctly */
> +       .macro  __badr, c, rd, sym
> +       .eqv    .Lsym\@, \sym
> +       adr\c   \rd, .Lsym\@ + 1

Wild shot, but the following works for me.

	.eqv    .Lsym\@, \sym + 1
	adr\c	\rd, .Lsym\@

Does it make sense ?

Thanks,
Guenter